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A bacteriophage that is used often for E. coli transduction.
Genome is a linear double-standed DNA molecule of about 90kb, is terminally redundant, and cyclically permuted.
DNA libraries are sometimes kept in the phage and is then transfected into E. coli to grow up clones. Individual clones can be isolated by diluting the phage to produce sepratable plaques.


A 53KDa protein the product of a tumour suppressor gene.
The loss of this protein due to mutation is a primary event in the formation of many types of cancer (breast, colon, lung, leukaemia, liver). P53 is involved in regulating the activity of some other genes (eg. P21, a cell division kinase inhibitor). It also prevents cells entering cell division. P53 levels are increased after DNA is damaged by UV and ionizing radiation and cells are arrested in cell division until either the damage is repaired or they die by apoptosis (programmed cell death).

P1-derived artificial chromosome (PAC)

One type of vector used to clone DNA fragments (100- to 300-kb insert size; average, 150 kb) in Escherichia coli cells.
Based on bacteriophage (a virus) P1 genome.


A logarithmic measure of the potency of an antagonist; the negative log of the concentration of antagonist which would produce a 2-fold shift in the concentration-response curve for an agonist. It is calculated by extrapolating the line on a Schild plot to zero on the y-axis. If the slope of the Schild plot is not exactly 1, but does not differ significantly from it, some authorities prefer to constrain the slope of the line to 1 in calculating an estimated pA2 value (see fig. 1c in Jenkinson (1991)). This pA2 value then equates to the pKB value.


A group of cells that hve an inherent rhytmicity in the fluctuations of their membrane potential and therefore set the rhythm of activity (generally of muscle contraction or of neural activity) in a larger group of connected cells.
An example is a specialized region of the right atrium of the mammalian heart that sets the rate of contraction; also called the sinoatrial (SA) node.

Packed decimal (in computing)

A method of encoding 2 pieces of information in a single byte
For instance, instead of storing a digit in one byte and a sign in another byte using a traditional character encoding scheme, a packed decimal format might use a binary number to indicate the value of the digit in 4 bits of the byte and a code indicating whether the digit is positive or negative in the other 4 bits. The International Monetary Fund distributes data in Packed Decimal format.

Paired comparison

Comparison of a pair of samples that represent the standard or control and an experimental treatment.
e.g., Comparison of two samples as to which has the greater or lesser degree of intensity of a specified characteristic, such as sweetness and hardness. If more than two treatments are being considered, each treatment is compared with every other in the series


Pain is a subjective experience, the quality and magnitude of which can be difficult for the outsider to appreciate, yet there are certain features that allow it to be classified objectively. Its quality depends on the site of origin: the deep, dull ache of muscle or tendon pain is consistently different from the sharp or burning pain generated in the skin, despite the involvement of similar types of nociceptors. The experience of pain is not limited to the subject’s perception: both the autonomic and somatic motor systems respond to pain, and it is believed that these physical responses differ according to the type of tissue from which the noxious inputs originate.
It has been suggested that pain is part of a homeostatic mechanism that signals the presence of tissue damage and encourages the animal to alter its behavioural state. The sensory attributes of pain depend on the tissue of origin: superficial pain, originating in skin, is perceived as sharp and/or burning and is limited to a small well-defined area, whereas deep pain, such as that originating in muscle, is dull and aching and difficult to localize. In addition to these differences in the quality of superficial and deep pain, it has been observed that pain originating in deep structures evokes very different behavioural and cardiovascular responses to pain originating in superficial structures. Pain originating in skin evokes “a rise of pulse rate” and a “sense of invigoration” whereas pain originating in deep structures evokes quiescence, a “slowing of the pulse” and “falling of the blood pressure”. Similar studies showed that muscle pain was associated with a fall in blood pressure and bradycardia in awake human subjects.
Superficial pain causes a spinal reflex that results in a purposeful withdrawal of the body part from the offending stimulus. Conversely, deep pain elicits a passive, coping strategy aimed at promoting healing of the affected body part. Deep pain is more likely to transform into chronic pain than superficial pain.


Defines the upper boundary of the mouth.
The forward portion is the hard palate, hard because bone (maxillae and palatine) makes up this portion of it. Further back in the mouth, the soft palate consists of muscle and lacks any bone support. A conical muscular projection, the uvula, is suspended from the rear of the soft palate.

Palindromic sequence

A DNA locus whose 5'-to-3' sequence is identical on each DNA strand.
The sequence is the same when one strand is read left to right and the other strand is read right to left. Recognition sites of many restriction enzymes are palindromic. The term palindromic DNA ia applied to regions of DNA in which there are inverted repetitions of base sequences with two fold symmentry occuring over two strands.


Amplicillin-resitant plasmid.

Pan balance

Used to distribute weight evenly between more than one container.


In vertebrates, a small, complex gland located between the stomach and the duodenum, which produces digestive enzymes and the hormones insulin and glucagon. Also known as sweet bread when sold by the butcher. I ask you – sweet bread for an internal organ???

Panel testing

A laboratory procedure in which a series of tests is performed on one specimen, usually related to a single condition or disease, or for differential diagnosis.

Panic attack

Discrete periods of sudden onset of intense apprehension, fearfulness, or terror, often associated with feelings of impending doom. During the attacks there are such symptoms as dyspnea (shortness of breath), palpitations, chest pain or discomfort, choking or smothering sensations and fear of going crazy or losing control.
Panic attacks are characteristic of panic disorder, but may also occur in somatization disorder, major depression and schizophrenia.


Gram-negative rods are isolated from plant surfaces, seeds, soil, and water, as well as from animals and human clinical specimens.
They are opportunistic human pathogens.


A hump or swelling.
Plural = papillae
And Papillate = bearing a papilla.


Swelling of the optic nerve head.


The set of all points in a plane that are equally distant from a fixed point (called the  focus) and a fixed line, (called the directrix).


A surface that is formed by rotating a parabola about its axis.

Paracentric inversion

A chromosomal inversion not involving the centromere.
(Note Inversion =  A chromosomal mutation involving the removal of a chromosome segment, its rotation through l80 degrees, and its reinsertion in the same location. The replacement of a section of a chromosome in the reverse orientation.)

Parachute reaction

The automatic placing of hands on floor when infant is suddenly lowered from the prone position.


Two lines are parallel if they are in the same plane and never intersect.


A solid figure with six faces such that the planes containing two opposite faces are parallel.    Each face is a parallelogram.


A quadrilateral with opposite sides parallel.

Parallel Probability

The parallel probability (Par Prob) is the probability that the shape, or form, of the assay run's standard curve is not significantly different from the shape of the reference assay standard curves.
A parallel probability is only computed for logistic curves. Linear regressions are straight lines and a parallel probability is not computed.

Parallel processing (of sensory information)

The relaying and analysis of sensory information down a number of separate (parallel) pathways, each performing some relatively specific processing.
In all sensory systems nerve fibres from the receptor surface at the periphery (i.e., the skin, the ear, the eye, etc) carry to the Central Nervous System a diversity of information about an object that is perceived, e.g., it’s colour, or speed of motion, or loudness/intensity, its duration etc In parallel processing these different pieces of information are believed to be relayed by at least partially separate sets of nerve fibres coming from the same peripheral receptor surface and then processed in different sets of neurons in the same nucleus in the brain or perhaps even  in different nuclei and structures in the brain. (see Serial processing)


Parallelism is the extent to which two or more different substances produce parallel dilution (titration) curves in an immunoassay. Parallelism measurements are used to detect matrix effects in different substance solutions. Parallelism is also used to measure the crossreactivity of a binder to substances similar in structure to the ligand.

Paralogous genes

Two genes or clusters of genes at different chromosomal locations in the same organism that have structural similarities indicating that they derived from a common ancestral gene and have since diverged from the parent copy by mutation and selection or drift.


A parameter is a quantifiable characteristic or feature of a statistical population.


A rare condition characterized by the gradual development of an intricate, complex and elaborate system of thinking based on (and often proceeding logically from) misinterpretation of an actual event.  A person with paranoia often considers himself / herself endowed with unique and superior ability. Despite its chronic course, this condition does not seem to interfere with thinking and personality. To be distinguished from schizophrenia, paranoid type.

Paranoid ideation

Ideation, of less than delusional proportions, involving suspiciousness or the belief that one is being harassed, persecuted or unfairly treated. The term is sometimes used when the clinician is unsure whether the disturbances are actually delusional. Ideas of reference often involve paranoid ideation.

Paranoid personality disorder

Pervasive and long-standing suspiciousness and mistrust of others; hypersensitivity and scanning of environment for clues that selectively validate prejudices, attitudes, or biases. Stable psychotic features such as delusions and hallucinations are absent.

Paranoid schizophrenia

Characterized by a persistence of or preoccupation with persecutory or grandiose delusions, or hallucinations with a persecutory or grandiose content. In addition, there may be delusions of jealousy.

Parapatric speciation

Speciation in which the new species forms from a population contiguous with the ancestral species' geographic range

Paraphyletic (also Paraphyly)

Pertaining to a taxon that excludes some members that share a common ancestor with members included in the taxon
Term applied to a group of organisms which includes the most recent common ancestor of all of its members, but not all of the descendants of that most recent common ancestor. (cf. monophyletic; technically speaking, the distinction between polyphyletic and paraphyletic is not as clear as it might seem.)


Paralysis that involves the legs only (from the waist down).


An organism that absorbs nutrients and consumes part of the tissues of its living host, usually without killing the host.


A symbiotic relationship in which the symbiont (parasite) benefits at the expense of the host by living either within the host (endoparasite) or outside the host (ectoparasite). The parasite derives nutrients at the expense of the host. Parasites usually live for some time in association with their hosts rather than killing them soon after encounter.


Kind of insect whose larvae develop within and kill their host. An organism that is parasitic on a single host in its immature stages, usually kills its host at some point during its development, and is free-living as an adult. Most known parasitoids are insects, in particular wasps (Ichneumonidae, Chalcidae), flies (Tachinidae), and strepsipterans (Stylopidae).

Parasympathetic nervous system

One of the two branches of the Autonomic Nervous System (ANS).See Sympathetic Nervous System, against which the Parasympathetic Nervous System often (but not always) acts in opposition. The Parasympathetic is often viewed as preparing the body for producing and storing energy reserves while the Sympathetic Nervous System is best viewed as mobilizing the body for action.

Parathyroid glands

Four tiny endocrine glands located behind the thyroid gland.
These glands make hormones that help control calcium and phosphorous levels in the body. The parathyroid glands are necessary for proper bone development.

Paravertebral muscle spasm

Spasm in the muscles on either side of the spinous processes (midline of the back).

Paraxial rod

A rod of material lying within the flagellum parallel to the axoneme, only found in some protists (e.g. euglenids) causing the flagellum to appear relatively thick.


Members of the subkingdom of animals consisting of the sponges.


A relatively unspecialized plant cell type that carries most of the metabolism, synthesizes and stores organic products, and develops into more differentiated cell types.

Parental generation

In an experimental genetic cross, the parents of the F1 generation; homozygous for the trait(s) being studied.


Brought into the body in some other way than the digestive tract, as by subcutaneous or intravenous injection.


Weakness, partial paralysis


An abnormal sensation of burning, prickling or tingling

Parietal Lobe

One of the two parietal lobes of the brain located behind the frontal lobe at the top of the brain.

Parkinsonism / Parkinson’s disease

Basal ganglion disease characterized by hypokinesia, tremor at rest, and muscular rigidity.


Refers to a rule used to choose among possible alternative explanations for a phenomenon, which states that the explanation implying the least number of changes in character states is the best.


A type of reproduction in which females produce offspring from unfertilized eggs.
The artificial activation of an egg in the absence of a sperm; the egg begins to divide as if it has been fertilized.

Partial agonist

An agonist which, no matter how high a concentration is applied, is unable to produce maximal activation of the receptors. In a preparation with a low receptor reserve, it is therefore unable to produce a maximal response. See also efficacy

Partial pressures

A fraction of total pressure.
Used as a measure of the concentration of gases.

particle size

the diameter, in millimeters, of suspended sediment or bed material. Particle-size classifications are:
[1] Clay—0.00024-0.004 millimeters (mm);
[2] Silt—0.004-0.062 mm;
[3] Sand—0.062-2.0 mm; and
[4] Gravel—2.0-64.0 mm.

parts per billion

the number of "parts" by weight of a substance per billion parts of water. Used to measure extremely small concentrations.

parts per million

the number of "parts" by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations.

Passy-Muir valve

A specialized one-way valve that is designed to be placed onto a tracheostomy tube (when the cuff is deflated), which then allows the patient to exhale through the upper airway

Passage (in cell biology)

In cell culture, the process in which cells are disassociated, washed, and seeded into new culture vessels after a round of cell growth and proliferation.
The number of passages a line of cultured cells has gone through is an indication of its age and expected stability.

Passive-aggressive personality disorder

Aggressive behavior manifested in passive ways such as obstructionism, pouting, procrastination, intentional inefficiency and obstinacy.
The aggression often arises from resentment at failing to find gratification in a relationship with an individual or institution upon which the individual is overdependent.

Passive diffusion (sometimes called passive transport)

When no facilitating factor is involved in the net movement of molecules from a region of high concentration to a region of lower concentration.
With regards to transmembrane movement, certain molecules can simply dissolve in the phospholipid bilayer, diffuses across it, and then dissolve in the aqueous solution at the other side of the membrane. No membrane proteins are involved in process, and the net movement of molecules is driven simply by the concentration gradient through intermolecular collisions from Brownian motion.
Water readily diffuses according to concentration gradient in a process termed osmosis. Although a polar molecule, it can pass across the cell membrane readily because of its small size (which allows permeation through the lipid bilayer even though the latter is hydrophobic) and because of the prescence of transmembrane protein channels called aquaporins.


Process designed to reduce the population of pathogenic bacteria in a product, sufficient to ensure product safety but with modest impact on the nutritional properties and flavour of the product.
Traditionally, this term has been applied to thermal processes but it can also refer to emergent alternative technologies with the purpose of pathogens inactivation.
Pasteurised milk or milk product = been exposed to a process of pasteurization wherein every particle of that milk or milk product is heated in properly designed and operated equipment to a specified temperature and then held continuously at or above that temperature for at least the corresponding specified time. Pasteurization eliminates pathogen (disease causing bacteria) contamination in milk and products derived from milk.

Patch clamp recordings

A technique for recording the flow of current and voltage changes in a cell and of perturbing the intracellular contents, to understand the properties of membrane ion channels.
A pipette is used to just touch the cell membrane without penetrating it. Negative pressure is applied to form a very tight seal (a gigaohm seal) of the cell membrane against the pipette so that the cell becomes firmly attached to the pipette, i.e., a patch of membrane is “clamped” against the pipette. This can result in very stable recordings.
In the voltage clamp technique, the membrane potential is held constant while measuring currents across the cell membrane.
In some cases, the patch of membrane is “sucked” into the pipette, allowing a direct contact between the electrode solution and the intracellular medium. This “whole cell” configuration gives the same resolution as intracellular recording, but with more stability.

Pathfinding (Neuronal pathfinding)

The process of neuronal axons finding the right neuron or target to connect to.


Disease-causing agent, usually a living microorganism or virus.
Generally, any viruses, bacteria, or fungi that cause disease.

Pattern formation (in development)

The developmental processes by which the complex shape and structure of higher organisms occurs.
The ordering of cells into specific three-dimensional structures, an essential part of shaping an organism and its individual parts during development.


A common plasmid.
Along with the obligatory origin of replication, this plasmid has genes which make the E. coli host resistant to ampicillin and tetracycline. It also has several restriction sites (BamHI, PstI, EcoRI, HindIII etc.) into which DNA fragments could be spliced in order to clone them.


See Polymerase chain reaction (PCR)

PCR amplicons

DNA that is generated by PCR amplification.


The negative logarithm of the EC50 or IC50 value.

Peabody Picture Vocabulary Test - Revised (PPVT-R)

A testing instrument which assesses comprehension of single spoken words through a picture pointing task.

Peak expiratory flow rate

The maximal flow rate generated during a forceful exhalation.

Peak flow

the maximum instantaneous discharge of a stream or river at a given location. It usually occurs at or near the time of maximum stage.

Peak flowmeter

A portable medical device that measures a patient’s spontaneous peak expiratory flow rate during a forced exhalation.


Natural gelling agent and principally used in making jams and jellies.


The part of each side of the neural arch of a vertebra. It connects the lamina with the vertebral body. The first portion of the posterior spine arising from the vertebral body.


A family tree describing the occurrence of heritable characters in parents and offspring across as many generations as possible.


Large nerve tracts (bundles) that emerge from certain regions of the brain.


Pertaining to the water column of the open sea - as opposed to coastal areas or the sea floor.

Pelvic inflammatory disease (PID)

Ascending infection from the vagina or cervix to the uterus, fallopian tubes and broad ligaments.
Most often caused by bacterial infections of chlamydia and gonorrhea.

Penetration depth

Distance the electromagnetic waves (of a certain frequency) travel in a material before it loses 63% (1/e) of its energy.


In genetics, the proportion of individuals with a particular genotype that show the phenotype ascribed to that genotype. Quantitative concept of gene expression: the percentage with which a dominant or homozygous recessive gene expresses itself in the phenotype.
It depends both on genotype and environment. If all individuals of the genotype show the trait, penetrance is 100%.  Compare with expressivity,  a qualitative concept of gene expression.


Measurement of the gel strength (the penetration of a plunger of a defined size and weight into the gel is measured).


A digestive protease (EC released by the chief cells in the stomach that functions to degrade food proteins into peptides.
Pepsin is expressed as a pro-form zymogen, pepsinogen, whose primary structure has an additional 44 amino acids. In the stomach, chief cells release pepsinogen. This zymogen is activated by hydrochloric acid (HCl), which is released from parietal cells in the stomach lining.
The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and HCl from the stomach lining when food is ingested. HCl creates an acidic environment which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin (the active form). Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin.
Pepsin will digest up to 20% of ingested carbon bonds by cleaving preferentially after the N-terminal of aromatic amino acids such as phenylalanine and tyrosine. It will not cleave at bonds containing valine, alanine or glycine. Peptides may be further digested by other proteases (in the duodenum) and eventually absorbed by the body.
Pepsin is stored as pepsinogen so it will only be released when needed, and does not digest the body's own proteins in the stomach's lining. Pepsin functions best in acidic environments, particularly those in a pH of 3.  Pepsin denatures if the pH is more than 5.0. Pepsin is potently inhibited by the peptide inhibitor pepstatin.


A large molecule (macromolecule) made up of amino acids.
Peptides are the building blocks of proteins.

Peptide bond

A covalent bond joining two amino acids, formed by condensation synthesis.


A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides.

Peptidyl site

The site on the ribosome occupied by the peptidyl- tRNA just before peptide bond formation.

Peptidyl transferase

The enzymatic centre in the ribosome responsible for peptide bond formation during translation.

per capita use

the average amount of water used per person during a standard time period, generally per day.


The interpretation of sensations by the brain; The ability to make sense of what one sees, hears, feels, tastes or smells.


A fraction in which the denominator is assumed to be 100.


The movement of water through the openings in a substance, e.g., rock or soil. 1) The movement of water through the openings in rock or soil. (2) the entrance of a portion of the streamflow into the channel materials to contribute to ground water replenishment.

Perfect number

A number that is the sum of all its factors except itself.   For example, 6.


Sometimes used synonymously with blood flow, although perfusion can be applicable to the flow of a solution other than blood.


The sac that contains the heart and produces fluid which allows the heart to move easily.

Pericentric inversion

A chromosomal inversion that involves the centromere.


The region around or near the centromere (also known as kinetochore) of a nuclear chromosome.


The sum of the lengths of the sides of a polygon.


The measure of how often a function repeats its same values.

Periodic function

A function that keeps repeating the same values.


A fibrous membrane that covers the surface of bone except at the end of the bones where it is covered with cartilage as part of a joint.
In children, periosteum is involved in forming new bone and molding the configuration of bone; and in the adult, the periosteum forms new bone secondary to injury or infection.

Peripheral membrane protein

Proteins loosely attached or adhering to a membrane.
They do not span the lipid bilayer of the membrane, but attach indirectly, typically by binding to integral membrane proteins, or by interactions with the lipid polar head of the lipid bilayer of the cell mambrane.

Peripheral nervous system

The sensory and motor neurons that connect the body to the central nervous system.

Peripheral neuropathy

Damage to the neurons that carry sensory information from the arms and legs.

Peripheral pain pathways

Pain perception is initiated by excitation of unmyelinated (C-fibre) or thinly myelinated (Adelta) primary afferent neurons (nociceptors) with cell bodies in spinal dorsal root ganglia (DRG). The DRGs also contain temperature sensors, itch sensors, cutaneous mechanoreceptors and mechanosensitive afferents from the musculature, none of which is normally nociceptive. Nociceptors can be divided into subclasses defined by their receptive properties, neurochemical profiles and central projections. Each subclass of nociceptive neurons responds to a characteristic range of acute stimuli, such as tissue damage, extreme temperature, or chemical insult. For reasons that remain poorly known, peripheral nerve damage can lead to neuropathic pain: the on-going perception of pain in the absence of any other tissue damage.

Driven by transcription factors (TFs) including Islet1, Sox10 and Runx1, all nociceptors express TrkA, the receptor for nerve growth factor (NGF), during embryonic development. Postnatally, a subpopulation retains Runx1, extinguishes TrkA, and expresses Ret, the receptor for glial cell derived neurotrophic factor (GDNF). Unmyelinated TrkA+ nociceptors contain the peptides calcitonin-gene-related-peptide (CGRP) and/or substance P (SP) whereas thinly myelinated TrkA- nociceptors bind the lectin I-B4 and express P2X3 receptors. Further differentiation of nociceptors establishes different subclasses defined by their receptive properties, neurochemical profiles and central projections. Each subclass of nociceptive neurons responds to a characteristic range of acute stimuli, such as tissue damage, extreme temperature, or chemical insult. However, for reasons that remain poorly known, peripheral nerve damage can lead to neuropathic pain: the on-going perception of pain in the absence of any other tissue damage.


The entire assemblage of scales, spines and spicules which encase some heliozoa, chrysomonads, etc.


Regular contractions of a body or part of a body.
Mostly said of the intestinal system of vertebrates: a series of alternating contractions and relaxations of smooth muscle that lines the walls of the digestive organs and that forces food to move forward.
Also applied to the squirming behaviour of some euglenids.
See Segmentation.


The region of the body around, and external to, the mouth.
To deserve application of this term, the region must be modified to favour the acquisition of food.


A membrane that lines the body cavity and forms the external covering of the visceral organs.


Inflammation of the lining of the abdominal cavity.
Indications of peritonitis are called "peritoneal signs": tender abdomen, rebound pain (pain when manual pressure released from examining abdomen), board-like rigidity of abdominal muscles, no bowel sounds (gurgles). The peritoneal membrane is very sensitive to exposure to foreign substances. Contact with blood, bile, urine, pus will cause peritoneal signs. Before antibiotics, people would die from peritonitis if an inflamed appendix burst.

Peritubular capillaries

In the vertebrate kidney, the capillaries that surround the renal tubule.
Water and solutes are reabsorbed into the bloodstream through the peritubular capillaries and some substances are secreted from them into the renal tubule

Periventricular leukomalacia

The most common ischemic brain injury in premature infants, characterized by the death of white matter near the cerebral ventricles.
The lesion results from decreased cerebral blood flow to the periventricular white matter surrounding the lateral ventricles in the brain. The associated neurologic deficit is spastic dylegra. Frequently results in Cerebral Palsy.


A measure of the ease with which a substance can penetrate through a membrane.
An Ideal semi-permeable membrane is permeable only to solvent, usually water, ie. a water molecule is the only kind of particle that can penetrate, or pass through, the membrane.
Selectively permeable membrane is permeable to some substances and impermeable to others, i.e., certain particles other than water molecules may be able to penetrate such a membrane. This definition applies to a biological membrane.

Permissible Exposure

A human exposure limit that is published and enforced by OSHA as a legal standard. PEL may be either time-weighted-average (TWA). Limit: Exposure limit (8 hour), a 15 minutes short term exposure limit STEL, or ceiling © limit. Expressed in ppm or mg/m3.


The permutation of n things taken j at a time is:


An enzyme which catalyzes the transfer of oxygen from hydrogen peroxide to a suitable substrate and thus brings about oxidation of the substrate.

Peroxide Value (PV)

Measures the amount of peroxides and hydroperoxides in a sample of fat produced in the oxidation process.


A microbody containing enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.


Two lines are perpendicular if the angle between them is 90 degees.


1. The tendency to continue an activity once it has been started and to be unable to modify or stop the activity even though it is acknowledged to have become inappropriate.
2. Persistent repetition of words, ideas of subjects so that, once an individual begins speaking about a particular subject or uses a particular word, it continually recurs. Perseveration differs from the repetitive use of "stock words" or interjections such as "you know" or "like".
The inappropriate persistence of a response in a current task which may have been appropriate for a former task. Perseverations may be verbal or motoric. Perseveration is most commonly seen in organic mental disorders, schizophrenia and other psychotic disorders.

Persistent Vegetative State (PVS)

A long-standing condition in which the patient utters no words and does not follow commands or make any response that is meaningful.


Acute infectious disease characterized by a cough that has a "whoop" sound; also called whooping cough

A serious respiratory infection which can cause pneumonia, brain damage, or death.

Pervasive developmental disorder (PDD)

1. Extreme distortions or delays in the development of social behavior and language. 2. A term used to describe drug exposure to children while in the womb. Results of this exposure can cause extremely short attention spans. Behavior disorders, limited or no processing skills, and/or difficulty understanding spoken words.

PET scan (positron emission tomography scan)

A procedure in which a small amount of radioactive glucose is injected into a vein, and a scanner is used to make detailed, computerized pictures of areas inside the body where the glucose is used.
Because cancer cells often use more glucose than normal cells (as they more metabolically active than normal cells), the pictures can be used to find cancer cells in the body.
After receiving the radioactive drug, the patient lies still for about 45 minutes while the drug circulates throughout the body. If a tumor is present, the radioactive sugar will accumulate in the tumor. The patient then lies on a table, which gradually moves through the PET scanner 6 to 7 times during a 45-minute period. The PET scanner is used to detect the radiation. A computer translates this information into the images that are interpreted by a radiologist.
PET scans are more accurate in detecting larger and more aggressive tumors than in locating tumors smaller than 8 mm and/or less aggressive. They may also detect cancer when other imaging techniques show normal results. PET scans may be helpful in evaluating and staging recurrent disease.

Petit mal attacks

Epileptic seizures characterized by brief periods of fixed stare, unconsciousness, unresponsiveness, and lack of activity.


The negative logarithm of the hydrogen ion concentration in an aqueous solution.
A measure of the relative acidity or alkalinity of water. Water with a pH of 7 is neutral; lower pH levels indicate increasing acidity, while pH levels higher than 7 indicate increasingly basic solutions. See pH scale


A virus for which the natural host is a bacterial cell (i.e., it infects bacteria).
Also called a bacteriophage.


A type of plasmid which carries within its sequence a bacteriophage replication origin.
When the host bacterium is infected with "helper" phage, the phagemid is replicated along with the phage DNA and packaged into phage capsids.


Large white blood cells that contribute to the immune defenses by ingesting microbes, other cells, cell debris, microorganisms, and foreign particles.
The two principal phagocytes are neutrophils and monocytes. They emigrate out of the blood and into tissues in which an infection has developed.
After phagocytosis the ingested material is then degraded via enzymes.


A type of endocytosis (internalization into the cell) involving large, particulate substances; The ingestion of visible particles of food by enclosing them with a membrane to form a food vacuole..


An organism which feeds by phagocytosis.


Examines the effects on the body of a drug, specifically the study of the intake of drugs in the body including absorbtion, distribution, transformation and excretion.
Examines issues such as how quickly a drug is absorbed into the blood and how different dosages affect the absorption, how the drug is distributed into organs or tissues the body, how the body metabolizes the drug and whether what the drug is changed into by the body is active, as well how long it takes the body to metabolize half of the drug (the drug's half-life), and how long it takes the drug to clear the body and be excreted


The individualized drug selection and dosage profiling by a doctor based on patient's genotyping and haplotyping data for genes involved in pharmacodynamic and pharmacokinetic drug actions in the body.

Pharyngeal arches (Also known as branchial arches, gill arches, or visceral arches)

Columns of mesenchyme found in the neck of the developing vertebrate embryo derived from cranial neural crest.
In lower vertebrates, blood vessels formed here become part of the gills; in higher vertebrates derivatives include portions of the jaw and middle ear.


An area in the vertebrate throat where air and food passages cross.
A region of the ingestion apparatus lying internal to the mouth (of a metazoan organism) or internal to the cytostome of a protist. Involved in the swallowing process.
See Cytopharynx.

Phase changes

Freezing or boiling

Phase I, II, III or IV studies (Federal Drug Administration (FDA) Approval Process)

Different stages of a clinical trial necessary to prove the efficacy and absence of toxicity of a drug or treatment.
Preclinical study: Stage of drug research development, undertaken prior to the administration of the drug to humans. Consists of in vitro and in vivo screening, pharmacokinetics, toxicology, and chemical upscaling.

Phase I: If preclinical toxicity studies in animals reveal no harmful effects, a phase I study is usually then performed on a limited number of healthy human volunteers.These studies are designed on a small scale to show that the drug is safe in humans and set to determine the highest tolerated dose and to explore the safety, kinetics, interactions, and pharmacological effects of various doses. They seek to gain early evidence of effectiveness. Ultimately, all the information gleaned from a well designed Phase I trial will aid in developing a well designed Phase II trial. Phase Ia studies examine how the body reacts to a single, one-time dose of the drug in a monitored situation, usually involving healthy volunteers without the condition that the drug will be indicated for. As each dose is tested in a predetermined number of people, the trial progresses to the next dosing level with a different group of volunteers. Phase Ib trials examine how the body reacts to multiple doses of the drug over a period of time, from a few days to a few weeks. The average length of time that a compound is in Phase I testing is about one year.

Phase IIPhase II testing looks at the efficacy of a drug, provided that it has proven safe in Phase I testing. Phase II testing is conducted in a relatively small number of patients having the condition the drug is indicated for, using several doses that, based on the data obtained in Phase I, are hypothesized to be efficacious doses. A placebo is also tested to obtain a baseline value for the comparison of drug effectiveness. Phase II testing takes from several months to 3 years.
In clinical phase IIa studies, efficacy is tested on a limited group of patients, and the optimal administration regimen (dose, frequency) is determined. Often, a phase IIb study with a larger number of patients is required, or a combined phase IIb/III, in order to be able to make statistically justifiable recommendations with regard to the administration regimen.

Phase III: Here efficacy and safety of a single or a limited number of drug regimens are evaluated by applying them in a sufficiently large number of patients (usually a few hundred), who are more representative of the population as a whole. Efficacy and safety of the new treatment are compared with a placebo or with the existing standard treatment or to other approved drugs. Usually, several complementary phase III studies are performed simultaneously. The reports of the phase I to III studies are part of the drug registration file. Studies commenced after closing the registration file but before the product is released on the market are sometimes grouped under the name phase IIIb studies.
Testing takes on average 24 months, with the FDA requiring a longer period for drugs to treat some indications.

New Drug Application (NDA): When a drug has completed all three stages of testing, the company files an NDA with the FDA. Once the FDA has reviewed all the data from clinical testing, it can approve the NDA for the drug to be marketed and sold, or it can schedule a hearing to bring experts together to comment on the clinical data and the drug to be reviewed. Sometimes an additional study is requested to clarify the scientific data to show more proof that the drug works, or to compare the drug to existing medications for treating the disease.

Phase IV: Begins when the product is approved for release on the market. Phase IV consists of post marketing surveillance studies conducted to learn more about the drug's risks, benefits, optimal use, and how it compares to competitors. Studies on large numbers of people may be helpful after a product is brought into circulation, for instance, in order to trace rare side effects.

Phencyclidine hydrochloride (PCP)

An anesthetic agent used in veterinary medicine.
Also an illegal hallucinogenic street drug, "angel dust."


A phenotype that is not genetically controlled but looks like a genetically controlled phenotype.
An environmentally induced phenotype that resembles the phenotype produced by a mutation.


An approach to taxonomy based entirely on measurable similarities and differences in phenotypic characters, without consideration of homology, analogy, or phylogeny.


The physical and physiological traits of an organism ; Observable manifestation of a genetic trait, resulting from a specific genotype (the set of genes it possesses) and its interaction with the environment.
The term for the appearance of an organism with respect to a particular character or group of characters (physical, biochemical, and physiologic), as a result of the interaction of its genotype and its environment. Therefore, organisms with the same genotype may display different phenotypes due to environmental factors. Conversely, organisms with the same phenotypes may have different genotypes.
Often used to define the consequences of a paticular mutation.

Phenotypic variance

Variance of phenotype due to the combination of genotypic and environmental factors.

Phenylketonuria (PKU)

An inherited disease in which the body is unable to metabolize a part of protein called phenylalanine, which then collects in the blood stream. This abnormal build-up of phenylalanine can prevent the brain from developing as it should.
Children with PKU often are irritable, restless, and destructive. They may have a musty oder about them, and often have dry skin or rashes. Some have convulsions. Usually, they become physically well-developed children and have blonder hair than their relatives.
It can be detected through a routine screening soon after birth, and can cause mental retardation if strict diet management is not instituted.


A chemical signal that is used to pass information between individuals, generally of the same species.
A small, volatile chemical signal that functions in communication between animals and acts much like a hormone in influencing physiology and behaviour.


The distance between threads on a screw. Cancellous bone screws have a larger pitch than cortical bone screws.

Philadelphia chromosome

A translocation between the long arms of chromosomes 9 and 22, often found in the white blood cells of patients with chronic myeloid leukemia.

pH meter

An electrode system for measuring the free Hydrogen ion concentration in a solution.
Most pH systems consist of a delicate glass bulb which is sensitive to hydrogen ions, a reference electrode, a high input impedance volt meter, and often a temperature sensor.


Defective development of the limbs so that the hands and/or feet are attached close to the body and resemble flippers.


The production of sound by means of vocal cord vibration.

Phonic generalization

Refers to the ability to generalize information related to sounds from one word or configuration to another, predicting that which might follow, in order to approximate proper spelling.

Phonological impairment

A common speech disorder also known as misarticulation.
Here the child says the sounds wrong, or omits or duplicates certain sounds within a word. The problem may reflect poor neurological motor skills, a learning error, or difficulty in identifying certain speech sounds. Examples of common errors are
"wabbit" for "rabbit", "thnake" for "snake", "dood" for "good", and "poo" for "spoon".
Another phonological impairment is unstressed syllable deletion, in which a child simply skips over a syllable in a long word, as in "nana" for "banana", or "te-phone" for "telephone". Many of these misproductions are a part of normal development and are expected in the speech of very young children, but when they persist past the expected age they are considered abnormal and usually indicate brain dysfunction.

Phonological process analysis

The evaluation process in which the patterns of speech errors are carefully analyzed to determine if a developmental phonological disorder may be present.


An enzyme that hydrolyzes esters of phosphoric acid, removing a phosphate group.

Phosphatase and tensin homolog activity (PTEN activity)

Refers to activity of a pathway within the body, which helps to regulate insulin signaling and insulin sensitivity in adipose tissue.
That pathway's normal product is a specific lipid, but in some advanced cancers the PTEN activity is greatly altered. Thus, some pharmceutical research programs screen cancer-drug candidate compounds against PTEN activity.

Phosphatase and tensin homolog gene (PTEN gene)

A tumor-suppressor gene (in human DNA) that also serves a function in the regulation of insulin signaling and insulin sensitivity.
The presence of genistein has been shown to induce the PTEN gene (thereby resulting in apoptosis of breast cancer cells).

Phosphate group

A functional group consisting of a phosphorus ion combined to 4 oxygen.
A functional group important in energy transfer in reactions in cells.

Phosphodiester bond

A bond between a sugar group and a phosphate group.
Such bonds form the sugar-phosphate-sugar backbone of DNA and RNA. A diester bond (between phosphoric acid and two sugar molecules) links two different nucleotides together to form the nucleotide polymers DNA and RNA.

Phospholipase A2 (PLA2)

A key enzyme involved in the release of arachidonic acid (AA) from the cell membrane.
Inhibition of PLA2 by lipocortins results to a decrease in inflammation.


Molecules consisting of a phosphate group combined with 1 or more lipids, arranged having a polar, hydrophilic head and a nonpolar, hydrophobic tail (the lipid molecules).
These molecules make up the inner bilayer of biological membranes, and are arranged with polar, hydrophilic head pointing outwards with the nonpolar, hydrophobic tail pointing inwards.


The addition of a phosphate group (Phosphorous + 4 Oxygen) to another molecule.
Hence Dephosphorylation = the removal of a phosphate group from another molecule.


An organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide.
Hence Photoautotrophy (Synonymous with photosynthesis)


A semiconductor device used to detect light and generate an electrical current.

Photoelectron spectroscopy

Technique using photoionization and analysis of emitted photoelectrons to study composition and electronic state of a sample.

Photofragmentation spectrometer

Three-stage instrument that uses an electrospray ion source, a coolable ion trap, mass selection and laser light illumination, and a time-of-flight spectrometer to study solvation processes.


An organism that uses light to carbon generate ATP but that must obtain in organic form.


A photodetector, with adjustable voltage, that translates optical tube (PMT) signals into electrical current.
Increasing the PMT voltage, increases the output signal for a given amount of light.


A quantum, or discrete amount, of radiant energy.
Fundamental quantum particles.  It is the interaction of photons with other particles that drives the universe.


Regular alternation of night and day, the length of which varies in the course of the year.
Hence: Photoperiodism = A physiological response to day length, such as flowering in plants


Painful oversensitivity to light. For example, there is photophobia in measles (rubeola). Keeping the lights dim or the room darkened may be useful. Sunglasses may also help.
Progressive: Increasing in scope or severity. Advancing. Going forward. In medicine, a disease that is progressive is going from bad to worse.


The process of generating ATP from ADP and phosphate by means of a proton-motive force generated by the thylakoid membrane of the chloroplast during the light reactions of photosynthesis.


A cell or organ capable of detecting light.


A metabolic pathway in plants that consumes oxygen, releases carbon dioxide, generates no ATP, and decreases photosynthetic output.
Generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide.


The products of photosynthesis.


The biological synthesis of organic material using light as an energy source; The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.
Plants convert carbon dioxide and water, in the presence of chlorophyll and light energy, into carbohydrate food and oxygen.  It is a means of acquiring energy for metabolism which involves trapping radiant energy in chloroplasts, the use of that energy to break up water molecules (hydrolysis) and to convert released energy into an accessible form - such as the molecule ATP. The only form of autotrophy in eukaryotic cells. Some heterotrophic protists have symbiotic algae which allow them to exploit photosynthesis.
Synonymous with Photoautotrophy.

Photosynthetic pigments

Large molecules in chloroplasts that absorb radiant energy (hence they have colour), mostly chlorophylls and carotenes and, occasionally phycobilins.


The light-harvesting unit in photosynthesis, located on the thylakoid membrane of the chloroplast and consisting of the antenna complex, the reaction-center chlorophyll a, and the primary electron acceptor.
There are two types of photosystems, I and II; they absorb light best at different wavelengths.


Growth of a plant shoot toward or away from light.


a measure of the relative acidity or alkalinity of water. Water with a pH of 7 is neutral; lower pH levels indicate increasing acidity, while pH levels higher than 7 indicate increasingly basic solutions.

pH scale

Scale used to express acidity or alkalinity, from 1 (strong acid) through 7 (neutral) to 14 (strong alkali).
A measure of hydrogen ion concentration equal to –log [H+] and ranging in value from 0 to 14.
The scale is logarithmic, and each pH unit is 10 foild different from the previous or the next, e.g., pH 4 is ten times as acidic as pH 5 and pH 2 is ten times as acidic as pH 3, and so on.
pH is a measure of the activity of the hydronium ion (H3O+) which, according to the Debye-Huckel expression, is a function of the concentration of the hydronium ion [H3O+], the effective diameter of the hydrated ion and the ionic strength (µ m) of the solvent. For solutions of low ionic strength (µ m < 0.1) hydronium ion activity is nearly equivalent to [H3O+] which isnormally abbreviated to [H+]. Then, for a weak acid (HA) dissociating to H+ and A- with a dissociation constant, Ka and pKa equal to -log10 Ka, the most important relationships are defined the following two equations: Ka = [H+] [A-] / [HA]  and pH = log 1 / [H+] = pKa + log [A-] / [HA]

Phyletic change

The changes taking place in a single lineage of organisms over a long period of time; one of the principal patterns of evolutionary change.

Phylogenetic inference

The scientific process of collecting and analyzing data to provide the best estimate of the true phylogeny of a group of organisms.


The evolutionary history of a species or group of related species; The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered.
Any group of species are descended from a common ancestral species, which, over time, split into two species, with these descendents splitting again, and again, until the entire collection of species was produced through evolution and speciation (the splitting of a lineage). The genealogical connections thereby formed in general take the shape of a tree, called the phylogeny of the group.


The scientific discipline of resolving phylogenetic relationships between organisms.  
Field of biology that deals with the relationships between organisms. It includes the discovery of these relationships, and the study of the causes behind this pattern.

Phylum (Plural = phyla)

A taxonomic category; phyla are divided into classes.
The second highest level of taxonomic classification containing one or more Classes forming a subgroup of a Kingdom.


A physician who specializes in physical medicine and rehabilitation.
Pronounced Fizz ee at' rist. Some physiatrists are experts in neurologic rehabilitation, trained to diagnose and treat disabling conditions. The physiatrist examines the patient to assure that medical issues are addressed; provides appropriate medical information to the patient, family members and members of the treatment team. The physiatrist follows the patient closely throughout treatment and oversees the patient's rehabilitation program.

Physical map

Map indicating physical locations on a DNA molecule (e.g., restriction enzyme recognition sites, RFLPs, and genes); measured in base pairs (bp).
For the human genome, the lowest- resolution physical map is the banding patterns on the 24 different chromosomes; the highest- resolution map would be the complete nucleotide sequence of the chromosomes.

Physical record

A chunk of data that has a specified and constant size in bytes or that is clearly delimited from other records by a newline character (one or two bytes which denote the end of a line) or sector of a disk or other means identifiable to a computer program reading the file.
For example, a card-image data file has physical records of 80 bytes each, by definition. In a file in logical record length structure, each physical record is the same number of bytes in length as the "logical record length."

Physical record length

The length, in bytes , of a physical record.
In ICPSR Tape Information Forms and on CDNet, physical record length is referred to simply as "record length" (abbreviated "RecLen").

Physical therapist

A person who is licensed to assist in the examination, testing, and treatment of persons who are physically disabled or handicapped through the use of special exercise, application of heat or cold, use of sonar waves, and other techniques.
The physical therapist evaluates components of movement, including: muscle strength, muscle tone, posture, coordination, endurance, and general mobility. The physical therapist also evaluates the potential for functional movement, such as ability to move in the bed, transfers and walking and then proceeds to establish an individualized treatment program to help the patient achieve functional independence.


The study of function in cells, organs, or entire organisms; the processes of life.
The most superior of all disciplines, surpassing art in beauty, philosophy in  thoughtfulness, law in erudition, commerce in attention to detail,engineering in practicality, and footy in grace and skill.
Leaves as a speck in the distance pretenders like biochemistry, microbiology, pathology, genetics, immunology and other minor disciplines of some interest only to those who don’t get out much.


Chemical complex (large molecule) substance that is the dominant (i.e., 60 to 80%) chemical form of phosphorous within cereal grains, oilseeds, and their by products.
Monogastric animals (e.g., swine) cannot digest and utilize phosphorus within phytate, because they lack the enzyme known as phytase in their digestive system, so that phosphorus (phytate) is excreted into the environment.


An antibiotic, produced by plants, that destroys microorganisms or inhibits their growth.


Substances found in edible fruits and vegetables that may be ingested by humans daily in gram quantities and that exhibit a potential for modulating the human metabolism in a manner favorable for reducing risk of cancer.


A pigment involved in many responses of plants to light.


Particulate matter derived from phytoplankton and plants.


Aquatic, free-floating, microscopic, photosynthetic organisms.
Microscopic plant plankton that are the primary producers in the oceans, obtaining their energy through photosynthesis.


The ratio of the circumference of a circle to its diameter.

Pia mater (Latin for soft mother)

The innermost of the three connective tissue membranes (the meninges) that cover and protect the brain and spinal cord.
A vascular membrane that covers the surface of the CNS. The blood vessels provide nutrients to and remove waste products from the brain and spinal cord.
(see Arachnoid MaterDura Mater, Meninges)


The craving and eating of unusual foods or other substances.
Seen in a variety of medical conditions, pregnancy, and emotional disturbances.


A colored substance that absorbs light over a narrow band of wavelengths.

Pilus (Plural = pili)

A surface appendage in certain bacteria that functions in adherence and the transfer of DNA during conjugation.
A conjugation tube; a hollow hair-like appendage of a donor Escherichia coli cell that acts as a bridge for transmission of donor DNA to the recipient cell during conjugation

Pineal gland

A small endocrine gland on the dorsal surface of the vertebrate forebrain.
It secretes the hormone melatonin, which regulates body functions related to seasonal day length.


A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
A process of ingesting material by enclosing it with a membrane. The resulting structure is usually too small to be seen with the light microscope and is mostly suitable for the ingestion of fluid or mucus.

Piriformis syndrome

A clinical diagnosis based on complaints of pain and abnormal sensations in the buttocks region with extension into the hips and posterior thigh as would be seen in sciatica.

Pituitary gland (Hyophysis)

An endocrine gland at the base of the brain (i.e., below the hypothalamus) that is controlled by secretions from the hypothalamus and which secretes hormones into the blood.
The pituitary gland secretes hormones that control the endocrine organs of the body. It is attached to the bottom of the hypothalamus by a slender stalk called the infundibulum. The pituitary gland consists of two major regions—the anterior pituitary gland (anterior lobe or adenohypophysis) and the posterior pituitary gland (posterior lobe or neurohypophysis).

The posterior lobe (neurohypophysis) stores and releases two hormones produced by the hypothalamus, and the anterior lobe (adenohypophysis) produces and secretes many hormones that regulate diverse body functions.
(see HypothalamusEndocrine system)


Picture element
The smallest unit of a computer image or a digital TV screen


The negative logarithm of an equilibrium constant.


The negative logarithm of the acid dissociation constant, Ka.
Just like the pH, the pKa tells you of the acid or basic properties of a substance.
pKa <2 means strong acid
pKa >2 but <7 means weak acid
pKa >7 but <10 means weak base
pKa >10 means strong base
pH at which 50% of an acid is dissociated into H+ and base. High pKa values indicate a weak acid.


A measure of the potency of a competitive antagonist; the negative log of the molar concentration which at equilibrium would occupy 50% of the receptors in the absence of agonist. In an experiment in which a single concentration of antagonist has caused a parallel shift of the agonist concentration-response curve, the pKB value can be calculated using the Gaddum equation:
pkB = log(conc.ratio - 1) - log(antagonist conc.)
For a competitive antagonist (i.e. one where the slope of the Schild plot equals 1), the pKB is theoretically equal to the pA2 value. In practice, there may be some discrepancy. The pKB value should also be equal to the pKi value for the compound determined in a binding assay, although there may again be a discrepancy caused by the use of different media, etc.


The negative log of the Ki value. The pKi value for an antagonist should theoretically equal its pKB value at the same receptor, determined in an in vitro experiment.
Further information is available on receptor binding, including a useful radioactivity calculator for biochemical and pharmacological experiments.


An inactive ingredient or drug that is meant to appear like a genuine medicine.Placebos are often used in double blind studies as a control.  A dummy treatment in a clinical trial, designed to assess the extent to which factors other than the drug under test affect the outcome of the disease.


A structure in the pregnant uterus for nourishing a viviparous fetus with the mother's blood supply; formed from the uterine lining and embryonic membranes.

Placenta abrupto

A condition in which the placenta separates prematurely from the uterine wall.
This is a serious condition that may result in spontaneous abortion.

Placental mammal

A member of a group of mammals, including humans, whose young complete their embryonic development in the uterus, joined to the mother by a placenta.

Placenta previa

A condition characterized by a placental implantation in the lower portion of the uterus.  The position of the placenta may completely cover the internal cervical opening, or it may be just within the lower segment, or it may partially cover the internal cervical opening.


A member of an extinct class of fishlike vertebrates that had jaws and were enclosed in a tough, outer armour.


A flat surfaces that stretches off into infinity.


Organisms living in the water column (above the sediment); The plants or animals that live in the water column, incapable of moving against a current.
Mostly microscopic organisms that drift passively or swim weakly near the surface of oceans, ponds, and lakes. Includes larval stages of benthic and pelagic organisms, phytoplankton (in surface waters), zooplankton, jellies and other drifting or weakly swimming organisms. Planktonic organisms can de divided into holoplankton and meroplankton depending on how much of their life cycle they spend as plankton.


Organisms which feed on plankton.

Plantar Flexion

The bending of the toes (or fingers) downwards towards the sole (or palm).
(See Dorsi Flexion)


The liquid portion of blood, in which cells are suspeded.

Plasma cell

A derivative of B cells that secretes antibodies.

Plasma membrane

Outer membrane of a cell, sometimes called the cell membrane.
The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.
The term plasma membrane is used more frequently when discussing prokaryotes.

Plasma renin activity

The rate of production, in blood, of the precursor of angiotensin II, which is called angiotensin I.

Plasma volume

Plasma volume is the total volume of the non-cellular component of blood.
Blood is made up of plasma and cells.


Circular loop of DNA in prokaryotes. Eukaryotic DNA is organized into chromosomes.
A small ring of DNA that carries accessory genes separate from those of a bacterial chromosome. Also found in some eukaryotes, such as yeast.
Small DNA fragment found outside the chromosome, in almost all bacterial cells; autonomously replicating extra chromosomal circular rings of DNA present in monerans and some other organisms.
Escherichia coli, the usual bacteria in molecular genetics experiments, has a large circular genome, but it will also replicate smaller circular DNAs as long as they have an "origin of replication". Plasmids may also have other DNA inserted by the investigator. A bacterium carrying a plasmid and replicating a million-fold will produce a million identical copies of that plasmid. Common plasmids are pBR322, pGEM, pUC18
These DNA molecules are distinct from the normal bacterial genome and are nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors.
Plasmids carry between 2 and 30 genes. Some seem to be able to move in and out of the bacterial chromosome. They are self-replicating in a manner like the bacterial chromosome.
Some plasmids can temporarily associate with the nucleoid and become incorporated in the bacterial chromosome; these are known as episomes.
Plasmids carry non-vital genes which may or may not be useful to bacteria. Plasmids which do not confer any useful trait are called cryptic plasmids. Many plasmids have been recognized for E. coli, including the F plasmids ("fertility factors") and R plasmids (drug/antibiotic resistance). The F plasmid contains 25 genes, some of which control the production of F pili (proteins which extend from the surface of F+, or male, cells to the surface of F-, or female, cells). The R plasmid conveys drug resistance on cells having it. As many as 10 resistance genes can be contained on a single R plasmid. The R plasmids can be transferred to other bacteria of the same species, to viruses, and even to bacteria of different species. Drug (antibiotic) resistance has been found among pathogens causing the diseases typhoid fever, gastro-enteritus, plague, undulant fever, meningitis, and gonorrhea. In addition to the more common modes of transfer, R plasmids may be passed through the cell membrane. The resistance genes appear to operate by either breaking down the antibiotics or by circumventing the block the antibiotic places on a key bacterial metabolic pathway.
Plasmids are the principal tools for inserting new genetic information into microorganisms or plants.

Plasmid suicide vector

A plasmid that cannot replicate in a particular host.

Plasmodesma (Plural = plasmodesmata)

An open channel in the cell wall of plants through which strands of cytosol connect from adjacent cells.

Plasmodium (Plural = Plasmodia)

A type of amoeboid organization involving a large mass of cytoplasm and, usually, many nuclei.
A type of body form adopted by some slime moulds. The genus Plasmodium is the cause of malaria.


The fusion of the cytoplasm of cells from two individuals; occurs as the first step in syngamy.


The ability of cellular or tissue structures and their resultant function to be influenced by an ongoing activity.


One of a family of closely related plant organelles, including chloroplasts, chromoplasts, and amyloplasts (leucoplasts).
Any of several pigmented cytoplasmic organelles found in plant cells and other organisms, having various physiological functions, such as the synthesis and storage of food. From it come the terms aplastidic and plastidic for with and without chloroplasts respectively.

Plate (in cell culture studies)

To spread cells over the surface of solid medium in a plate.


A temporary or permanent leveling off in a function or process.


A small enucleated blood cell important in blood clotting; derived from large cells in the bone marrow.

Platelet activating factor (PAF)

A compound that reduces inflammation by increasing permeability of blood vessels and contracting various involuntary muscles such as those in airways.

Plato, degrees

Expresses the specific gravity as the weight of extract in a 100 gram solution at 17.5°C.

Platykurtic Distribution

A platykurtic distribution is a gaussian distribution having a negative kurtosis and a broad shaped peakedness.

Play audiometry

A play activity such as dropping a block, putting rings on a peg, etc., is utilized as the response to acoustic stimulus.

Pleated sheet

One form of the secondary structure of proteins in which the polypeptide chain folds back and forth, or where two regions of the chain lie parallel to each other and are held together by hydrogen bonds.


The effect of a particular gene on several different traits.
The ability of a single gene to have multiple effects.

Pleiotropic mutation

A mutation that has effects on several different characters.


An ancestral character state.
A trait from which an evolutionary novelty (apomorphy) is derived.

Plesiomorphic character

A primitive character state for the taxa under consideration; A primitive phenotypic character possessed by a remote ancestor.


A symptom exhibited by tissues not yet dead but in the process of dying (e.g., wilting).


The membrane that surrounds the lungs that secretes a fluid which allows the lungs to move as they inhale and exhale.


Excision of part of the pleura, usually parietal

Pleurisy, pleuritis

An inflammation of the pleura, the lining of the lungs and chest cavity


Having the ability to give rise to all of the various cell types of the body.
Pluripotent cells are rare and generally small in number but can be found in a number of tissues including umbilical cord blood. They cannot make extra-embryonic tissues such as the amnion, chorion, and other components of the placenta. Pluripotency is demonstrated by providing evidence of stable developmental potential, even after prolonged culture, to form derivatives of all three embryonic germ layers from the progeny of a single cell and to generate a teratoma after injection into an immunosuppressed mouse.
(See MultipotentTotipotent)

Pluripotent stem cell

A cell within bone marrow that is a progenitor for any kind of blood cell.


A network or interjoining of nerves and blood vessels or of lymphatic vessels.


The number of chromosomes present in the cell.
The amount of DNA in a cell gives an indication of the ploidy, but is not directly proportional. See HaploidDiploid, and Aneuploid.

Pneumocystic carinii pneumonia (PCP)

A rare form of pneumonia involving a tiny parasite.


Excision of a lung or lobe (lobectomy) of the lung


Accumulation of air or gas in the space between the lung and chest wall, resulting in partial or complete collapse of lung.
The air enters either by an external wound, a lung perforation, from burrowing abscesses, or from the rupture of a superficial lung cavity. Pneumothorax is attended with sudden and severe pain and rapidly increasing dyspnea.

Point mutation

A mutation that can be mapped to one specific site within a locus.
A change in a gene at a single nucleotide pair.
A small mutation that consists of the replacement (transition or transversion); addition; or deletion ( frameshift) of one or a few bases.

Point scale

Numerical form of a category scale, which has established intervals and a starting and end point, e.g. a 5-point scale.

point-source pollution

water pollution coming from a single point, such as a sewage-outflow pipe.

Poisson distribution

A poisson distribution is a distribution of random occurrences in which one occurrence has no influence on any other occurrence.
A mathematical expression giving the probability of observing various numbers of a particular event in a sample when the mean probability of that event on any one trial is very small.
The variance of a poisson distribution is equal to its mean and therefore the standard deviation is equal to the square root of the mean of the distribution. Radioactive decay measurements follow a poisson distribution and therefore have a lower measurement error when more counts are accumulated.


Having parts or areas with opposed or contrasting properties, such as positive and negative charges, or a head and a tail.

Polar body

A polar body is a structure produced when an early egg cell, or oogonium, undergoes meiosis.
Minute, nonfunctioning cell produced during those meiotic divisions that lead to egg cells; contains a nucleus but very little cytoplasm.
In the first meiosis, the oogonium divides its chromosomes evenly between the two cells but divides its cytoplasm unequally. One cell retains most of the cytoplasm, while the other gets almost none, leaving it very small. This smaller cell is called the first polar body. The first polar body usually degenerates. The ovum, or larger cell, then divides again, producing a second polar body with half the amount of chromosomes but almost no cytoplasm. The second polar body splits off and remains adjacent to the large cell, or oocyte, until it (the second polar body) degenerates. Only one large functional oocyte, or egg, is produced at the end of meiosis.

Polar coordinate

A coordinate system of ordered pairs in which the first number of the pair represents distance from the origin, and the second number of the pair represents the angle of inclination from the horizontal axis.

Polar covalent bond

A bond in which electrons are shared between elements having a difference in electronegativity of between 0.5 and ~2.0.
A type of covalent bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive.  It is a bond in which the electron density is unsymmetrical.

Polar gene conversion

A gradient of conversion  (i.e., the correction of misplaced bases in DNA) frequency along the length of a gene.

Polarity of characters

The states of characters used in a cladistic analysis, either original or derived.
Original characters are those acquired by an ancestor deeper in the phylogeny than the most recent common ancestor of the taxa under consideration. Derived characters are those acquired by the most recent common ancestor of the taxa under consideration.

Polarity gene

A mitochondrial gene with alleles that are preferentially found in daughter mitochondria after a recombinational event between mitochondria.

Polarized Light

Polarized light is light that travels in a single plane.

Polar molecule

A molecule (such as water) with opposite charges on opposite sides
Hydrophilic, or "water-loving", describing molecules or groups that are soluble in water.
These molecules would be composed of elements having polar covalent bonds that do not cancel each other out.

Polar mutation

A mutation that affects the transcription or translation of part of the gene or operon downstream of the mutant site.
For example, nonsense mutations, frameshift mutations, and insertion sequence(IS)-induced mutations.


A viral disease with three known strains which can cause permanent paralysis, deformity, or death.
Immunization with the OPV vaccine protects children against this disease.


A material used to make gels for separation of mixtures of macromolecules by electrophoresis.

Polyacrylamide gel

A semisolid suspension of acrylamide monomers cross-linked to form polyacrylamide chains, in water
These gels are cast between a pair of glass plates by polymerizing a solution of acrylamide monomers into polyacrylamide chains and simultaneously cross-linking the chains into a semisolid matrix.  The pore size of a gel can be varied by adjusting the concentration of polyacrylamide and the cross-linking reagent.
When a mixture of proteins is applied to a gel and an electric current applied, smaller proteins migrate faster than larger proteins through the gel.  The rate of movement is influenced by the gel’s pore size and the strength of the electric field.  The pores in a highly cross-linked polyacrylamide gel are quite small.  Such a gel could resolve small proteins and peptides, but large proteins would not be able to move through it.

Polyacrylamide gel electrophoresis

Electrophoresis through a matrix composed of a synthetic polymer, used to separate proteins, small DNA, or RNA molecules of up to 1000 nucleotides.
Used in DNA sequencing.


A polygamous mating system involving one female and many males.

Poly(A) polymerase

Catalyzes the addition of adenine residues to the 3' end of pre-mRNAs to form the poly(A) tail.

Poly A tail (poly adenosine tail)

A sequence of repeating adenosine ribonucleotides added to the 3' end of a newly transcribed pre- mRNA before it exits the nucleus.
This modification can be up to 200-300 bases long and functions to increase the stability and translatability of the mRNA.
After an mRNA is transcribed from a gene, the cell adds a stretch of A residues (typically 50-200) to its 3' end. It is thought that the presence of this "polyA tail" increases the stability of the mRNA (possibly by protecting it from nucleases). Note that not all mRNAs have a polyA tail; the histone mRNAs in particular do not.


Marine segmented worms.
Some are planktonic although most are benthic. The dominant component of the macrofauna. Synonymous with Polychaete.

Polychlorinated biphenyls (PCBs)

A group of synthetic, toxic industrial chemical compounds once used in making paint and electrical transformers, which are chemically inert and not biodegradable.
PCBs were frequently found in industrial wastes, and subsequently found their way into surface and ground waters. As a result of their persistence, they tend to accumulate in the environment. In terms of streams and rivers, PCBs are drawn to sediment, to which they attach and can remain virtually indefinitely.

Polycistronic mRNA

An mRNA that codes for more than one protein.
Prokaryotic messenger RNAs that contain several cistrons within the same mRNA transcript.

Polyclonal antibodies

A mixture of immunoglobulin molecules, which arise from more than one clone of B-lymphocyte cells, secreted against a specific antigen, each recognizing a different epitope. Polyclonal antibodies are usually present when antisera from a conventional immunization is used.

Polycystic Ovary Syndrome (PCOS)

An endocrine disorder with a host of symptoms related to small painful cysts on the ovaries.  It is marked by the overproduction of male hormones in females.


Excessive drinking persisting for long periods of time as occurs in diabetes mellitus.

Poly-dA/poly-dT technique

A method of inserting DNA into a vector by adding poly-dA to the linearised vector and poly-dT to the DNA of interest.
This technique is also feasible with poly-dG and poly-dC.


A string of epitopes (immunological targets; see Epitope) linked together to form a powerful immunological
package capable of stimulating a strong immune response

Polyethylene terephthalate (PET)

A light-weight clear plastic with acceptable barrier properties to gas and water vapour.

Polygenic disorder

Genetic disorder resulting from the combined action of alleles of more than one gene (e.g., heart disease, diabetes, and some cancers).
Although such disorders are inherited, they depend on the simultaneous presence of several alleles; thus the hereditary patterns usually are more complex than those of single-gene disorders.

Polygenic inheritance

An additive effect of two or more gene loci on a single phenotypic character.


The union of several line segments that are joined together so as to completely  enclose an area.
decagon:  a polygon with ten sides.
heptagon:    a polygon with seven sides.
hexagon:  a polygon with six sides.
octagon:   a polygon with 8 sides.
pentagon:   a five sided polygon.
quadrilateral: a polygon with 4 sides.
rectangle:   a quadrilateral with four 90 degree angles.
regular polygon: a polygon in which all the angles are equal and all of the sides are equal.
rhombus:   a quadrilateral with four equal sides.
square:   a quadrilateral with four equal sides and four 90 degree angles.
triangle:   a three sided polygon.
tetrahedron: a polyhedron with four faces.


A polygamous mating system involving one male and many females.


A solid that is bounded by plane polygons.
dodecahedron:  a polyhedron with twelve faces.
heptagon:    a polygon with seven sides.
hexahedron:  a polyhedron with six faces.   A regular hexahedron is a cube.
icosohedron:  a polyhedron with 20 faces.
octahedron:   a polyhedron with 8 faces.
regular polyhedron:  a polyhedron whose faces are congruent, regular polygons.
tetrahedron: a polyhedron with four faces.


A large molecule composed of repeated subunits, of many identical or similar monomers linked together.

Polymerase (DNA p. or RNA p.)

Enzyme that catalyzes the synthesis of of a polymer from its subunits, i.e., of nucleic acids on preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.
It links individual nucleotides together into a long strand, using another strand as a template. There are two general types of polymerase — DNA polymerases (which synthesize DNA) and RNA polymerase (which makes RNA). Within these two classes, there are numerous sub-types of polymerase, depending on what type of nucleic acid can function as template and what type of nucleic acid is formed. A DNA-dependant DNA polymerase will copy one DNA strand starting from a primer, and the product will be the complementary DNA strand. A DNA-dependant RNA polymerase will use DNA as a template to synthesize an RNA strand.

Polymerase chain reaction (PCR)

A powerful method for amplifying specific DNA segments in vitro which exploits certain features of DNA replication, by incubating with special primers, DNA polymerase molecules and nucleotides.
A method of amplifying or copying a short sequence of DNA (DNA fragments) that is faster than cloning, in order to obtain sufficient amounts for diagnostic testing.
PCR is the basis for a number of extremely important methods in molecular biology. It can be used to detect and measure vanishingly small amounts of DNA and to create customized pieces of DNA. It has been applied to clinical diagnosis and therapy, to forensics and to vast numbers of research applications.  The technique derives its name from one of its key components, a DNA polymerase (to unzip the DNA helix by breaking the H-bonds between bases) used to amplify a piece of DNA.  As PCR progresses, the DNA generated is used as a template for replication.  It is possible to amplify a single piece of DNA generating many copies.

The technique involves combining DNA fragments with 5’- and 3’- primers, DNA polymerase, nucleotides, and other components to form a mixture in which the DNA is cyclically amplified.

The method amplifies specific DNA base sequences by cycles of template denaturation, primer addition, primer annealing and replication using thermostable DNA polymerase. It uses a heat- stable polymerase and two 20- base primers, one complementary to the (+)- strand at one end of the sequence to be amplified and the other complementary to the (- )- strand at the other end. Replication requires a primer which is added to initiate the copying of each strand, along with nucleotides and Taq polymerase, and specificity is determined by the sequence and size of the primer. By cycling the temperature, the target DNA is repetitively denatured and copied. A single copy of the target DNA, even if mixed in with other undesirable DNA, can be amplified to obtain billions of replicates.

The degree of amplification achieved is set at a theoretical maximum of 2^N, where N is the number of cycles, eg 20 cycles gives a theoretical 1048576 fold amplification. In addition to primers and DNA polymerase, PCR reactions must contain template DNA (the DNA to be amplified) and the DNA "building blocks", deoxynucleotide triphosphates (dNTPs, which include dATP, dTTP, dGTP, and dCTP).
Because the newly synthesized DNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence.

PCR can be used to amplify RNA sequences if they are first converted to DNA via reverse transcriptase. This two-phase procedure is known as ‘RT-PCR’. PCR also can be used to detect the existence of the defined sequence in a DNA sample.

Polymerase slippage

During replication, the slipping of DNA polymerase III from the DNA template strand at the repeat region and the subsequent reattachment at a more distant site.
Polymerase slippage can cause the newly created DNA strand to contain an expanded section of DNA.
The polymerase slippage model suggests that polymerase slippage is the cause of increased repeat regions.


The repetitive bonding of small molecules (monomers) to produce large molecules (polymers).


Long chain molecules such as PVC, nylon or DNA produced by the polymerisation of monomers.


Referring to a population in which two or more physical forms are present in readily noticeable frequencies.


Difference in DNA sequence among individuals.
The coexistence of two or more distinct forms of individuals (polymorphic characters) in the same population.
Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis.

Polymorphonuclear Leukocytes (PMN)

Formerly named microphages, they are phagocytic (i.e., foreign particle-ingesting) white blood cells that have a lobed nucleus.
For example, during an attack of the common cold (when virus first invades mucous membranes of the human nose), the body responds by making Interleukin-8 (IL-8); a glycoprotein that attracts large quantities of polymorphonuclear leukocytes to the mucous membranes of the nose (to try to combat the infection).
Another example is when polymorphonuclear leukocytes (PMN) migrate-in to a female pig's uterus within six hours after semen is introduced via breeding. PMN remove exces sperm and bacteria, resulting in a "friendly" envronment for embryos to develop in the uterus.


Inflammation of two or more nerves simultaneously.


An algebraic expression of the form: axn + bx(n-1) + .......+ cx3 + dx2 + ex + k


A polymer made up of many nucleotides covalently bonded together.

Polynucleotide phosphorylase

An enzyme that can polymerize nucleotide diphosphates without the need for a primer.
The function of this enzyme in vivo is probably in its reverse role as an RNA exonuclease.


Type of sweetener used in reduced-calorie foods.
They differ from intense sweeteners in that they are considered nutritive; that is, they do contribute calories to the diet. Polyols are incompletely absorbed and metabolized, however, and consequently contribute fewer calories than sucrose.
The polyols commonly used include sorbitol, mannitol, xylitol, maltitol, maltitol syrup, lactitol, erythritol, isomalt and hydrogenated starch hydrolysates. Most are approximately half as sweet as sucrose; maltitol and xylitol are about as sweet as sucrose. Polyols are found naturally in berries, apples, plums and other foods. They also are produced commercially from carbohydrates such as sucrose, glucose, and starch for use in sugar-free candies, cookies and chewing gum.
Along with adding a sweet taste, polyols perform a variety of functions such as adding bulk and texture, providing a cooling effect or taste, preventing the browning that occurs during heating and retaining the moisture in foods.


A polymer (chain) of many amino acids linked together by peptide bonds; A chain of linked amino acids (usually < 100 amino acids in length).
Polypeptides are the building blocks of proteins.


Feeding on many different kinds of food.
In the case of parasites or parasitoids, the use several different groups of organisms as hosts.


Pertaining to a taxon whose members were derived from two or more ancestral forms not common to all members; A group of organisms that does not include their most recent common ancestor.
Term applied to a group of organisms which does not include the most recent common ancestor of those organisms; the ancestor does not possess the character shared by members of the group. Technically speaking, the distinction between polyphyletic and paraphyletic is not as clear as it might seem.


Cell with more than two complete sets of chromosomes per nucleus.
Ploploidy = A chromosomal alteration in which the organism possesses more than two complete chromosome sets


A growth that projects from the lining of mucous membrane, such as the intestine.


An aggregation of several ribosomes attached to one messenger RNA molecule.


A polymer of up to over a thousand monosaccharides, formed by condensation synthesis.


See Polyribosome

Polytomous tree

A tree that has at least one branch point that is a polytomy.
A tree with only one polytomy is called polytomous, as is a tree with multiple polytomies. This is in contrast to a dichotomous tree.

Polyvalent vaccine

A recombinant organism into which has been cloned antigenic determinants from a number of different disease-causing organisms.


1. The universe, or statistical population, of data under investigation from which a data sample will be taken.
2. A group of individuals of one species that live in a particular geographic area.

Population bottleneck

Type of genetic drift that occurs as the result of a population being drastically reduced in numbers by an event having little to do with the usual forces of natural selection.

Population density

The number of individuals of a population per unit area or volume of living space.

Population variability analysis

A method of predicting whether or not a species will persist in a particular environment.


Abnormal cavity within brain tissue, usually resulting from outpouching of a lateral ventricle.


a measure of the water-bearing capacity of subsurface rock. With respect to water movement, it is not just the total magnitude of porosity that is important, but the size of the voids and the extent to which they are interconnected, as the pores in a formation may be open, or interconnected, or closed and isolated. For example, clay may have a very high porosity with respect to potential water content, but it constitutes a poor medium as an aquifer because the pores are usually so small.

Portable water

water of a quality suitable for drinking.

Positional bias

A type of contextual influence in which a preceding test product or sample influences the evaluation of following products/samples.

Positional cloning

A technique used to identify genes, usually those that are associated with diseases, based on their location on a chromosome.

Positional information

Signals, to which genes regulating development respond, indicating a cell's location relative to other cells in an embryonic structure.

Position effect

A situation in which the phenotype expressed by a gene is altered by changes in the position of the gene within the genome.
For instance genes translocated to regions of heterochromatin are often not expressed.

Positive control

Regulation mediated by a protein that is required for the activation of a transcription unit.

Positive End of Expiration Pressure (PEEP).

A certain amount of positive pressure is always left in the lungs, even at the end of expiration. Very useful when treating the patient with congestive Heart Failure.

Positive feedback

A physiological control mechanism in which a change in some variable triggers mechanisms that amplify the change.
The process whereby a disturbance about a homeostatic set-point results in an increase in that disturbance.
This form of feedback is rarely used in biological systems because of its explosive and damaging nature, but such a system operates in the generation of the upstroke (the depolarization) of the action potential.

Positive number

A real number greater than zero.

Positron-emission tomography (PET).

A major technique to image brain structures to investigate functional and, to a lesser degree, anatomical details within the brain.
PET imaging requires cyclotron-produced radionuclides which emit positrons e.g. 18-Fluorine and a PET camera. The positron emitted from the radionuclide is annihilated and results in the release of two annihilation radiation photons of 511keV, at 180 degrees.  Data is reconstructed and reformatted into multiple planes. The most commonly used PET agent currently I 18F-deoxyglucose (FDG).

Possible outlier

A possible outlier is a parameter probability which is equal to or lower than the possible probability limit but higher than the probable probability limit.
Possible outlier parameters have shifted enough from the reference assay group to warrant inspection but are not as severely shifted as a probable outlier.

Postauricular hearing aid

A behind-the-ear hearing aid.

Posterior neuropore

Posterior open end of the neural tube that must close during the development of the embryo. Failure to close results in spina bifida.

Posterior spinal fusion

A fusion of the cervical, thoracic, or lumbar regions primarily fusing the lamina and sometimes the facet joints, using iliac or other bone graft.

Post-ictal sleep

Sleep that follows an epileptic seizure.

Postreplicative repair

A DNA repair process initiated when DNA polymerase bypasses a damaged area.
Enzymes in the rec system are used.

Post-synaptic membrane

The surface of the cell on the opposite side of the synapse from the synaptic terminal of the stimulating neuron. The post-synaptic membrane must contain receptor proteins to bind the neurotransmitter that is rleased from the synaptic terminal of the pre-synaptic neuron to relay messages across the synaptic cleft.  It  ust also contain degradative enzymes to break down the \neurotransmitter and thereby limit it’s duration of action.

Post-transcriptional Gene Silencing (PTGS)

An automatic natural response (e.g., in certain plants) to the high buildup (i.e., within such plant cells) of identical mRNA molecules. Because such a high buildup typically occurs as a result of viral infection (of plant), the plant's natural defense system systematically breaks-down those mRNA molecules (to fight the viral infection). Triggering the plant’s 'attack' on an unwanted mRNA can be employed by genetic engineers to "silence" a given gene (i.e., by destruction of that gene's mRNA), via the (cosuppression of) plant's natural PTGS response.
See also Gene silencing.

Post-transcriptional regulation

Any process occurring after transcription which affects the amount of protein a gene produces. Includes RNA processing efficiency, RNA stability, translation efficiency, protein stability. For example, the rapid degradation of an mRNA will reduce the amount of protein arising from it. Increasing the rate at which an mRNA is translated will increase the amount of protein product.

Post translational modification

Changes in eukaryotic mRNA, tRNA or other RNAs made after transcription has been completed. The changes to mRNA include addition of a 5'cap and 3' polyA tail and removal of introns, and to tRNA include modification of bases and removal of introns.

Post translational processing

The reactions which alter a protein's covalent structure, such as phosphorylation, glycosylation or proteolytic cleavage.

Post translational regulation

Any process which affects the amount of protein produced from a gene, and which occurs after translation in the grand scheme of genetic expression.
Actually, this is often just a buzz-word for regulation of the stability of the protein. The more stable a protein is, the more it will accumulate.

Post Traumatic Amnesia (PTA)

A period of hours, weeks, days or months after the injury when the patient exhibits a loss of day-to-day memory.
The patient is unable to store new information and therefore has a decreased ability to learn. Memory of the PTA period is never stored, therefore things that happened during that period cannot be recalled. May also be called Anterograde Amnesia.


A fundamental statement that is assumed to be true without proof.

Postural drainage

A technique in which the patient is positioned in specific ways such that gravity assists with the drainage of pulmonary secretions from a lobe or segment
Done in cases of cystic fibrosis in particular.


The attitude of the body. Posture is maintained by low-grade, continuous contraction of muscles which counteract the pull of gravity on body parts. Injury to the nervous system can impair the ability to maintain normal posture, for example holding up the head.

Postzygotic barrier

Any of several species-isolating mechanisms that prevent hybrids produced by two different species from developing into viable, fertile adults.

Potassium channels

Potassium channels
(a) Voltage-dependent potassium channels
Derived from a large gene family whose diversity of channel properties may arise from alternative splicing or post-translational modifications. These channels are associated with a number of well characterised currents: delayed rectifier, transient outward and inward rectifier. 
(b) Calcium-activated potassium channels
Three currents have been associated with this type of potassium channel: the large, the intermediate and the small conductance currents. 
(c) ATP-sensitive potassium channels
ATP has been found to inhibit the opening of some potassium channels through which the IK(ATP) current flows. Such channels are inward-rectifying and voltage dependent. 
(d) Receptor-linked potassium channels
Some potassium channels are opened by muscarinic receptor activation through coupling to a G-protein. The M-current is associated with such channels.


A measure of the concentrations of a drug at which it is effective. A much-abused, vague term which should always be further defined. For agonists, EC50IC50KA or pD2 are usually used, while pA2KB or pKB are used for antagonists. Other terms are used in binding studies (see section 3) which do not distinguish between agonists and antagonists. It is important to realise that the potency of an agonist does not give any information about its affinity for the receptor, because the pharmacological response is rarely directly proportional to receptor occupancy (see efficacy).


The amount of electrification of a point with reference to some standard

Potential difference

The voltage difference between two points.  Electricity flows from a high to low level of potential.

Potential energy

The energy stored by matter as a result of its location or spatial arrangement. Amount of useable energy within a body at rest.

Potentiometric titration

The titrant is automatically dispensed and the end point is determined electronically by use of an electrode.

Pot of gold

a hidden treasure, many of which can be found in Reeko's Mad Scientist Lab.


In physics: Amount of work done per second.
In mathematics: A number that indicates the operation of repeated multiplication.

Power spectrum

The Fourier transform of the kinetic energy field. In acoustics power spectra represent the amount of energy at each frequency component within the signal that is beign analysed.


Parts per billion.


Parts per million.


Parts per thousand.

Prader-Willi syndrome

A genetic disorder that can cause a growth hormone deficiency.
A syndrome characterized by severe hypotonia (floppiness), poor suck and feeding problems in early infancy followed later in infancy by excessive eating that, if unchecked, leads gradually to huge obesity. All children with Prader-Willi syndrome (PWS) show developmental delay and mild-to-moderate mental retardation with multiple learning disabilities. Hypogonadism is present in both females (with small labia minora and clitoris) and males (with underdeveloped scrotum and nondescent of the testes). Short stature and small hands and feet are common.

Pragmatic (in regards to speech)

A component of language that is concerned with the use of language in social contexts, including rules that govern language functions and forms of messages when communicating.


When a chemical reaction taking place in a liquid results in the formation of a solid product, then the solid product tends to fall (i.e., it "precipitates") out of the liquid. The process is called “precipitation" and the product is called a "precipitate."


Precision is a measurement of the reproducibility of replicate sample responses in an assay run. Precision is composed of both the random error and the systematic error of the measurement.

Precision Probability

The precision probability is the computed probability resulting from a statistical comparison between the reproducibility of the specimen replicate responses (measured as variance or CV) and the reproducibility predicted for that response point.
The predicted reproducibility is calculated from a variance line computed from the variances or CV's of the reference assays in Test Method Analysis. A separate variance line is computed for the standards and for the unknowns. Expected control sample variances are computed from the unknown variance line.

Precision PI

A precision PI is a probability index computed using the individual precision probabilities of the sample responses from each baseline standard and standard specimen, control specimen, and unknown specimen.


An interaction between organisms in which one organism, the predator, kills and eats the other organism, the prey.


An organism that eats other living organisms.
Predation = An interaction between species in which one species, the predator, eats the other, the prey.
Prey = An organism eaten by another organism.

Preemptor stem

A configuration of leader transcript mRNA that allows transcription in attenuator-controlled amino acid operons.

Preference tests

Preference or acceptance tests determine representative population preferences.

Pre-ganglionic and post-ganglionic neurons

Neurons (nerve cells) of the Autonomic Nervous System.
The Autonomic Nervous System is that branch of the nervous system whereby the brain controls internal body organs and structures (heart, gut, bladder, liver, etc.  It consists of two divisions, the Sympathetic and Parasympathetic Nervous Systems.  In both divisions the final output from the brain to tissues/organs is a 2 neuron system consisting of Pre-ganglionic neurons from the brain or spinal cord to a ganglion (a collection of nerve cell bodies) where they contact Post-ganglionic neurons which have axons (extensions of the nerve cell bodies) running from the ganglion to the target organs or tissues.

Prelingual deafness

Loss of hearing sensitivity that occurred at birth or earlier than the development of speech and language (before 2 - 5 years of age).
Hearing loss may be congenital (born with) or adventitious (having occurred after birth, acquired).

Premature Adrenarche

The early growth of pubic hair before the age of 8 years in girls and the age of 9 years in boys.

Premature Thelarche

The early appearance of breast development in females who show no other signs of puberty.

Premenstrual syndrome

Describes the appearance of physical and emotional symptoms during the second half of the menstrual cycle.

Pre-Morbid Condition

Characteristics of an individual present before the disease or injury occurred.


The first (primary) transcript from a protein coding gene is often called a pre-mRNA and contains both introns and exons.
Pre-mRNA requires splicing (removal) of introns to produce the final mRNA molecule containing only exons.


A loss of hearing sensitivity with increasing age.
Presbycusis is characterized by hearing loss at higher frequencies gradually extending towards lower frequencies with increasing age.

Press fit

A method for implanting orthopedic devices. For a press fit, a device is inserted without cement or hardware fixation. The geometry of adjacent structures holds the device in place. The acetabular component of a bipolar femoral prosthesis is press fitted into the acetabulum. The stem of uncemented femoral prostheses are press fitted into the medullary canal of the proximal femur. Also referred to as interference fit.


the application of a steady force upon another object.

Pressure of speech

Speech that is increased in amount, accelerated and difficult or impossible to interrupt.
Usually it is also loud and emphatic. Frequently, the individual talks without any social stimulation and may continue to talk even though no one is listening. Pressure of speech is most often seen in manic episodes, but may also occur in some cases of organic mental disorders, major depression with psychomotor agitation, schizophrenia, other psychotic disorders, and, occasionally, acute reactions to stress.

Pressure natriuresis

The phenomenon whereby increased arterial pressure increases salt and water excretion by the kidney.
This mechanism is widely believed to dominate long-term regulation of blood pressure.

Prezygotic barrier

A reproductive barrier that impedes mating between species or hinders fertilization of ova if interspecific mating is attempted.

Pribnow box

Relatively invariant sequence of six nucleotides (concensus TATAAT) in prokaryotic promoters centred at the position minus 10 (10 base pairs upstream from the transcription start site).

Primary (or Direct) active transport mechanisms

Transport of substances across the cell membrane that requires energy expenditure by the cell because the substance cannot get across the cell membrane readily or because the transported solutes have to flow against the thermodynamic potential (i.e, the thermodynamic potential created by the electro-chemical gradient). Here energy input drives the transport – hence “Active” (or Direct) Transport Systems.
Active transport systems use integral membrane proteins.
Some transport of substances across the cell membrane requires energy expenditure because the transported solutes have to flow against the thermodynamic potential (i.e, the thermodynamic potential created by the electro-chemical gradient). In these systems, the energy source and transport machinery are "coupled".
The energy source may be ATP, light or a concentration gradient.
The transport system uses integral membrane proteins.
There are 3 classes of Direct or Primary active transport mechanisms:
Ion pumps (also known ATPase Ion pumps) – transport of ions.
ABC proteins/system (ATP-binding cassette) – involves ATP and 3 proteins to bind, transport and release the transported molecule.
Group translocation: a sequence of molecules is involved & the transported substance is chemically modified during the transport process.

Primary consumer

An herbivore; an organism in the trophic level of an ecosystem that eats plants or algae.
heterotrophic organism that feeds directly on a primary producer (autotroph). Synonymous with herbivore.

Primary germ layers

The three layers (ectoderm, mesoderm, endoderm) of the late gastrula, which develop into all parts of an animal.

Primary growth

Growth initiated by the apical meristems of a plant root or shoot.

Primary hyperalgesia

Increased sensitivity to nociceptive (painful) stimuli in a region of the body that has been recently subjected to painful stimuli.
see Hyperalgesia for details

Primary immune response

The initial immune response to an antigen, which appears after a lag of several days.

Primary organ rudiments

The larger body structures that form initially during development of the embryo (the later formed ones are referred to as secondary organ rudiments).

Primary producer

An autotroph, which collectively make up the trophic level of an ecosystem that ultimately supports all other levels; usually a photosynthetic organism.

Primary production

Amount of organic material produced from inorganic material by primary producers in a given area in a given period of time.

Primary productivity

The rate at which light energy or inorganic chemical energy is converted to the chemical energy of organic compounds by autotrophs in an ecosystem.

Primary structure

The level of protein structure referring to the specific sequence of amino acids.

Primary transcript

The product of eukaryotic transcription before post-transcriptional modifications take place.
When a gene is transcribed in the nucleus, the initial product is the primary transcript, an RNA containing copies of all exons and introns. This primary transcript is then processed by the cell to remove the introns, to cleave off unwanted 3' sequence, and to polyadenylate the 5' end. The mature message thus formed is then exported to the cytoplasm for translation.

primary wastewater treatment

the first stage of the wastewater-treatment process where mechanical methods, such as filters and scrapers, are used to remove pollutants. Solid material in sewage also settles out in this process.


An enzyme that creates an RNA primer for initiation of DNA replication.


A member of the order of mammals that includes anthropoids and prosimians.

Prime mover

The muscle that has the major role in any movement
Prime movers may also act as fixators, allowing one type of movement by opposing another. For example, the flexor carpi radialis and the flexor carpi ulnaris normally flex the wrist, but they also act as fixators to prevent extension of the wrist when the extensor digitorum acts to extend the fingers. 
Muscles that assist the prime mover are called synergists and muscles that oppose the prime mover are called antagonists.

Prime number

A number whose only factors are itself and 1.


A set of oligonucleotide sequences (anywhere from 6-50 nucleotides long and typically <20 nucleotides) complementary to the 5’- and 3’- ends of the DNA fragment to be amplified by polymerase chain reaction (PCR).
An already existing short RNA chain bound to template DNA to which DNA nucleotides are added during DNA synthesis.
Short segment of DNA or RNA that anneals to a single strand of DNA in order to initiate template-directed synthesis and extend a new DNA strand by the enzymatic action of DNA polymerase to produce a duplex-stranded molecule.
The DNA polymerases are only able to extend a pre-existing strand along a template; they are not able to take a naked single strand and produce a complementary copy of it de-novo. A primer which sticks to the template is therefore used to initiate the replication. Primers are necessary for DNA sequencing and PCR

Primer extension

A method used to figure out how far upstream from a fixed site the start of an mRNA is.
For example, perhaps you have isolated a cDNA clone, but you don't think that the clone has all of the 5' untranslated region. To find out how much is missing, you would first sequence the part you have, and figure out which strand is coding strand (usually the coding strand will have a large open reading frame). Next, you ask the DNA Synthesis Facility to make an oligonucleotide complementary to the 5'-most region of the coding strand (and thus complementary to the mRNA). This "primer" is hybridized to mRNA (say, a mixture of mRNA containing the one in which you are interested), and reverse transcriptase is added to copy the mRNA from the primer out to the 5' end. The size of the resulting DNA fragment shows how far away from the 5' end your primer is.


Not specialized; at an early stage of evolution or development.
Describes a character state that is present in the common ancestor of a clade. A primitive character state is inferred to be the original condition of that character within the clade under consideration. For example, "presence of hair" is a primitive character state for all mammals, whereas the "hairlessness" of whales is a derived state for one subclade within the Mammalia.

Primitive streak

Thickening of the epiblast cell layer caused my movement of mesodermal cells into the blastocoel.
This structure is characteristic of avian, reptilian and mammalian gastrulation.

Primordium (Plural = Primordia)

A cell or organ in its earliest stage of differentiation.


A complex of two proteins, a primase and a helicase, that initiates RNA primers on the lagging DNA strand during DNA replication.

Principal component analysis

A special type of factor analysis.
The principal component analysis or Karhunen-Loeve transform is a mathematical way of determining that linear transformation of a sample of points in L-dimensional space which exhibits the properties of the sample most clearly along the coordinate axes. Along the new axes the dimensions consist of a linear combination of variables, the sample variances are extremes (maxima and minima), and uncorrelated.
The name comes from the principal axes of an ellipsoid (e.g. the ellipsoid of inertia), which are just the coordinate axes in question.
By definition, the principal axes will include those along which the point sample has little or no spread (minima of variance). Hence, an analysis in terms of principal components can show (linear) interdependence in data. A point sample of L dimensions for whose L coordinates M linear relations hold, will show only (L-M) axes along which the spread is non-zero. Using a cutoff on the spread along each axis, a sample may thus be reduced in its dimensionality.
The principal axes of a point sample are found by choosing the origin at the centre of gravity and forming the dispersion matrix .
Principal component analysis has in practice been used to reduce the dimensionality of problems, and to transform interdependent coordinates into significant and independent ones. An example used in several particle physics experiments is that of reducing redundant observations of a particle track in a detector to a low-dimensional subspace whose axes correspond to parameters describing the track. In practice, non-linearities of detectors, frequent changes in detector layout and calibration, and the problem of transforming the coordinates along the principal axes into physically meaningful parameters, set limits to the applicability of the method.

Principle of allocation

The concept that each organism has an energy budget, or a limited amount of total energy available for all of its maintenance and reproductive needs.


Protein infectious agent associated with several neurological diseases.
An infectious form of protein that may increase in number by converting related proteins to more prions
Each disease (scrapie; kuru; Creutzfeld-Jakob syndrome; Alzheimer's disease) has a different prion.
Prion rods: The microscopic rods that appear when prions, that have been broken up with proteinase K but then allowed to come back together into crystalline forms.

Prion protein (PrP)

The prion protein can exist in various forms. One is called PrPc and is the normal type of the protein that is found in a cell (i.e. chromosomal PrP). One is called PrPsc (or PrPscrapie) that is found in the infected cells. It may be called PrP-res, indicating that it is difficult to break down with proteinases. PrP27-30 is the name of the prion protein after it has been broken up by protease K.

prior appropriation doctrine

the system for allocating water to private individuals used in most Western states. The doctrine of Prior Appropriation was in common use throughout the arid West as early settlers and miners began to develop the land. The prior appropriation doctrine is based on the concept of "First in Time, First in Right." The first person to take a quantity of water and put it to beneficial use has a higher priority of right than a subsequent user. The rights can be lost through nonuse; they can also be sold or transferred apart from the land. Contrasts with riparian water rights.


Type of tannin found in cranberries, cranberry products, cocoa and chocolate which may provide the health benefits of improving urinary tract health and of reducing the risk of cardiovascular disease.

Probable outlier

A probable outlier is a parameter probability which is equal to or lower than the probable probability limit.
Probable outlier parameters are severely shifted and require inspection.


Probability is the likelihood that the difference between two or more groups of the same parameter is due to random error alone. Probabilities are expressed as a decimal value between 0 and 1, which is the calculated probability of no significant difference between the parameters. A high probability means that there is no significant difference between the parameters. A low probability signals a significant difference between the parameters. Probability limits are used to define the probability values, below which the parameter being tested is considered significantly different from the reference.

Probability Limits

A probability limit is a probability which forms a boundary between the acceptance (greater than) or rejection (less than) of the hypothesis that there is no significant difference between the groups. The possible probability limit signals a parameter which may be compromised.
The probable probability limit signals a parameter which is severely shifted.

Probability Index

The probability index (PI) for a control group is calculated by averaging the inverse of all the probabilities within the group and then inverting this average.


Single-stranded DNA or RNA molecule of specific base sequence, either radioactively or fluorescently labeled; a fragment of DNA or RNA which is labeled in some way (often incorporating 32P or 35S), and which is used to hybridize with the nucleic acid in which you are interested.Used to identify the complementary nucleotide sequence by hybridization to the DNA fragment or gene of interest.
For example, if you want to quantitate the levels of alpha subunit mRNA in a preparation of pituitary RNA, you might make a radiolabeled RNA in-vitro which is complementary to the mRNA, and then use it to probe a Northern blot of the pit RNA. A probe can be radiolabeled, or tagged with another functional group such as biotin. A probe can be cloned DNA, or might be a synthetic DNA strand. As an example of the latter, perhaps you have isolated a protein for which you wish to obtain a cDNA or genomic clone. You might (pay to) microsequence a portion of the protein, deduce the nucleic acid sequence, (pay to) synthesize an oligonucleotide carrying that sequence, radiolabel it and use it as a probe to screen a cDNA library or genomic library..

Probe amplification

The increase in the amount of probe molecules specific to a given target so that they can be easily detected. Examples include Q-beta and ligase chain reaction.

Problem-Solving Skill

Ability to consider the probable factors that can influence the outcome of each of various solutions to a problem, and to select the most advantageous solution.People with deficits in this skill may become "immobilized" when faced with a problem. By being unable to think of possible solutions, they may respond by doing nothing.

Procedural memory

The type of memory used in performing skills, learned behaviors, or procedures; remembering how to do something like tie a shoelace.
Procedural memories are easy to do but difficult to explain to others. For example, it is easy to demonstrate how to ride a bike but it is not easy to describe how to do it. Procedural memories are less likely to be forgotten.


The reactions occurring in the nucleus which convert the primary RNA transcript to a mature mRNA. Processing reactions include capping, splicing and polyadenylation. The term can also refer to the processing of the protein product, including proteolytic cleavages, glycosylation, etc.


The ability of an enzyme to repetitively continue its catalytic function without dissociating from its substrate.

Process schizophrenia

A type of schizophrenia attribed more to organic factors than to environmental ones; typically begins gradually, continues chronically, and progresses (either rapidly or slowly) to an irreversible psychosis.


Early signs or symptoms of a disorder.


In chemistry: The substances produced in a chemical reaction.
In mathematics: The result of two numbers being multiplied.

Product rule

The rule stating that the probability of two independent events occurring simultaneously is the product of the individual probabilities.


A mutagen that tends to produce frameshift mutations.


A steroid hormone secreted by the corpus luteum of the ovary; maintains the uterine lining during pregnancy.

Progesterone Response Element (PRE)

A binding site in a promoter to which the activated progesterone receptor can bind.
The progesterone receptor is essentially a transcription factor which is activated only in the presence of progesterone . The activated receptor will bind to a PRE, and transcription of the adjacent gene will be altered. See also "Response element".


A synthetic form of progesterone.
This class of drugs was originally developed to allow absorption by mouth for use in birth control pills.


The prospect as to recovery from a disease or injury as indicated by the nature and symptoms of the case.

Prognostic factor

A situation or condition, or a characteristic of a patient, that can be used to estimate the chance of recovery from a disease or the chance of the disease recurring.

Pro-inflammatory cytokines

Cytokines produced predominantly by activated immune cells such as microglia and are involved in the amplification of inflammatory reactions.
These include IL-1, IL-6, TNF-a, and TGF-ß.

Projectile vomiting

Vomiting with the material ejected with great force.


Cell or organism lacking a membrane-bound, structurally discrete nucleus and other subcellular compartments, usually having their DNA in a single molecule and not organized in chromosomes; A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles
prokaryotic = Literally "before the nucleus".
Prokaryotic cells have no internal membranes or cytoskeleton. Their DNA is circular, not linear. This type of cell is found only in the domains Bacteria and Archaea.
Contrast with eukaryote.


A hyperplastic symptom of disease in which organs appear before the natural time.


The phase of mitosis in which the nuclear envelope breaks into fragments. Some of the spindle fibers reach the chromosomes and attach to protein structures at the centromeres, called kinetochores, while others make contact with microtubules coming from the opposite pole. The opposing spindle fibers move the chromosomes toward the metaphase plate, an imaginary plane equidistant from the poles.


A specific nucleotide sequence in DNA that binds RNA polymerase and indicates where to start transcribing RNA.
A regulatory region that is a short distance upstream from the 5' end of a transcription start site and to which RNA polymerase will bind in order to initiate transcription; the first few hundred nucleotides of DNA "upstream" (on the 5' side) of a gene, which control the transcription of that gene.
The promoter is part of the 5' flanking DNA, i.e. it is not transcribed into RNA, but without the promoter, the gene is not functional. Note that the definition is a bit hazy as far as the size of the region encompassed, but the "promoter" of a gene starts with the nucleotide immediately upstream from the cap site, and includes binding sites for one or more transcription factors which can not work if moved farther away from the gene.


Movement of the forearm so that the palm is turned backward or downward. (see also Supination and Rotation)


Lying on one's stomach.


The nucleus of a sperm or egg prior to fertilization.

Proper fraction

A fraction whose numerator is less than its denominator.


A phage genome that has been inserted into a specific site on the bacterial chromosome.
A temperate phage chromosome inserted as part of the linear structure of the DNA chromosome of a bacterium.


The first stage of mitosis, during which duplicated chromosomes condense from chromatin, and the mitotic spindle forms and begins moving the chromosomes toward the center of the cell.


An equation of fractions in the form: a/b = c/d


If y = kx, then y is said to be proportional to x.


The sensory awareness of the position of body parts with or without movement. Combination of kinesthesia and position sense.


The act of moving an object and maintaining its motion


A lower primate; includes lemurs, lorises, tarsiers, and bush babies, as well as many fossil forms.


One of a group of modified fatty acids secreted by virtually all tissues and performing a wide variety of functions as messengers.

Prostate gland

A walnut-sized gland in the male reproductive system just below the bladder, that secretes an acid-neutralizing component of semen.
It surrounds part of the urethra, the duct that empties the bladder. The main function of the prostate is to supply fluid for the sperm during ejaculation.

Prostate Specific Antigen (PSA)

A marker protein for prostate cell secretions.


Protein that binds DNA in sperm, replacing histones and allowing chromosomes to become more highly condensed than possible with histones.


An enzyme that hydrolyses (cleaves) peptide bonds that link amino acids in protein molecules.
Proteases, also known as proteinases or proteolytic enzymes, are a large group of enzymes. Proteases belong to the class of enzymes known as hydrolases, which catalyse the reaction of hydrolysis of various bonds with the participation of a water molecule.

They are currently classified into six groups:

  • Serine proteases
  • Threonine proteases
  • Cysteine proteases
  • Aspartic acid proteases
  • Metalloproteases
  • Glutamic acid proteases

The threonine and glutamic acid proteases were not described until 1995 and 2004, respectively. The mechanism used to cleave a peptide bond involves making an amino acid residue that has the cysteine and threonine (peptidases) or a water molecule (aspartic acid, metallo- and glutamic acid peptidases) nucleophilic so that it can attack the peptide carbonyl group. One way to make a nucleophile is by a catalytic triad, where a histidine residue is used to activate serine, cysteine or threonine as a nucleophile.

Proteases occur naturally in all organisms and constitute 1-5% of the gene content. These enzymes are involved in a multitude of physiological reactions from simple digestion of food proteins to highly regulated cascades (e.g., the blood clotting cascade, the complement system, apoptosis pathways, and the invertebrate prophenoloxidase activating cascade). Peptidases can break either specific peptide bonds (limited proteolysis), depending on the amino acid sequence of a protein, or break down a complete peptide to amino acids (unlimited proteolysis). The activity can be a destructive change abolishing a protein's function or digesting it to its principal components; it can be an activation of a function or it can be a signal in a signalling pathway.

Proteases are also a type of exotoxin, which is a virulence factor in bacteria pathogenesis. Bacteria exotoxic proteases destroy extracellular structures. Protease enzymes are also found used extensively in the bread industry in Bread improver.

Proteases are involved in digesting long protein chains into short fragments, splitting the peptide bonds which link amino acid residues. Some of them can detach the terminal amino acids from the protein chain (exopeptidases, such as aminopeptidases, carboxipeptidase A); the others attack internal peptide bonds of a protein (endopeptidases, such as trypsin, chymotrypsin, pepsin, papain, elastase).

Proteases are divided into four major groups according to the character of their catalytic active site and conditions of action: serine proteinases, cysteine (thiol) proteinases, aspartic proteinases and metalloproteinases. Attachment of a protease to a certain group depends on the structure of catalytic site and the amino acid (as one of the constituents) essential for its activity.

Proteases are used throughout an organism for various metabolic processes. Acid proteases secreted into the stomach (such as pepsin) and serine proteases present in duodeum (trypsin and chymotrypsin) enable us to digest the protein in food; proteases present in blood serum (thrombin, plasmin, Hageman factor, etc.) play important role in blood clotting, as well as lysis of the clots, and the correct action of the immune system. Other proteases are present in leukocytes (elastase, cathepsin G) and play several different roles in metabolic control. Proteases determine the lifetime of other proteins playing important physiological role like hormones, antibodies, or other enzymes -- this is one of the fastest "switching on" and "switching off" regulatory mechanisms in the physiology of an organism. By complex cooperative action the proteases may proceed as cascade reactions, which result in rapid and efficient amplification of an organism's response to a physiological signal.

Inhibitors of proteases:
The function of peptidases is inhibited by protease inhibitor enzymes. Examples of protease inhibitors are the class of serpins (serine protease or peptidase inhibitors), incorporating alpha 1-antitrypsin. Other serpins are complement 1-inhibitor, antithrombin, alpha 1-antichymotrypsin, plasminogen activator inhibitor 1 (coagulation, fibrinolysis) and the recently discovered neuroserpin.

Natural protease inhibitors include the family of lipocalin proteins, which play a role in cell regulation and differentiation. Lipophilic ligands, attached to lipocalin proteins, have been found to possess tumor protease inhibiting properties. The natural protease inhibitors are not to be confused with the protease inhibitors used in antiretroviral therapy. Some viruses, with HIV among them, depend on proteases in their reproductive cycle. Thus, protease inhibitors are developed as antiviral means.

Proteases, being themselves proteins, are known to be cleaved by other protease molecules, sometimes of the same variety. This may be an important method of regulation of peptidase activity.


A giant protein complex that recognizes and destroys many endogenous proteins tagged for elimination by the small protein ubiquitin.


Large, three-dimensional biological polymer complex, containing nitrogen, found in food and essential for the human body.  Constructed from a set of 20 different monomers called amino acids organised as one or more chains of amino acids in a specific order; the order of amino acids is determined by the base sequence of nucleotides in the gene coding for the protein.  
Proteins are required for the structure, function, and regulation of the bodys cells, tissues, and organs, and each protein has unique functions. Examples are hormones, enzymes, and antibodies.
Some amino acids can be manufactured in the body. Others must be supplied by the diet. The body breaks down food proteins into their amino acid constituents and reassembles the amino acids into the proteins needed for normal functioning.
Proteins have many different functions: structure(collagen); movement (actin and myosin); catalysis (enzymes); transport (hemoglobin); regulation of cellular processes (insulin);  hormones; and response to the stimuli (receptor proteins on surface of all cells).
The information for making proteins is stored in the sequence of nucleotides in the DNA molecule.

Protein aggregate

A misfolded, rigid protein grouping.

Protein engineering

A technique used in the production of proteins with new or artificial amino acid sequences.

Protein gradient

The product of the electron transport chain.
A higher concentration of protons outside the inner membrane of the mitochondria than inside the membrane is the driving force behind ATP synthesis.

Protein kinase

An enzyme that transfers phosphate groups from ATP to a protein.

Protein phosphatase

An enzyme that removes phosphate groups from proteins, often functioning to reverse the effect of a protein kinase.

Protein translocation

Spatial movement of protein within a cell (e.g., from the cytoplasm to nucleus, or into organelles).


A glycoprotein in the extracellular matrix of animal cells, rich in carbohydrate.


The ability to break down proteins.
Hence Proteolytic enzymes = Enzymes which catalyze the hydrolysis (break-down) of proteins or peptides. Proteins (enzymes) that destroy the structure (by peptide bond cleavage) and hence the function of other proteins. These other proteins may or may not themselves be enzymes.


The full complement of proteins present in an organism at any one time (i.e., coded by the genome at that time).
The proteome can change during the life cycle of an organisms and often proteins active during development are different from those used in adulthood          (See Genome)


Study of proteins and their biochemical function in an organism


paraphyletic group consisting of those eukaryotes which are not animals, true fungi or green plants.

Protocol Specifications

Protocol specifications are the physical details which define the test method.
Protocol specifications include sample information such as the number of standards (controls) and their respective concentrations, the number of replicates for each unknown specimen, and the placement of control samples. Protocol specifications also include information necessary for the data reduction and quality control and analysis of data runs, such as the typed of curve fitting selected, the variance line coefficients, and subpopulation categories.


A normal cellular gene concerned with the regulation of cell growth and division, which carries out a normal cellular function, but which can become an oncogene under certain circumstances.
Mutations to proto-oncogenes produce oncogenes that often cause or promote the unregulated growth and division of a cell (i.e., make a cell cancerous). (see Oncogene).
The prefix "c-" indicates a cellular gene, and is generally used for proto-oncogenes (examples: c-mybc-mycc-fosc-jun , etc).


A subatomic particle with a single positive electrical charge, found in the nucleus of the atom.
Protons, along with other subatomic particles called Neutrons, make up the nucleus of a atom.  The number of protons in an atom is called the atomic number of the element.
3 quarkhadrons.

Proton acceptor / Proton donor

Proton acceptor = An anionic compound capable of accepting a proton from a proton donor; that is, a Bronsted-Lowry base.
Proton donor = The donor of a proton in an acid-base reaction; that is, a Bronsted-Lowry acid.

Proton emission

Ejection of a proton from an atom's nucleus. Since an atom loses a proton during proton emission, it changes from one element to another. For example, after undergoing proton emission, an atom of nitrogen (with 7 protons) becomes an atom of carbon (with 6 protons).

Proton motive force

The potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ions (the Proton pump) across biological membranes during chemiosmosis.


All the contents of a cell, including the nucleus.
See Cytoplasm

Proton pump

An active transport mechanism in cell membranes that consumes ATP to force hydrogen ions out of a cell and, in the process, generates a membrane potential.


Bound, water-insoluble pectin, as it occurs native in fruits.


Living matter

Protozoan (Plural = Protozoa)

Unicellular heterotrophic animals; A protist that lives primarily by ingesting food, an animal-like mode of nutrition. From a phylogenetic point of view, an outmoded concept because it included unrelated eukaryotes - various amoebae, flagellates, ciliates and sporozoa.


A device for measuring angles.


Gram negative rods isolated from human clinical specimens and from penguins


A virus chromosome (DNA) integrated into the DNA of the host cell.


Compound that the human body can convert into a vitamin.
For example, beta-carotene is a provitamin because the body can convert it into vitamin A, as needed.

Proximate causation

The hypothesis about why natural selection favored a particular animal behaviour.


Near to

Pruning (Neuronal pruning)

The process of shortening or reducing number of neuronal synapses, axons, or dendrites in response to use or growth signals.


Increased pain and decreased strength in lower limbs associated with physical activity.
Complaints are similar to those caused by insufficient blood supply to the limb but are caused by diminished blood supply to the nerves in a narrowed spinal canal.


A body cavity consisting of a fluid-filled space between the endoderm and the mesoderm; characteristic of the nematodes.
Pseudocoelomate = An animal, such as a rotifer or roundworm, whose body cavity is not completely lined by mesoderm


The apparent disappearance of a taxon. In cases of pseudoextinction, this disappearance is not due to the death of all members, but the evolution of novel features in one or more lineages, so that the new clades are not recognized as belonging to the paraphyletic ancestral group, whose members have ceased to exist. The Dinosauria, if defined so as to exclude the birds, is an example of a group that has undergone pseudoextinction.


Clinical features resembling a dementia that are not due to organic brain dysfunction or disease. Pseudodementia may occur in a major depressive episode or may be seen in factitious disorder with psychological symptoms.


The sudden appearance of a recessive phenotype in a pedigree, due to deletion of a masking dominant gene.The recessive allele shows itself in the phenotype when only one copy of the allele is present, as in hemizygous alleles or in deletion heterozygotes .


An inactive gene derived from an ancestral active gene.


A nonmalignant intraocular disturbance resulting from the detachment of the retina.


Gram-negative rod is widely distributed in nature. Some species are pathogenic for humans, animals, or plants (e.g. P. aeruginosa).

Pseudopodium (Plural = Psuedopodia)

A cellular extension of amoeboid cells used in moving and feeding.
Fingerlike extensions from an amoeboid cell; literally "false feet". Transient extensions of the cell surface, used for locomotion or feeding. They may be supported internally (actinopods) or not (rhizopoda), they may be thread like (filose) or broad (lobose), may or may not bear extrusomes (nudipodia, extrusopodia) and they may be one (monopodial) to many (polypodial) produced at one time.

Pseudotumor cerebri

Raised intracranial pressure, usually causing only headache and papilledema.
No clear underlying structural abnormality.

Psoriatic arthritis

Joint inflammation associated with psoriasis . Psoriatic arthritis is a potentially destructive and deforming form of arthritis that affects approximately 10% of persons with psoriasis.

Psychogenic deafness

Auditory impairment which may result from emotional stress as an unconscious means of escape from an intolerable situation.
Also referred to as conversion or hysterical deafness.


A professional specializing in counselling, including adjustment to disability.
The psychologist may provide individual or group psychotherapy for the purpose of cognitive retraining, management of behavior and the development of coping skills by the patient/client and members of the family.


The broad field of psychological and mental testing.

Psychomotor agitation

Excessive motor activity associated with a feeling of inner tension; the activity is usually nonproductive and repetitious.  When the agitation is severe, it may be accompanied by shouting or loud complaining. The term should be used in a technical sense to refer only to states of tension or restlessness that are accompanied by observable excessive motor activity. Examples: Inability to sit still, pacing, wringing of hands, pulling at clothes.

Psychomotor retardation

1. A slowed development of abilities to perform acts involving cognitive and physical processes. 2. A generalized slowing down of physical reactions, movements and speech.

Psychomotor seizure

A type of seizure in which the child displays inappropriate, purposeless behavior (such as lip smacking, chewing, or other automatic reactions) for the setting and automatic or involuntary movements and actions.


The scientific technique of measurement involving systematic measures of relationships between stimuli and perceptions.


A general term used to describe any of several mental disorders
characterized by social withdrawal, distortions of reality, delusions, hallucinations,
illusions, loss of contact with environment, disintegration of the personality, and unclear thinking to the degree that any one of these interferes with the capacity to cope with everyday life.


Gram-negative rod associated with fish, processed meat and poultry products.
Some strains have been isolated from pathological specimens from humans and animals.


Microorganisms that grow best at cold temperatures, with optimum growth at 5° -20°C (41°-68°F) and are capable of growing at refrigerated and room temperatures.


Microorganism able to grow well between 0°C and 7°C, having an optima of 20°C to 30°C.

public supply

water withdrawn by public governments and agencies, such as a county water department, and by private companies that is then delivered to users. Public suppliers provide water for domestic, commercial, thermoelectric power, industrial, and public water users. Most people's household water is delivered by a public water supplier. The systems have at least 15 service connections (such as households, businesses, or schools) or regularly serve at least 25 individuals daily for at least 60 days out of the year.

public-water use

water supplied from a public-water supply and used for such purposes as firefighting, street washing, and municipal parks and swimming pools.

Pull-down assays

In vitro methods used to determine physical interaction between two or more proteins (between bait and preys).


Pertaining to the lungs.

Pulmonary artery

In birds and mammals, an artery that carries deoxygenated blood from the right ventricle of the heart to the lungs, where it is oxygenated.

Pulmonary artery wedge pressure (PAWP)

The pressure obtained when the balloon of the pulmonary artery catheter is inflated, wedging it in the pulmonary artery

Pulmonary embolus

Mass of undissolved matter (such as a blood clot, tissue, air bubbles, and bacteria) in the pulmonary arteries or its branches

Pulmonary fibrosis

Scarring throughout the lungs which can be caused by many conditions such as, sarcoidosis , hypersensitivity pneumonitis , asbestosis, and certain medications. Pulmonary fibrosis can also occur without an identifiable cause, in which case it is referred to as idiopathic pulmonary fibrosis . Symptoms include shortness of breath, coughing and diminished exercise tolerance. Treatment involves the use of corticosteroids (such as prednisone ) and/or other medications that suppress the body's immune system. Interferon gamma-1b treatment may help patients with idiopathic pulmonary fibrosis who do not respond to corticosteroid therapy. The goal of treatment is to decrease lung inflammation and subsequent scarring. Responses to treatment vary. Toxicity and side effects of treatments can be serious. Therefore, patients with pulmonary fibrosis are generally cared for by lung specialists.

Pulmonary hypertension

High blood pressure in the pulmonary artery that conveys blood from the right ventricle to the lungs. The pressure in the pulmonary artery is normally low compared to that in the aorta . Pulmonary hypertension can irrevocably damage the lungs and cause failure of the right ventricle.

Pulmonary vein

In birds and mammals, a vein that carries oxygenated blood from the lungs to the left atrium of the heart, from which blood is pumped into the left ventricle and from there to the body tissues.


A measurement of heart rate; distention of an artery that can be felt each time the heart contracts.

Pulse chase experiment

An experiment in which cells are grown in radioactive medium for a brief period (the pulse) and then transferred to nonradioactive medium for a longer period (the chase).

Pulsed-field gel electrophoresis (PGFE)

An electrophoretic technique in which the gel is subjected to electrical fields alternating between different angles. A gel technique which allows size-separation of very large fragments of DNA, in the range of hundreds of kb to thousands of kb. As in other gel electrophoresis techniques, populations of molecules migrate through the gel at a speed related to their size, producing discrete bands. In normal electrophoresis, DNA fragments greater than a certain size limit all migrate at the same rate through the gel. In PFGE, the electrophoretic voltage is applied alternately along two perpendicular axes, which forces even the larger DNA fragments to separate by size .This allows very large DNA fragments to snake through the gel, and hence permitting efficient separation of mixtures of such large fragments.

Pulse oximeter 

A medical instrument that allows the measurement of oxygen saturation non-invasively using the infrared absorption spectra of haemoglobin
Generally the device is found in a finger cuff


With a dimpled or spotted appearance.

Punctuated equilibrium

A theory of evolution advocating spurts of relatively rapid change in relatively rapid bursts followed by long periods of stasis.

Punnett square

The checkerboard diagram used for analysis of allele segregation.


In metamorphozing insects, a stage between the larva and adult during which the organism undergoes major developmental changes.


the “hole” or circular opening in the iris through which light passes to the lens and the retina; located slightly to the nasal side of the center of the iris; lies behind the anterior chamber of the eye and the cornea and in front of the lens; diameter changes with contraction and relaxation of the muscular fibers of the iris as the eye responds to changes in light, emotional states and other kinds of stimulation.

Pure-Tone audiometry

Measurement of hearing sensitivity based on the use of pure tones of various frequencies and intensities as auditory stimuli.


A nitrogen- containing, single- ring, basic compound, such as adenine or guanine, , with a characteristic two-ring structure, that occurs in nucleic acids. One of the components of nucleic acids, the purines in DNA and RNA are adenine and guanine.


A system involving a sac and channels and found in some dinoflagellates.
The function is not understood, but it may act as an osmoregulatory organelle.


A nucleus that is part of the basal ganglia in the brain. It acts with the caudate nucleus to influence motor activity.

PyelogramIntravenous or Retrograde

Special X-rays showing the drainage pattern of the kidneys. In the intravenous method a dye opaque to X-rays is injected into a vein. After a waiting period for the blood and dye to pass through the kidneys, X-rays can be taken of the collecting system of the kidney, ureter and bladder.
In the retrograde method a dye opaque to X-rays is flushed backwards up the urethra and bladder and up the ureters to the kidneys.


Shrinkage and condensation of nuclear chromatin, associated with cell necrosis.


A layer of water in which there is a steep gradient in density with depth. It separates the well-mixed surface waters from the dense waters of the deep ocean.
Density of the water is a function of temperature, salinity and, to a lesser extent, pressure.


The union of all line segments that connect a given point and the points that lie on a given polygon.

Pyramid of energy

A diagram of the energy flow between the trophic levels of an ecosystem; plants or other autotrophs (at the base of the pyramid) represent the greatest amount of energy, herbivores next, then primary carnivores, secondary carnivores, etc.


protein body lying inside some types of chloroplasts.




A nitrogen- containing, double- ring, basic compound, such as cytosine, thymine, or uracil, with a characteristic single-ring structure, that occurs in nucleic acids.
One of the components of nucleic acids, the pyrimidines in DNA are cytosine and thymine; in RNA, cytosine and uracil.

Pythagorean Theorem

The theorem that relates the three sides of a right triangle: a2 + b2 = c2

Pythagorean triple

Three natural numbers that satisfy the pythagorean theorem.