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NAD+ (nicotinamide adenine dinucleotide)

Abbreviation of nicotinamide adenine dinucleotide. NAD+ is a coenzyme present in all cells that helps enzymes transfer electrons during the redox reactions of metabolism

NADP (nicotinamide adenine dinucleotide phosphate)

Abbreviation of nicotinamide adenine dinucleotide phosphate, a coenzyme that functions as an electron acceptor in the lightdependent reactions of photosynthesis.
NADP+ is the oxidized form of nicotinamide adenine dinucleotide phosphate which is used as an electron carrier.

NADPH (nicotinamide adenine dinucleotide phosphate)

The reduced form of nicotinamide adenine dinucleotide phosphate which is used as an electron carrier.


A unit of measure; one millionth (10-9) of a meter.


A technology that creates small materials at the scale of molecules by manipulating single atoms. The name nano comes from the size of molecules which is measured in nanometers - or one billionth of a meter (0.000000001 meter). The dimension of single atoms is ten fold smaller. The molecular processes of life, particularly the activity of proteins (enzymes) and the self-organizing behavior of many biological molecules has greatly inspired nanotechnology and molecular motors (i.e. protein complexes) could be considered the result of natures nanotechnology.


A non-steroidal anti-inflammatory drug (NSAID) used for the management of mild to moderate pain, fever, and inflammation . Naproxen blocks the enzyme cyclooxygenase that makes prostaglandins, resulting in lower concentrations of prostaglandins. As a consequence, inflammation, pain and fever are reduced. Brand names for naproxen include Anaprox, Naprelan, Naprosyn, and Aleve.

Narcissistic personality disorder

Grandiose sense of self-importance of uniqueness; preoccupation with fantasies of limitless success; need for constant attention and admiration; disturbances in interpersonal relationships, such as lack of empathy, exploitativeness, and waivering between extremes of overidealization and devaluation.


A sleep disorder syndrome consisting of excessive daytime sleep and disordered regulation of REM sleep.
Usually begins in teenage years. Components of REM sleep intrude into the Non-REM sleep and the waking states.  There are 4 main symptoms:
(a) Chronic excessive daytime sleepiness not relieved by any amount of sleep,
(b) Cataplexy which is the sudden loss of postural muscle tome, while awake, similar to the atonia of REM sleep (consciousness is still preserved throughout the attack), triggered by strong emotion especially laughter
© Sleep paralysis which, like cataplexy, consists of the same type of atonia as in REM sleep; it occurs at the onset of sleep or on awakening
(d) Vivid auditory or dream-like hallucinations (hypnagogic or hypnopompic hallucinations) at the onset of sleep or on awakening.

Nasal cannula

A low flow oxygen delivery device designed to administer oxygen through the nose, filling the anatomic reservoir with oxygen enriched gas.
This device is used with flows of less than 6 L/min.


A voice disorder in which a structural or functional inadequacy of the palate
may cause a person to sound either nasal or denasal.  Nasality may result from clefts of the palate, including submucous clefts; related craniofacial anomalies such as Pierre-Robin and Treacher Collins syndromes; neuropathies such as cerebral palsy; or, simply functional inadequacy. Denasality may result from physical blockage of air through the nasal passages or less frequently from idiosyncratic speech production styles.

Nasogastric tube (NG Tube)

A plastic feeding tube placed in the nose and extended to the stomach. This tube allows for direct "tube feeding" to maintain the nutritional status of the patient or removal of stomach acids.

Nasopharyngeal airway (see also Oropharyngeal airway)

An artificial airway that passes through the nose and nasopharynx and rests just behind the tongue, where it separates it from the posterior pharynx

Native microflora

Microorganisms that are normally found within a food source (often referred to as spoilage organisms).

Natural killer cell

A nonspecific defensive cell that attacks tumor cells and destroys infected body cells, especially those harboring viruses.

Natural logarithm

The logarithm (log) to base e where e ≈ 2.718. The natural logarithm is represented by the symbol “ln”.

Natural numbers

The counting numbers

Natural selection

The process described by Darwin's theory of evolution that favors certain genotypes and disfavors others: Process by which the genotypes in a population that are best adapted to the environment increase in frequency relative to less well-adapted genotypes over a number of generations.
Differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Evolution occurs when natural selection causes changes in relative frequencies of alleles in the gene pool. This process is entirely guided by the interaction of an organism with its environment.

Near infrared spectroscopy

A spectroscopic method utilising the near infra-red region of the electromagnetic spectrum (from 1100nm to 2500nm).

Nearest-neighbour analysis

A technique of transferring radioactive atoms between adjacent nucleotides in DNA used to demonstrate that the two strands of DNA run in opposite directions.


The death of living cells or tissues . Necrosis can be due, for example, to ischemia (lack of blood flow). From the Greek "nekros" (dead body).

Necrotizing enterocolitis

A condition in which the intestine becomes inflamed, and there is sometimes death of the intestinal wall.  This death of the intestinal wall results in a shortening of the intestine and subsequent nutritional malabsorption and poor growth.


A ubiquitin-like small protein modifier.


The Nedd8 conjugation process, called neddylation, is similar to ubiquitination.
Neddylation utilizes the E1 activating-enzyme complex composed of two subunits, APP-BP1 and UBA3, and the E2 conjugating-enzyme, UBC12 (Yeh, 2000). The only known substrates of neddylation are Cullin family proteins -- Cul1, Cul2, Cul3, Cul4A, Cul4B, and Cul5 -- which have been shown to be modified by Nedd8 in mammalian cells. Cullins directly interact with Roc1, a Ring finger protein, and the Cullin-Roc1 complex comprises the core module of a series of ubiquitin E3 ligases, which confer substrate specificity and therefore regulate the degradation process. Among Cullins, many studies focused on Cul1, an essential component of the SCF complex which functions as ubiquitin E3 ligase. The SCF complex consists of core subunits: Skp1, Cul1/Cdc53, Roc1/Hrt/Rbx1, and a substrate-recognition F-box protein. Cul1 functions as a scaffold protein within the SCF complex; the N-terminal domain of Cul1 interacts with the adaptor protein Skp1 that links with the F-box protein, and the C-terminal domain interacts with Roc1 and the ubiquitin E2 enzyme

Negative feedback

A process whereby a change in a physiological variable that is being monitored triggers a response that counteracts the initial fluctuation.
This, with positive feedback, is a primary mechanism of homeostasis.

Negative numbers

A real number less than zero.


A type of ingestion rod. Stiff aggregates of many microtubules found around the cytostome of some ciliates and used during the ingestion of food.


A threadlike stinger, containing a poisonous or paralyzing substance, found in the cnidocyte of cnidarians.


phylum in the animal kingdom. Scientific name for nematode. A dominant group within the meiofauna. Scientific name for nematode. Commonly called roundworms.

N-end rule

The life span of a protein is determined by its amino-terminal (N-terminal) amino acid.


Paying little or no attention to a part of the body.


Clusters of closely related species and subspecies that have evolved relatively recently.


New words invented by the subject, distortions of words or standard words to which the subject has given new, highly idiosyncratic meaning; Nonsense or made-up word used when speaking.  The person often does not realize that the word makes no sense. The judgement that neologisms are present should be made cautiously, taking into account the subject's educational and cultural background. Examples: "I was accused of midigation" (meaning the subject was accused of breaking the law). "They had an insinuating machine next door" (explaining how her neighbors were bothering her). Neologisms may be observed in schizophrenia and other psychotic disorders.

Neonatal seizures

Seizures in newborns evidenced by alternating contractions of various muscle groups.

Nephelometric turbidity unit (NTU)

Unit of measure for the turbidity of a solution such as water.
Essentially, a measure of the cloudiness of water as measured by a nephelometer. Turbidity is based on the amount of light that is reflected off particles in the water.


A kidney stone


The tubular excretory unit of the vertebrate kidney.


A non-inflammatory disease of kidneys

Nernst equation

An equation for calculating the electrical potential difference across a cell membrane for a single permeating ion.
The Nernst equation relates the electrical potential across a permeable membrane to the distribution of a charged ion that it separates, provided that the ion is permeating. (i.e., an ion that can diffuse across the cell membrane). The equation considers only one permeating ion at a time in a cell:

Where V = voltage difference, R = Universal gas constant, z = valency of the ion,
F = Faraday’s constant, Co = concentration on one side of the membrane, and
Ci = concentration on one side of the membrane.
(see also Resting Membrane Potential and Goldman equation)


A collection of neuron fibres (axons and dendrites) wrapped up together tightly in connective tissue.

Nerve fibre

A word to describe the axon of a neuron .
The axon is a  filamentous process extending from the cell body of a neuron and conducting the nerve impulse.

Nerve impulse

Another name for the Action Potential: a rapid, transient, self-propagating change in electric potential across the membrane of an axon
The Action Potential or nerve impulse  is a rapid series of changes in the membrane potential of a neuron that occurs in response to a stimulus or input that causes the Resting Membrane Potential to reach a threshold level.

Nervous system

All the nerve cells of an animal; the receptor-conductor-effector system;.
In humans, the nervous system consists of the Central nervous system (brain and spinal cord) and the Peripheral nervous system (the rest of the nerves in the body).

Net energy (of food)

Energy actually available for the body’s cells taking into account the extra heat produced by the transformation of nutrients after their absorption in the bloodstream.


A network is a set of hardware and software that allows more than one computer to share the same programs, data files, and peripheral equipment.

Network File System (NFS)

A process for mounting magnetic disks on a network so that disks not physically attached to a computer appear as if they were physically attached.

Neural crest

A band of cells along the border where the neural tube pinches off from the ectoderm.  The cells of the neural crest migrate to various parts of the embryo and form the pigment cells in the skin, bones of the skull, the teeth, the adrenal glands, and parts of the Peripheral nervous system.


Paroxysmal pain that extends along the course of one or more nerves

Neural groove

A dorsal, longitudinal groove that forms in a vertebrate embryo. It is bordered by two neural folds; preceded by the neural-plate stage and followed by the neural-tube stage.

Neural plate

Thickened strip of ectoderm in early vertebrate embryos that forms along the dorsal side of the body.  It gives rise to the Central nervous system.

Neural stem cell

A stem cell found in adult neural tissue that can give rise to neurons and glial (supporting) cells

Neural transplantation

A surgical procedure involving the transplantation of stem cells directly into the brain, close to the site of damage which is identified using brain-imaging technology.
Following implantation, the stem cells move into the site of damage and mature into the functional brain cells needed to repair the damage.

Neural tube

Primitive, hollow, dorsal nervous system of the early vertebrate embryo.
It is formed by fusion of neural folds around the neural groove. The rudiment of the brain and spinal cord; its lumen gives rise to the cavities, or ventricles, of the brain and to the central canal of the spinal cord.

The nervous system develops from a part of ectoderm lying originally on the dorsal side of the embryo, above the notochord and the somites. This part of the ectodermal layer thickens and becomes the neural plate, whose edges rise as neural folds that converge toward the midline, fuse together, and form the neural tube. In vertebrates the neural tube lies immediately above the notochord and extends beyond its anterior tip. The remainder of the ectoderm closes over the neural tube and becomes, in the main, the covering layer (epithelium) of the animal’s skin (epidermis).

As the neural tube detaches itself from the overlying ectoderm, groups of cells pinch off and form the neural crest, which plays an important role in the development of, among other things, the segmental nerves of the brain and spinal cord.


A chemical agent that is released by a neuron and diffuses through a local region of the central nervous system, acting on neurons within that region; generally has the effect of modulating the response to neurotransmitters rather than itself producing a response.

Neuromuscular junction (NMJ)

The junction between an axon terminal of a motor neuron and a muscle fiber innervated by that motor neuron.  At the NMJ the axon terminal of a motor neuron is typically branched, forming neuromuscular junctions with a number of different muscle fibers.


Having to do with the nerves or the nervous system.


Nerve cell. The fundamental unit of the nervous system, with structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane.


A psychologist who specializes in evaluating (by tests) brain/behavior relationships, planning training programs to help the survivor of brain injury return to normal functioning and recommending alternative cognitive and behavioral strategies to minimize the effects of brain injury.

Neurosecretory cells

Cells in the hypothalamus that receive signals from other nerve cells, but instead of signaling to an adjacent nerve cell or muscle, release hormones into the bloodstream.


A chemical messenger released from the end of one neuron (it’s synaptic terminal) to move across a gap (the synaptic cleft)  and then bind to receptors on a post-synaptic cell where it produces a response. Neurotransmitters are needed to carry information across the gap (the synaptic cleft) between one neuron and the next (the pre-synaptic and post-synaptic neurons) in the form of signalling known as chemical synaptic signalling. This is one of the most common methods of signalling in the nervous system. Binding of the neurotransmitter to the corresponding receptors on nearby cell surfaces (post synaptic membrane) causing a physiological stimulus in form of a membrane current (triggering action potentials) or second messenger cascade activating channels, pumps, kinases, or proteases. The molecular mechanisms of activation are similar to those of hormones.


A reaction in which the characteristics of an acid or base disappear.

Neutral variation

Genetic diversity that confers no apparent selective advantage.


An electrically neutral particle (a particle having no electrical charge), found in the atom.
Particles with zero charge forming part of an atomic nuclei. 3 quarkhadrons.


Phagocytic (ingesting, scavenging) white blood cells that are produced in the bone marrow.They ingest and destroy invading microorganisms and facilitate post- infection tissue repair.  Upon ingestion of pathogens, the neutrophil generates reactive oxygen species such as O2- (also known as free radicals). That O2- causes an influx of potassium ions into the portion of the neutrophil that contains the pathogen, which thereby releases proteases from existing granules ('storage' sites) in the neutrophil. The proteases then kill the pathogen. In addition to generating proteases, neutrophils can secrete collagenase and plasminogen activator. They are the immune system's "first line" of defense against invading pathogens, and large reserves are called forth within hours of the start of a "pathogen invasion."

Neutrophil Extracellular Trap

A sort of mesh that is formed (e.g., in blood vessels) via the pathogen-triggered "programmed death" of immune system neutrophils. Chromatin and some protein molecules from the dead neutrophils adhere to each other; thereby forming the mesh (which entraps pathogens such as bacteria, for other immune system cells to kill).

Neurocutaneous syndromes

A group of conditions that combine skin and sensory or nervous difficulties.

Neurofibromatosis (NF)

An inherited, progressive neurological disorder in which tumor(s) are present in the tissues of the central nervous system (e.g., skin, brain).

Neuropathic pain

Pain originating from a central or peripheral nervous system site rather than from a tissue or organ. The most common pain sensation, nociceptive pain, results from physical tissue damage, which is then transmitted to the brain  by nerves to ultimately result in the perception of pain. A second type of pain, neuropathic pain, stems from a malfunction in the central and/or peripheral nervous system. Because its source is internal, neuropathic pain may lack an obvious origin. (see Nociceptive pain)


A general term used to describe the adaptive changes in the structure or function of nerve cells or groups of nerve cells in response to injuries to the nervous system or alterations in patterns of their use and disuse.


Behavior that involves a partial disorganization, characterized by combinations of anxieties, compulsions, obsessions, and phobias.

Neurotic disorder

A mental disorder in which the predominant disturbance is a symptom or group of symptoms distressing to the individual, recognized by him/her as unacceptable and alien (ego-dystonic); reality testing is grossly intact.  Behavior does not actively violate gross social norms (though it may be quite disabling). The disturbance is relatively enduring or recurrent without treatment, and is not limited to a transitory reaction to stressors. There is no proven organic etiology or factor.

Neurotic process

A specific etiological process involving the following sequence:
(1) unconscious conflicts between opposing desires or between desires and prohibitions, which cause (2) unconscious perception of anticipated danger or dysphoria, which leads to (3) use of defense mechanisms that result in (4) either symptoms, personality disturbance or both.


The environment of the interface between water and air, often rich in bacteria and protists.


Substance with a pH of 7.0. Substances with a pH rating close to neutral include meats and milk products (pH 6.4).


A chemical released from nerves that activates or inhibits the next nerve in the network, or some other type of cell. Neurotransmitters are used to transmit information (relayed along a nerve in the form of action potentials) from a neuron to the next cell in the series.  They can act on the target cell generally only if the target cell has appropriate receptors to bind these chemicals.  However, some neurons release gas molecules (nitric oxide and carbon monoxide have been discovered to carry out this role) which can act on the target cell without binding to specific receptors.

Newlne character

One or two bytes which denote the end of a line. In DOS a newline character is two bytes: a carriage return and a linefeed. In UNIX a newline character is one byte: a linefeed.

Newtons Laws of Motion

Classical laws which enable the prediction of the path of any object from a grain of sand to entire galaxies.
1.    A body will remain at rest or move with a constant velocity unless acted upon by an outside force.
2.    The acceleration of a body is proportional to the applied force.  This is expressed by the universal formula:  Force = mass × acceleration.
3.    For every action there is an equal and opposite reaction.


National Geodetic Vertical Datum. (1) As corrrected in 1929, a vertical control measure used as a reference for establishing varying elevations. (2) Elevation datum plane previously used by the Federal Emergency Management Agency (FEMA) for the determination of flood elevations. FEMA current uses the North American Vertical Datum Plane.


National Geodetic Vertical Datum of 1929. A geodetic datum derived from a general adjustment of the first order level nets of the United States and Canada. It was formerly called "Sea Level Datum of 1929" or "mean sea level" in the USGS series of reports. Although the datum was derived from the average sea level over a period of many years at 26 tide stations along the Atlantic, Gulf of Mexico, and Pacific Coasts, it does not necessarily represent local mean sea level at any particular place.


The slope of the Hill plot; the Hill coefficient. See Hill analysis.

Niacin and niacinamide

Forms of Vitamin B3. Niacinamide can be made from niacin in the body. Niacin and niacinamide are required for the proper function of fats and sugars in the body and to maintain healthy cells. Only niacin seems to lower cholesterol, not niacinamide. It is  most effective in increasing HDL cholesterol and modestly effective in lowering LDL cholesterol, and triglyceride levels.


An organism’s role in its environment; the ecological position occupied by a species.

Nick translation

A method for incorporating radioactive isotopes (typically 32P) into a piece of DNA.
The DNA is randomly nicked by DNase I, and then starting from those nicks DNA polymerase I digests and then replaces a stretch of DNA. Radiolabeled precursor nucleotide triphosphates can thus be incorporated.

Niskin bottle

Quantitative sampling device for taking water samples below the surface.
Comprised of a cylindrical container with spring loaded stoppers at both ends. Often used in conjunction with CTD sensors.

Nitrogen fixation

Biological assimilation of atmospheric nitrogen to form organic nitrogen-containing compounds.

Nitric oxide

A gas that acts on blood vessels to relax them.
In the kidney, nitric oxide promotes salt and water excretion


The oxidation of ammonia or ammonium to nitrites and nitrates, as by nitrifying bacteria.


Safe food additive that has been used for centuries to preserve meats, fish and poultry.  It also contributes to the characteristic flavour, colour and texture of processed meats such as hot dogs. Because nitrite safeguards cured meats against the most deadly foodborne bacterium of all, Clostridium (C.) botulinum, its use is supported by the public health community. The human body generates much greater nitrite levels than are added to food. Nitrates consumed in foods such as carrots and green vegetables are converted to nitrite during digestion. Nitrite in the body is instrumental in promoting blood clotting, healing wounds and burns, and boosting immune function to kill tumor cells.


An enzyme, unique to certain prokaryotes, that reduces N2 to NH3.

Nitrogenous base

An organic base that contains the element nitrogen. A nitrogen- containing molecule having the chemical properties of a base.


Digestive reaction-product of nitrite, a food additive used to preserve meats, fish and poultry.

NMDA (N-methyl-D-aspartate)

A glutamate analog that is the agonist of Glutamte NMDA-type receptors. Glu-NMDA receptors are important for learning and memory and for particular long-term processes in the brain.

Noble gases

Elements with zero valency.  They form group 0 in the periodic table and are non-reactive.

Nociceptive pain

Pain originating from injury to a peripheral tissue or organ. The most common pain sensation.  Results from physical tissue damage, which is then transmitted to the brain by nerves to ultimately result in the perception of pain. (see Neuropathic pain)


Applied to organisms that are active during the hours of darkness.


A branching point in a phylogenetic tree, representing the common ancestor of the lineages descending from this branching point.

Nodes of Ranvier

The small gaps in the myelin sheath between successive glial cells along the axon of a neuron. The nodes are the sites of of high concentration of voltage-gated ion channels, particularly voltage-gated Na+ channels.  They are therefore important in regenerating the current for transmission of Action Potentials down the axon of a neuron.

Noise signals

A signal (e.g., a sound or a visual stimulus) containing a wide range of frequencies with a specific distribution of energy across these frequencies. Different noise signals can vary systematically in the range of frequencies and the energy distribution across these frequencies.
A noise with a particular distribution of parameters is given a different “colour” in its name, e.g.,
White noise: Power density is constant over a finite frequency range. (See White Noise)
Pink noise: power density decreases 3dB per octave with increasing frequency (density proportional to 1/f) over a finite frequency range which does not include DC.  Thus it contains more energy at lower frequencies than higher; however each octave contains the same amount of power (not a trivial filtering problem!)
Red noise: Oceanic ambient noise (ie, noise distant from the sources) is often described as "red" due to the selective absorption of higher frequencies."
Orange noise: quasi-stationary noise with a finite power spectrum with a finite number of small bands of zero energy dispersed throughout a continuous spectrum. These bands of zero energy are centered about the frequencies of musical notes in whatever system of music is of interest. Since all in-tune musical notes are eliminated, the remaining spectrum could be said to consist of sour, citrus, or "orange" notes. Orange noise is most easily generated by a roomfull of primary school students equipped with plastic soprano recorders.
Green noise: A really long term power spectrum averaged over several outdoor sites. Rather like pink noise with a hump added around 500Hz. Supposedly the background noise of the world.
Blue noise: Power density increases 3dB per octave with increasing frequency (density proportional to f) over a finite frequency range. This can be good noise for dithering.
Purple or Violet noise: Power density increases 6dB per octave with increasing frequency (density proportional to f^2) over a finite frequency range. Differentiated white noise.
Grey noise: Noise subjected to a psychoacoustic equal loudness curve (such as an inverted a-weight curve) over a given range of frequencies, so that it sounds like it is equally loud at all frequencies. This would be a better definition of "white noise" than the "equal power at all frequencies" definition, since real "white light" has the power spectrum of a 5400K black body, not an equal power spectrum.
Brown noise: Power density decreases 6dB per octave with increasing frequency (density proportional to 1/f^2) over a frequency range which does not include DC. It is not named for a power spectrum that suggests the color brown, rather, the name is a coruption of Brownian motion. AKA "random walk" or "drunkard's walk" noise.
Black noise: Power density is constant for a finite frequency range above 20kHz. Ultrasonic white noise. This black noise is like the so-called "black light" with frequencies too high to be perceived as sound, but still capable of affecting you or your surroundings.

Noise suppression (in hearing aids)

Technologies implemented in hearing aids to reduce interference from unwanted “noise” in the environment, allowing the listener to be able to hear the desired signals more clearly. Some hearing aids have circuitry designed to control unwanted background noise, for example; automatic gain control, compression amplification, automatic signal processing, and noise suppression switches. Some hearing aids are programmable and use digital technology to automatically adjust hearing aid settings for different listening environments.


A graph that allows a third variable to be measured when the values of two related variables are known.


Not able to walk.

Non-coding sequences

Sequences of nucleotides that do not signal for proteins.
Non-coding sequences contain regulatory sequences, sequences of old, inactive genes, repeat sequences that allow recombination of genetic information (DNA pieces) from different chromosomes, locations, or even foreign DNA introduced by microbial or viral infection.
Many non-coding sequences are transposable elements (see Transposon) meaning that they can copy and insert themselves at many different sites within chromosomes. These rearrangements of physical location of DNA strands affect number, location, and sequence of genes coding for proteins and RNA, and are vital for generating mutations important for evolutionary fitness of an organism.
The human genome contains only 5% coding sequences (genes that make proteins), while half of all non-coding portions are made of transposable elements reminiscent of viral DNA.  (see also Coding sequences)

Noncompetitive inhibitor

A substance that reduces the activity of an enzyme by binding to a location remote from the active site, changing its conformation so that it no longer binds to the substrate.

Noncyclic electron flow

A route of electron flow during the light reactions of photosynthesis that involves both photosystems and produces ATPNADPH, and oxygen; the net electron flow is from water to NADP

Noncyclic photophosporylation

The production of ATP by noncyclic electron flow.


Chromosome pairs do not disjoin (separate) during cell division so that both chromosomes go to one daughter cell and none to the other.
An accident of meiosis or mitosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly. Nondisjunction causes errors in chromosome number (trisomy) such as a type of Down syndrome in which the chromosomal pairs do not separate properly as the sperm or egg cells are formed; also known as Trisomy 21.

Non-equilibrium theory

Suggests that the number of species increases or decreases depending on how the environment influences species production, exchange and extinction at any particular time.


A nonimmunoassay test (NIA) is a single set of samples which are measured on a detector instrument as a single batch but is not computed as an immunoassay. The measurements are printed as formatted raw data in a single report.

Non-insulin-dependent diabetes (Type 2 diabetes)

A condition in which the body either makes too little insulin or cannot properly use the insulin it makes to convert blood glucose to energy.
Type 2 diabetes may be controlled with diet, exercise, and weight loss, or may require oral medications and/or insulin injections.
(see Insulin-dependent diabetes)

Non-ionizing radiation

Rays of energy that move in long, slow wave patterns and do not penetrate cells.

Nonisotopic Assays

Nonisotopic assays are immunoassays which do not have radioactive labels for their tracer material.

Nonlinear dynamics.

Seemingly random behaviour that can actually be described by chaos theory to actually be well defined, orderly, and sensitive to initial conditions.

Nonparametric Tests

Nonparametric tests are statistical tests which make no assumptions about the distribution of a statistical population. Nonparametric tests are generally more robust but less sensitive than parametric tests

Nonpolar covalent bond

A type of covalent bond in which electrons are shared equally between two atoms of similar electronegativity.

non-point source (NPS) pollution

pollution discharged over a wide land area, not from one specific location. These are forms of diffuse pollution caused by sediment, nutrients, organic and toxic substances originating from land-use activities, which are carried to lakes and streams by surface runoff. Non-point source pollution is contamination that occurs when rainwater, snowmelt, or irrigation washes off plowed fields, city streets, or suburban backyards. As this runoff moves across the land surface, it picks up soil particles and pollutants, such as nutrients and pesticides.

Non self tapping screw

A screw that requires a tapping procedure in a predrilled guide hole before the screw can be inserted. See self-tapping screw, tap.

Nonsense mutation

A mutation that changes an amino acid codon to one of the three stop codons, resulting in a shorter and usually nonfunctional protein.

Nonspecific Binding

Nonspecific binding is the measurement of label or labeled conjugate present in the bound fraction of a ligand-binder reaction which occurred from nonimmune-specific means.Nonspecific binding is the only type of binding found in competitive binding immunoassay samples (NSB) having no binder present; and in sandwich binding immunoassay samples (Zero Blank) having no ligand present. Nonspecific binding samples as baseline standards can be used to represent infinitely high standard concentrations (competitive binding) or infinitely low standard concentrations (sandwich binding).

Noonan syndrome

The adolescent growth spurt is often blunted or delayed and 60 percent of males have undescended testes. In addition to delay in growth, sexual development is variable and may also be delayed. Occurs in about 1 in 1,000 to 1 in 2,500 live births. The actual genetic abnormality has not yet been discovered.


A hormone, produced by the medulla of the adrenal gland. The principal neurotransmitter in sympathetic nerves of the Autonomic Nervous System. It increases the concentration of glucose in the blood, raises blood pressure and heartbeat rate, and increases muscular power and resistance to fatigue; also one of the principal neurotransmitters; also called norepinephrine.

Noradrenaline spillover

A measure of the rate of net production of the neurotransmitter noradrenaline from an organ. We use this as an index of the release of noradrenaline.

Normal (in geometry)


Normal distribution

See Gaussian distribution

Normal Solution

An outdated term referring to the concentration of an acid or base. It equals molarity times the number of hydrogen ions or hydroxyl ions generated by each molecule of acid or base respectively. Molarity is the preferred term.

Normalized Response

The normalized response is the percentage of bound ligand-binder complex of the sample relative to a specified baseline standard, or standard, response within the assay run. Whereas the adjusted response measures the actual amount of ligand-binder binding which occurred in the sample, the normalized response measures the amount of binding relative to another sample within the assay run. This means that all of the binding variation between assay runs, which is due to binder deterioration, incubation differences, buffer changes, etc., are included in the adjusted response. These binding differences between assay runs are factored out of the normalized response. This allows the integrity of the sample itself to be examined without being distorted by differences in the binding conditions.

Normalized variance

The normalized variance is the variance of the mean of the replicate samples' untransformed normalized response.

Normalized coefficient of variance (CV)

The normalized CV is the coefficient of variation (standard deviation/mean) of the replicate samples' untransformed normalized response.

Norm of reaction

The range of phenotypic possibilities for a single genotype, as influenced by the environment.

Northern blot

A technique for analyzing mixtures of RNA, whereby the presence and rough size of one particular type of RNA (usually an mRNA) can be ascertained.
Technique involves separating and transferring mRNA from a gel to a filter in order to identify and locate mRNA sequences that are complementary to and hybridize with a labeled DNA probe. After Dr. E. M. Southern invented the Southern blot, it was adapted to RNA and named the "Northern" blot.

Norwalk virus

Virus that contaminates raw oysters/shellfish, water and ice, salads, frosting, person-to-person contact.


A longitudinal, flexible rod formed from dorsal mesoderm and located between the gut and the nerve cord in all chordate embryos.


Gram-negative rods


Abbreviation for non-specific binding. It is the proportion of radioligand binding which is not displaced by other drugs specific for the receptor. It may consist of binding to other receptor types, to other proteins in the membrane, partitioning into lipids, or many other things. It is determined experimentally using a saturating concentration of a drug which, ideally, is chemically dissimilar to the radioligand but is specific for the same receptor.


Sequence of nucleotides added in a template-independant fashion at the joining junctions of heavy-chain immunoglobulin genes.

Nuclear envelope (or Nuclear membrane)

The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm. The double membrane which surrounds the eukaryotic nucleus. It has many pores in its surface which regulate the flow of large compounds into and out of the nucleus.

Nuclear magnetic resonance (NMR)

The selective absorption of very high-frequency radio waves by certain atomic nuclei that are subjected to an appropriately strong stationary magnetic field. Nuclei in which at least one proton or one neutron is unpaired act like tiny magnets, and a strong magnetic field exerts a force that causes them to precess in somewhat the same way that the axes of spinning tops trace out cone-shaped surfaces while they precess in the Earth’s gravitational field. When the natural frequency of the precessing nuclear magnets corresponds to the frequency of a weak external radio wave striking the material, energy is absorbed from the radio wave. This selective absorption, called resonance, may be produced either by tuning the natural frequency of the nuclear magnets to that of a weak radio wave of fixed frequency or by tuning the frequency of the weak radio wave to that of the nuclear magnets (determined by the strong constant external magnetic field).

Nuclear run-on

A method used to estimate the relative rate of transcription of a given gene, as opposed to the steady-state level of the mRNA transcript (which is influenced not just by transcription rates, but by the stability of the RNA).This technique is based on the assumption that a highly-transcribed gene should have more molecules of RNA polymerase bound to it than will the same gene in a less-active state. If properly prepared, isolated nuclei will continue to transcribe genes and incorporate 32P (radio-labelled phosphorus) into RNA, but only in those transcripts that were in progress at the time the nuclei were isolated. Once the polymerase molecules complete the transcript they have in progress, they should not be able to re-initiate transcription. If that is true, then the amount of radiolabel incorporated into a specific type of mRNA is theoretically proportional to the number of RNA polymerase complexes present on that gene at the time of isolation. A very difficult technique, rarely applied appropriately.


An enzyme which degrades nucleic acids. A nuclease can be DNA-specific (a DNase), RNA-specific (RNase) or non-specific. It may act only on single stranded nucleic acids, or only on double-stranded nucleic acids, or it may be non-specific with respect to strandedness. A nuclease may degrade only from an end (an exonuclease), or may be able to start in the middle of a strand (an endonuclease). To further complicate matters, many enzymes have multiple functions; for example, Bal31 has a 3'-exonuclease activity on double-stranded DNA, and an endonuclease activity specific for single-stranded DNA or RNA.


Having a nucleus.

Nucleic acid

The collective name for DNA and RNA molecules found in every cell: A large molecule composed of nucleotide subunits.
A polymer consisting of many nucleotide monomers; the two types are DNA and RNA.
The genetic component of cells (DNA, RNA polymers), but also important for cellular energy metabolism, signaling, and protein biosynthesis (RNA, single nucleotides). A biological molecule composed of a long chain of nucleotides. DNA is made of thousands of four different nucleotides repeated randomly.
It serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities.

Nucleic acid probe

In DNA technology, a labeled single-stranded nucleic acid molecule used to tag a specific nucleotide sequence in a nucleic acid sample. Molecules of the probe hydrogen-bond to the complementary sequence wherever it occurs; radioactive or other labeling of the probe allows its location to be detected.


A dense region of DNA in a prokaryotic cell.

Nucleoid region

The region in a prokaryotic cell consisting of a concentrated mass of DNA.
Unlike a nucleus, it is not bound by a membrane.


A specialized structure in the nucleus, formed from various chromosomes and active in the synthesis of ribosomes. An optically dense region (or regions) in a nucleus, associated with RNA synthesis. Not always visible.


An organic molecule consisting of a nitrogenous base joined to a five-carbon sugar.


The basic, beadlike unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.


The building block of a nucleic acid, consisting of a five-carbon sugar covently bonded to a nitrogenous base and a phosphate group;   A subunit of DNA or RNA consisting of a nitrogenous base (a purine or pyrimidine), a phosphate molecule, and a sugar molecule. Thousands of nucleotides are linked to form a DNA or RNA molecule.
A nucleotide, the basic building block of nucleic acids, is a monomeric molecule of DNA or RNA composed of a pentose sugar (with 5-carbons such as deoxyribose in DNA, ribose in RNA), an organic nitrogenous base, and a phosophate group. DNA consists of the four bases: adenine (A), guanine (G), cytosine (C), and thymine (T); l RNA consists of the four bases: adenine (A), guanine (G), cytosine (C), with the substitution of uracil (U) for T.


1. An atom's central core, containing protons and neutrons.
2. A cluster of neurons (principally a cluster of neuron cell bodies) in some part of the brain.
3. The chromosome-containing organelle of a eukaryotic cell.
Membrane-bound organelle which contains the DNA in the form of chromosomes. It is the site of DNA replication, and the site of RNA synthesis. residual extra-nuclear genes are found in mitochondria and chloroplasts). A double membrane envelope separates the genetic material (chromosomes) from the cytoplasmic compartment. Nuclear pores allow for the exchange of proteins and RNA, but not chromosomes, during gene regulation and expression. The nuclear compartment separates the processes of transcription (messenger RNA formation) from translation (protein biosynthesis).


A nuclide is the nucleus of an atom having a specific atomic number and atomic mass number. A nuclide may be radioactive.


A type of unsupported pseudopodium without evident extrusomes.

Null hypothesis

In statistical analysis, a hypothesis proposing that there is no statistically significant difference between the observed results of an experiment and the expected results.

Null set

Same as empty set.  A set with no elements.

Number line

A line on which every point represents a real number.


A symbol that stands for a number.


The top number in a fraction.
a is the numerator  in


Chemicals that have beneficial effects (pharmacological effects) on our physiology if taken in appropriate amounts with food. Plants are the major source of nutraceuticals also known as phytochemicals. Plants produce those molecules for self-defense or to attract insects and animals to facilitate pollen distribution. For the latter purpose, they often are colorants while tasting bitter to ward of animals.


Molecules that can be used by cells or living organism to extract energy through metabolic processes. Although nutrients are often sought off only as energy providers, they can also be used as molecular building block for the biosynthesis of cellular structures.

Nutrient density

Nutrient dense foods are those that provide substantial amounts of vitamins and minerals and relatively fewer calories. The opposite of nutrient dense is calorie dense which are foods that mainly supply calories and relatively few nutrients.


Congenital night blindness caused by a lack of rod functioning in the retina


In aquatic insects, the larval stage.


Uncontrolled, involuntary rhythmic horizontal, vertical, or rotary rapid eye movements which may be congenital or acquired. Usually occurs in response to movement of the head.