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A bacteriophage which infects certain strains of E. coli .
The salient feature of this phage is that it packages only a single strand of DNA into its capsid. If the investigator has inserted some heterologous DNA into the M13 genome, copious quantities of single-stranded DNA can subsequently be isolated from the phage capsids. M13 is often used to generate templates for DNA sequencing.


An abnormally large head.


Evolutionary change on a grand scale, encompassing the origin of novel designs, evolutionary trends, adaptive radiation, and mass extinction; Large-scale evolution, entailing major changes in biological traits.
Contrast against Microevolution.


Animals which are smaller than 2cm but are retained on a sieve of mesh size 250mm.
Positioned in size between the meiofauna and megafauna. Dominated by polychaetes

Macula lutea

an oval yellow spot at the center of the retina, near the optic nerve; aroundfovea (blind spot); region of retina richest in photoreceptors; contains a pit,fovea centralis, and has no blood vessels; central vision occurs when an image is focused directly on the fovea centralis.


A giant molecule of living matter formed by the joining of smaller molecules, usually by condensation synthesis.
Polysaccharides, proteins, and nucleic acids are macromolecules.


One of two types of nuclei found in ciliates (the other being the micronucleus).
Typically the larger of the two, may be rounded, like a long sausage, or like a string of beads. Involved in production of proteins but not in sexual reproduction. Essential for the day to day activities of the ciliate


Inorganic nutrients that the body needs in relatively large amounts; Also called caloric nutrients including proteins, carbohydrates, and fats and some definitions include water and alcohol.
In animals the major macronutrients are protein, carbohydrate, fat, and water. In plants the major macronutrients are nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur. Note here that “macro” does not refer to the size of the nutrient but the fact that it is needed in relatively large amounts. The term is used by gardeners and in agriculture referring to common minerals needed for proper plant growth.
Contrast against Micronutrient.


A type of white blood cell that ingests dead tissue and cells and is involved in producing Interleukin 1; A phagocytic cell that is the counterpart of the monocyte.
A monocyte which has left the bloodstream and has moved into the tissues. Macrophages have basically the same functions as monocytes, but they carry these out in the tissues. In summary, they engulf and kill microorganisms, present antigen to the lymphocytes, kill certain tumor cells, and their secretions (e.g., leukotrienes) regulate inflammation. Macrophages utilize nitric oxide and hydrogen peroxide (which they synthesize) to kill the microorganisms they engulf (via oxidation), and the nitric oxide also helps to regulate the immune system.

In the spleen, macrophages engulf and destroy old red blood cells. When they reside in the bone marrow they store iron and then transfer it to red blood cells. In the lungs and GI tract they are scavengers and keep tissues clean. They also serve as a reservoir for the AIDS virus. They (and other phagocytic cells) are largely responsible for the localization and degradation of foreign materials at inflammatory sites.

Macrophages display chemotaxis (i.e., the sensing of, and movement toward or away from a specific chemical). For example, consumption (in food/feed) of mannanoligosaccharides by mammals causes macrophages (within that mammal's bloodstream) to depart from the bloodstream and move toward the gastrointestinal tract (tissues) where those macrophages eliminate some pathogens (i.e., those growing/reproducing in the gastrointestinal tract).

Macrorestriction map

Map depicting the order of and distance between sites at which restriction enzymes cleave chromosomes.


Used in science to describe large scale processes like the temperature, volume, pressure,and energy of a system characterizing the behavior of a very large number of molecules.
The macroscopic values tend to be predictable and represent the average behavior of a system. They give no detailed information about the behavior of individual molecules or units of a system.


Molten rock below the Earth’s surface.


A body which produces a magnetic field. 
All magnets are di-pole and follow the rule that like poles repel and unlike poles attract.

Magnetic flux density

Force that an electromagnetic source exerts on charged particles.
Magnetic flux density is measured in Telsa (1 Telsa =104 gauss).

Magnetic resonance imaging (MRI)

Diagnostic technique which uses a magnetic field and radio waves to provide computerized images of internal soft body tissues such as central nervous system and musculoskeletal systems. Also called MRI, NMRI, and nuclear magnetic resonance imaging.
A hazard-free, non-invasive way to generate visual images of thin slices of the body by measuring the nuclear magnetic moments of ordinary hydrogen nuclei in the body’s water and lipids. The patient's body is placed in a strong magnetic field. Radio pulses affect the resonance or "spin" of atoms in the tissues. A computer analyzes this information to show subtle differences in tissue molecular structure producing very high resolution and better differentiation of soft tissue, such as a tumor within the liver.
Magnetic resonance imaging makes better images of organs and soft tissue than other scanning techniques, such as computed tomography (CT) or x-ray. The technique is especially useful for imaging the brain, the spine, the soft tissue of joints, and the inside of bones. The technique presents no known health hazards, but it cannot be used on people who have cardiac pacemakers or certain other metal-containing devices implanted in their bodies.


The force that electric currents exert on other electric currents


The magnitude of a vector is its length.

Major arc

An arc of measure greater than 180 degrees.

Major axis

The line segment connecting the two vertices that are farthest apart in an ellipse.

Major depressive episode

A mood disorder with a depressed mood that may accompany a manic episode.

Major histocompatibility complex (MHC)

A large set of cell surface antigens encoded by a family of genes.
A genetic loci, or chromosomal region (approximately 3,000 Kb) which encodes for three classes of transmembrane (cell) proteins. A set of molecules displayed on cell surfaces that are responsible for lymphocyte recognition and "antigen presentation".
The Class I and Class II MHC molecules belong to a group of molecules known as the Immunoglobulin Supergene Family, which includes immunoglobulins, T-cell receptors, CD4, CD8, and others.

The MHC is encoded by several genes located on human chromosome 6.The MHC molecules control the immune response through recognition of "self" and "non-self" and, consequently, serve as targets in transplantation rejection. It is important in regulating the activity of all immune responses involving T-lymphocytes, as the T-lymphocyte receptor requires the presentation of foreign antigen with a class of MHC molecule. Foreign MHC markers trigger T-cell responses that may lead to the rejection of transplanted tissues and organs.

All cells express MHC class I but only a few specialised cells express MHC class II. These latter cells are termed antigen presenting cells and are important in controlling the extent to which an immunological reaction can be mounted. In the CNS, MHC class II expression is low and probably restricted to microglial cells.

MHC I proteins (located on the surface of nearly all cells) present foreign epitopes (i.e., fragments of antigens that have been ingested; peptides) to cytotoxic T cells (killer T cells).
MHC II proteins (located on the surface of immune system lymphocyte/dendritic cells and phagocytes) present foreign epitopes to helper T cells. That presenting of epitopes induces the organism's immune response.
MHC III proteins are components of the complement cascade. Genes in the MHC must be matched (between an organ donor and organ recipient) to prevent rejection of organ transplants.


To pretend inability so as to avoid duty or work.


Refers to an abnormal fit of the upper and lower dental structures.

Malphigian tubule

A unique excretory organ of insects that empties into the digestive tract, removes nitrogenous wastes from the blood, and functions in osmoregulation.


A type of bipolar disorder that is characterized by excitement, euphoria, expansive or irritable mood, hyperactivity, pressured speech, flight of ideas, decreased need for sleep, distractibility and impaired judgement. Delusions consistent with elation and grandiosity may be present.

Mannose-binding lectin

A soluble protein that acts as a key component of the innate immune system.
MBL acts by recognizing the presence of infectious pathogens in the body at an early stage and initiates steps to eliminate them

Manually coded English

Translation of the English language into signs.
Generally, the vocabulary of American Sign Language (ASL) is used. ASL signs are supplemented with other signs and fingerspelling to correspond directly to English words and to maintain English grammar and syntax.

Mann-Whitney Test

The Mann-Whitney test is a nonparametric statistical test which computes the probability that the rank values of two sets of a single parameter are members of the same population.
The ranking of the original parameter data is lowest to highest. The Mann-Whitney test is the nonparametric equivalent of the t test and is used when the data are not sufficiently gaussian in distribution or when the variances of the two groups are too unequal.

Map unit

A unit for distance in a linkage map. The distance equal to 1% recombination between two loci.

Marfan syndrome

A genetic disorder with an incidence rate of 1 in 5,000 individuals that affects the body's connective tissues, or the tissues in between the main cells of each organ of the body.
A person with Marfan syndrome will usually be tall, slender and somewhat loose jointed or limber. The arms, legs, fingers and toes may be disproportionately long when compared to the trunk.

Margin of error

A measurement of the accuracy of the results of a survey.
A margin of error of plus or minus 3.5% means that there is a 95% chance that the responses of the target population as a whole would fall somewhere between 3.5% more or 3.5% less than the responses of the sample (a 7% spread). However, for any specific question, the margin of error could be greater or less than plus or minus 3.5%.


Pertaining to the sea.

Marker (in DNA)

An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored.
Markers can be expressed-regions of DNA (genes) or some segment of DNA with no known coding function but whose pattern of inheritance can be determined.


The blocking of an object or stimulus or sensation by another object or stimulus or sensation.
Masking as in hiding or wearing a mask to hide something.


The amount of matter in an object.
Note that mass and weight are not the same thing.  Weight is the force on an object due to the gravitational pull of a planet or other heavenly body.  Mass on the other hand, remains constant, no matter where it is.

Mass number

The sum of the number of protons and neutrons in an atom's nucleus.

Mass spectrometry

A method of determining the chemical composition of a substance based on measuring the mass-to-charge ratio of ions.
The atoms or molecules in a substance are ionised by a beam of high-energy electrons. The ions are focussed by electrostatic and magnetic fields to give a spectrum, which can then be analysed. It can be used to identify unknown compounds, determine the isotopic composition of elements in a compound, and quantify the amount of a compound in a sample.

Mast Cells

Fixed (noncirculating) leukocyte cells that are present in many different kinds of body tissues.
When two IgE molecules of the same antibody "dock" at adjacent receptor sites on a mast cell, then (the two IgE molecules) capture an allergen (e.g., a particle of pollen) between them, a chemical-energetic signal is sent to the interior (inside mast cell) portion of receptor molecules, which causes that interior portion of molecule to change (i.e., transduction). That signal transduction causes a protein named "syk" to set off a chemical chain reaction inside the mast cell; thereby causing that mast cell to release leukotrienes, histamine, serotonin, bradykinin, and "slow reacting substance." Release of these chemicals into the body causes the blood vessels to become more permeable (leaky) and causes the nose to run, itchy and watery eyes. These chemicals also cause smooth muscle contraction, causing sneezing, breath constriction coughing, wheezing, etc.


Surgery to remove the breast (or as much of the breast tissue as possible).

Maternal rubella German measles

German measles contracted by a woman during the early stages of pregnancy. Produces a high probability of severe handicaps in
offsprings, including mental retardation, cardiac abnormalities, cerebral palsy, and sensory (hearing and vision) handicaps. It is a highly preventable disease that can be eradicated by vaccinating children under 12 years of age.


Projection of a cell that supports one or more flagella.


1. In biology: The nonliving component of connective tissue, consisting of a web of fibers embedded in homogeneous ground substance that may be liquid, jellylike, or solid.
2. In mathematics: A table of numbers arranged in rows and columns.
3. In immunoassays: The matrix of an immunoassay reaction is everything in the incubation medium except the ligand and the binder. Many of these matrix components influence the kinetics of the ligand-binder reaction.


Anything that takes up space and has mass.


The points on a curve where the value is greater than that of the surrounding points.

maximum contaminant level (MCL)

the designation given by the U.S. Environmental Protection Agency (EPA) to water-quality standards promulgated under the Safe Drinking Water Act. The MCL is the greatest amount of a contaminant that can be present in drinking water without causing a risk to human health.

McCune Albright Syndrome

A genetic condition affecting bone health, puberty and thyroid.
Characteristics may include early puberty, bone fracture, and café au lait spots (irregular brown spots on the skin). Evaluation by endocrinology to determine medications required to optimize bone health and monitoring of pubertal advancement.


Mean is the arithmetic average of a set of data and is a measurement of the central tendency of that data.
Also known as Average.


A sensory receptor that detects physical deformations in the body's environment.
Mechanoreceptors are associated with the sensations of pressure, touch, stretch, and motion, as well as of sound (sound waves are pressure waves that cause the displacement of specialised structures associated with sepcialised receptor cells which therefore are classed as mechanoreceptors).


The greenish fecal matter that forms the first bowel movement of a fetus.

Meconium aspiration

A situation where meconium filled amniotic fluid is breathed into the infant's lungs during the birth process. There may or may not be subsequent anoxia resulting in brain damage.
Meconium aspiration is rare in babies born at less than 34 weeks gestation, and may be as high as 40 percent in post term babies.


The number in a group of numbers such that there are an equal number of numbers in the set greater than the number as are less than the number.


Visual examination of the mediastinal structures including the heart, trachea, esophagus, bronchus, thymus, and lymph nodes


The inner, as opposed to the outer, part of an organ, as in the adrenal gland.
The “medulla oblongata” is a part of the brainstem. More generally though, the term “medulla” is applied to the inner part of an organ – thus for example, the kidney is described as having an outer “cortex” and an inner “medulla”.

Medulla oblongata

The lowest part of the vertebrate brain and located just above the spinal cord ; a swelling of the hindbrain dorsal to the anterior spinal cord.
The medulla oblongata is considered to control many vegetative or autonomic, homeostatic functions needed to maintain life, such as including breathing, heart and blood vessel activity, swallowing, digestion, and vomiting.


Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 centiMorgan (cM).


Defined as animals larger than 2cm.
These animals can be observed from photographs. The largest size class of animals. Comprised of errant and sessile organisms.


A condition in which the head is enlarged and the brain is enlarged and abnormal.
Usually associated with mental retardation.

Megapascal (MPa)

A unit of pressure equivalent to 10 atmospheres of pressure.


Animals that are intermediate in size between macrofauna and microfauna. Operationally defined as organisms which pass through a sieve of mesh size 250mm but are retained on a mesh size of 32mm.
Dominated by the foraminifera, copepoda and nematode.


A two-stage type of cell division in sexually reproducing organisms that results in gametes with half the chromosome number of the original cell; thus a process that allows one diploid celll to divide in a special way to generate haploid cells in eukaryotes.
Consists of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes (half the original chromosome content). For this reason, meiosis is often called a "reduction division". In organisms with a diploid life cycles, the products of meiosis are usually called gametes. In organisms with an alternation of generations, the products of meiosis are caled spores).
“Meiosis” comes from the Greek word “meioun” which means “to make smaller”.

Melanocyte stimulating hormone (MSH)

Peptides produced by the brain which are thought to stimulate reproductive hormones when body weight is low
There are three types of MSH: alpha- beta- and gamma-. These peptides are also anorexigenic, i.e., they inhibit food intake.


Semi-fluid structure which bounds all cells, and partitions the interior of eukaryotic cells; A planar structure surrounding cells and organelles within eukaryotic cells (e.g. membranes of cell nucleus of mitochondrion) separating aqueous compartments which carry out different metabolic processes.
It consists primarily of two lipid layers, with proteins "dissolved" in the lipids. Cell membranes are electrical insulators but permeable to hydrophobic molecules such as steroidal hormones and small gases (carbon dioxide, molecular oxygen, nitric oxide). All other water soluble and charged molecules depend on the presence of membrane proteins which provide transport pathways across the phospholipid bilayer.


A compound structure comprised of many cilia and associated with the mouth of a ciliate.
Either present in groups of three (oligohymenophora) or as a band of many more (polyhymenophora).

Membrane potential

The charge difference between the cytoplasm and extracellular fluid in all cells, due to the differential distribution of ions.
See Resting Membrane Potential.
Note that the membrane potential affects the activity of excitable cells and controls the transmembrane movement of all charged substances.

Memory, Episodic

Memory for ongoing events in a person's life.
More easily impaired than semantic memory, perhaps because rehearsal or repetition tends to be minimal.

Memory, Immediate

The ability to recall numbers, pictures, or words immediately following presentation.
Patients with immediate memory problems have difficulty learning new tasks because they cannot remember instructions. Relies upon concentration and attention.

Memory, Long Term

In neuropsychological testing, this refers to recall thirty minutes or longer after presentation.
Requires storage and retrieval of information which exceeds the limit of short term memory.

Memory, Short Term

Primary or 'working' memory; its contents are in conscious awareness.
A limited capacity system that holds up to seven chunks of information over periods of 30 seconds to several minutes, depending upon the person's attention to the task.

Memory cell

A clone of long-lived lymphocytes, formed during the primary immune response, that remains in a lymph node until activated by exposure to the same antigen that triggered its formation.
Activated memory cells are responsible for producing the secondary immune response.

Meninges (Singular = menynx)

Three connective tissue membranes covering the brain and the spinal cord.
The three layers of membranes are called (from out to in): Dura MaterArachnoid Mater, and Pia Mater.


An inflammation of the meninges.


One type of spina bifida in which the meninges around the spinal cord, has pushed out through the opening in the vertebrae in a sac called the "meningocele". However, the spinal cord remains intact.
This form of spina bifida can occur with little or no damage to the nerve pathways.
(see also Meningomyelocele and Myelomeningocele)


Hernial protrusion of part of the meninges and substance of the spinal cord through a defect in the vertebral column.
(see also Meningocele and Myelomeningocele)


The curved top surface of a column of liquid.

Menstrual cycle

A type of reproductive cycle in higher female primates, in which the nonpregnant endometrium is shed as a bloody discharge through the cervix into the vagina.
Comes from the Latin word “mensis” meaning “monthly” in keeping with the fact that the menstrual cycle in humans is about 28 days long (a widely-quoted average, but with a wide distribution of values, even in regularly-menstruating healthy young women).

Mental disorder

In the Diagnostic and Statistical Manual of the American Psychological Association, a mental disorder is conceptualized as a clinically significant behavioral or psychologic syndrome or pattern that occurs in an individual and that typically is associated with either a painful symptom (distress) or impairment in one or more important areas of functioning (disability). There is also an inference of a behavioral, psychological or biological dysfunction, and of disturbance beyond the relationship between the individual and society.
A disturbance limited to a conflict between an individual and society may represent social deviance, which may or may not be commendable, but it is not by itself a mental disorder.

Mental status examination

The process of estimating psychological and behavioral function by observing the patient, eliciting his description of self and formally questioning him.
Included in the examination are: (1) evaluation and assessment of any psychiatric condition, including provisional diagnosis and prognosis and determination of degree of impairment, suitability for treatment and indications for particular types of therapeutic intervention; (2) formulation of the personality structure of the subject, which may suggest the historical and developmental antecedents of whatever psychiatric condition exists; (3) estimation of the ability and willingness to participate appropriately in treatment. The mental status is reported in a series of narrative statements describing such things as affect, speech, thought content, perception and cognitive functions. The mental status examination is part of the general examination of all patients, although it may be greatly abbreviated in the absence of psychopathology.

Meroblastic cleavage

A type of cleavage in which there is incomplete division of yolk-rich egg, characteristic of avian development.


Temporary members of the plankton, usually during their early life stages, but not as adults.

Mesenchymal stem cells

Cells from the immature embryonic connective tissue.
A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce cartilage.


A membrane that suspends many of the organs of vertebrates inside fluid-filled body cavities.


The middle primary germ layer of a group of cells derived from the inner cell mass of the blastocyst.
It gives rise to bone, muscle, connective tissue, the notochord, the lining of the coelom, skeleton, gonads, kidneys, most of the circulatory system, and related structures.


Nuclear condition unique to the dinoflagellates in which the chromosomes remain permanently condensed.

Mesokurtic Distribution

A mesokurtic distribution is a gaussian distribution having no kurtosis (peakedness).


Two quarkhadrons, the product of radioactive decay.


Microorganisms that grow best at moderate temperatures, with optimum growth at 77°-113°F (25°-45°C).


A terminal cancer of the cell membrane surrounding the lungs, abdomen, and heart.


A membrane of cells that covers several internal organs and body cavities that secretes a lubricating fluid that aids in proper movement.

Messenger RNA (mRNA)

A type of RNA synthesized from DNA in the genetic material that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein. Thus mRNA contains sequences coding for a protein and serves as a template for protein synthesis by.
The term mRNA is used only for a mature transcript with polyA tail and with all introns removed, rather than the primary transcript in the nucleus. As such, an mRNA will have a 5' untranslated region, a coding region, a 3' untranslated region and (almost always) a poly(A) tail. Typically about 2% of the total cellular RNA is mRNA. See also RNAtRNA.

Metabolic disorder

A condition or disease related to dysfunction in the chemical processes and activities of the body.

Metabolic Syndrome

A cluster of metabolic risk factors that increase the chances of developing heart disease stroke.

Metabolisable energy

Quantity of calories provided by nutrients absorbed in the intestine, less the losses associated with faecal and urine evacuation.


The totality of an organism's chemical processes, consisting of catabolic and anabolic pathways.
Metabolism is the chemistry of energy extraction from nutrients and the biosynthesis of the building blocks of life (amino acids, sugars, lipids). A process by which absorbed nutrients are converted to chemical energy and building blocks for vital processes or cellular structures. Metabolism regulates the rate at which you burn Calories.

Metabotropic receptors for transmitters

A receptor in cells that is linked to G-proteins and does not contain an ion channel.
Binding of the transmitter to the binding site results in a change in the shape of the receptor that allows the binding and subsequent activation of a G protein. There is a diversity of G proteins so that a variety of cellular end effects may be produced, including effects exerted by 2nd messengers which are chemical agents activated by the G proteins.


Elements characterised by their opacity, malleability and thermal and electrical conductivity.


The resurgence of development in an animal larva that transforms it into a sexually mature adult.
A process of developmental change whereby a larva reaches adulthood only after a drastic change in morphology; occurs in most amphibians and insects, for some insects, this change may include another stage (pupa) before the adult stage


The second stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.
During metaphase, all the cell's duplicated chromosomes are lined up at an imaginary plane equidistant between the poles of the mitotic spindle.


A subdivided population of a single species.


To spread from one part of the body to another; When a cancerous growth invades a healthy organ or tissue from a diseased organ or tissue .
Metastasis is the spread of cancer beyond their original site. When cancer cells metastasize and form secondary tumors, the cells in the metastatic tumor are like those in the original (primary) tumor.


Multicelled animals.


The science and study of the physics, chemistry, and dynamics of the Earth's atmosphere, especially weather and climate.


Essential amino acid; furnishes (to organism) both labile methyl groups and sulfur necessary for normal metabolism.

Method Controls (MC)

The method controls are standard curve parameters that measure, directly or indirectly, the actual analytical methodology itself.
The method (standard curve) controls consist of the adjusted and normalized standard responses, standard curve coefficients, monitor points (min/max detectable concentrations, ED 20/50/80, etc.), dilution curve measurements, and replicate precision probabilities. From the method control parameters, assessments can be made about the performance of the label material, the tracer, the binder, the standards, the buffer matrix, the incubation conditions, and the separation of bound and free ligand. The individual method control parameters are listed in the statistical quality control tables from an assay report. All method control probabilities are weighted equally in the probability index calculation


Mostly isolated from water and leaf surface microflora, and are facultative methylotrophs, that is capable of growing on one-carbon compounds such as formate, formaldehyde, and methanol as the sole source of carbon and energy, as well as on a wide range of multicarbon substrates.


A drug that acts as an antimetabolite and specifically as a folic acid antagonist that inhibits the synthesis of DNA , RNA , and protein .

Michaelis-Menten equation/kinetics

Describes the kinetics of many enzymes, provided the concentration of enzyme is much less than the concentration of substrate (i.e., enzyme concentration is the limiting factor), and when the enzyme is not allosteric.
The equation is:

P = product
S = substrate
Vmax = maximum velocity of the enzyme
Km = Michaelis-Menten constant
EO = Free enzyme in solution (not bound to substrate, which is expressed as ES)
[ ] = concentration

The modern relationship between substrate and enzyme concentration was proposed in 1903 by Victor Henri. A microscopic interpretation was thereafter proposed in 1913 by Leonor Michaelis and Maud Menten, following earlier work by A V Hill. It postulated that enzyme (catalyst) and substrate (reactant) are in fast equilibrium with their complex, which then dissociates to yield product and free enzyme.
The current derivation, based on the quasi steady state approximation, that the concentration(s) of intermediate complex(es) do(es) not change, has been proposed by Briggs and Haldane.

To determine the maximum rate of an enzyme mediated reaction, the substrate concentration ([S]) is increased until a constant rate of product formation is achieved. This is the maximum velocity (Vmax) of the enzyme. In this state, enzyme active sites are saturated with substrate. Note that at the maximum velocity, the other factors that affect the rate of reaction (i.e., pH, temperature, etc.) are at optimal values.

The speed V means the number of reactions per second that are catalyzed by an enzyme. With increasing substrate concentration [S], the enzyme is asymptotically approaching its maximum speed Vmax, but never actually reaching it. Because of that, no [S] for Vmax can be given. Instead, the characteristic value for the enzyme is defined by the substrate concentration at its half-maximum speed (Vmax/2). This KM value is also called the Michaelis-Menten constant.

Since Vmax cannot be reached at any substrate concentration (because of its asymptotic behaviour, V keeps growing at any [S], albeit ever more slowly), enzymes are usually characterized by the substrate concentration at which the rate of reaction is half its maximum. This substrate concentration is called the Michaelis-Menten constant (KM) a.k.a. Michaelis constant.

For enzymatic reactions which exhibit simple Michaelis-Menten kinetics and in which product formation is the rate-limiting step (i.e., when k2 << k-1) KM≈k-1/k1=Kd, where Kd is the dissociation constant (affinity for substrate) of the enzyme-substrate (ES) complex. However, often k2 >> k-1, or k2 and k-1 are comparable, in which case nothing can be said about the enzyme affinity from the Michaelis constant alone.

The equation may be analyzed experimentally with a Lineweaver-Burk plot or a Hanes-Woolf plot.

Microaerophilllic environment

Environment with reduced oxygen concentrations, often below 5%. Carbon dioxide levels may approach 10%.


An experimental technique that is used to study the presence or activity of a full set of genes or proteins found in a cell or organism.
When studying the presence of genes, the microarray (DNA microarray) is used for genotyping, i.e., assessing the full genetic complement of an organism compared to the genetic complement of a closely related organism.


Gram-positive rod is found in dairy products, sewage, and insects


A microscopic organism.
Hence:Microbial = pertaining to microbes.

Microbial load

Total number of living microorganisms in a given volume or mass of microbiological media or food


Gram-positive cocci occur primarily on mammalian skin and in soil, but are commonly isolated from foodproducts and the air


A condition in which the head and brain are significantly smaller than normal for age and sex (head circumference less than the 5th percentile for age).
May be associated with mental retardation.


Evolutionary changes on the small scale, such as changes in gene frequencies within a population over a succession of generations.
Contrast against Macroevolution.


Organisms invisible to the naked eye, smaller than meiofauna. Operationally defined as organisms which pass through a sieve of mesh size 32mm


A solid rod of actin protein in the cytoplasm of almost all eukaryotic cells, making up part of the cytoskeleton.
They act alone or with myosin to cause cell contraction. They are integral to giving and maintaining the structure of the cell and, to a lesser extent, in transport of substances down the cell.


The bacterial colonies found in the large intestine.
These bacteria are important for proper digestion and fecal excretion of waste products.


One of two types of nuclei found in ciliates (the other being the maronucleus).
Dividing to produce two similar nuclei during asexual reproduction and producing nuclei with half the complement of DNA for sexual activity. Usuaully the smaller of the two types of nuclei, but many may be present. some cells lack a micronucleus and survive quite well without it.


Inorganic nutrients that the body needs in small amounts.
Vitamins and minerals are micronutrients in animals and iron, chlorine, copper, manganese, zinc, molybdenum, and boron are micronutrients for plant growth. Note here that “micro” does not refer to the size of the nutrient but the fact that it is needed in relatively small amounts.
Contrast against Macronutrient.


Usually single-celled creature that is too small to be seen without the aid of a microscope. The term may be applied to bacteria, archaea, and viruses, as well as certain fungi and protists.
Independent organisms of microscopic size, including bacteria, archaea (yeast and mold) and microscopic eukaryotes. When alive in a suitable environment, they grow rapidly and may divide or reproduce every 10 to 30 minutes. Therefore, they reach high populations very quickly. Undesirable microorganisms cause disease and food spoilage. Microorganisms are sometimes intentionally added to ferment foods, make antibiotics and for other reasons.


An abnormally small eyeball.

MicroRNA (miRNA)

Small non-coding RNAs that modulate diverse biological functions by post-transcriptional repression or degradation of target mRNAs.
They regulate mRNA transcribed by up to two-thirds of human genes. many miRNAs lie within introns of protein-coding genes and are commonly transcribed by RNA polymerase II. A single variety of miRNA can regulate up to 200 target mRNAs.
MicroRNA function consists of the following steps. Initially, primary (pri) miRNA is generated in a folded hairpin stem loop structure. A 60-80 nucleotide (nt) stem loop is excised and processed to a 70 nt hairpin-like premiRNA in the nucleus by a protein complex called microprocessor containing the RNAseIII Drosha and DGCR8 protein. After export into the cytoplasm by exportin 5 and RanGTP, double stranded pre-miRNA is loaded into a protein complex containing Dicer (which removes the loop and produces a miRNA duplex) and TRBP. It is then cleaved into a mature miRNA guide strand and the passenger or miRNA* strand which is usually degraded. Mature miRNA is incorporated into the RNA-induced silencing complex, RISC, with its Argonaute (Ago) core proteins. Once loaded with the mature miRNA, the RISC complex has two functions. In case of perfect complementarity of the mature miRNA the target mRNA (typically within the 3’UTR) is cleaved and subsequently degraded. If complementarity with target mRNA is imperfect, the mRNA is not degraded but arrests at the initiation step of translation and therefore is not translated into protein (translational repression). The important region for complementarity is the “seed” region at the 5’ end of mature miRNA. RISC complexes and target mRNAs accumulate in cytoplasmic foci called processing bodies or P-bodies.
MiRNAs have been investigated in CNS function and neural development. They are critically involved in proper expression of a differentiated neuronal phenotype, but are largely dispensable for early neural fate decision. For example, MiR-134 promotes dendritic arborization in hippocampal neurons. Conversely, inhibition of miR-124 by the transcription factor, RE1, halts neuronal differentiation. MiRNAs have been linked to disorders like Rett syndrome, where the mutated MeCP2 gene is controlled by miR-132, and Parkinson’s Disease since they maintain dopaminergic neuron integrity. These observations suggest that miRNAs can be key elements in the modification of neuronal phenotype.


A microsatellite is a simple sequence repeat (SSR).
It might be a homopolymer ('...TTTTTTT...'), a dinucleotide repeat ('....CACACACACACACA.....'), trinucleotide repeat ('....AGTAGTAGTAGTAGT...') etc. Due to polymerase slip (a.k.a. polymerase chatter), during DNA replication there is a slight chance these repeat sequences may become altered; copies of the repeat unit can be created or removed. Consequently, the exact number of repeat units may differ between unrelated individuals. Considering all the known microsatellite markers, no two individuals are identical. This is the basis for forensic DNA identification and for testing of familial relationships (e.g. paternity testing).


The description of the behavior of individual molecules (at the molecular level).
The behavior of an individual molecule may be very different from other identical molecules. In systems with a very large number of identical molecules, these individual deviations are negligible and contribute to the noise (random fluctuation) of a property of this system. The decay of a radioactive material is a good example of such an internal fluctuation. While the decay of isotopes can be predicted for a large clump of radioactive material (e.g. its half-life time, which is a macroscopic property), the actual time point of decay of an individual isotope cannot be predicted with accuracy and is random. However, we can assign a probability that it will decay within a certain amount of time (similar arguments can be made for individuals who have risk factors indicating the chance of developing a disease like cancer or a heart attack).

Microscopic ordering principle

At constant temperature, an increase in pressure increases the degree of ordering of the molecules of a substance.


Type of filament in eukaryotic cells composed of units of the protein tubulin; Filaments about 25 nanometers in diameter found in ciliaflagella, and the cell cytoskeleton; a hollow rod of tubulin protein in the cytoplasm of all eukaryotic cells and in cilia, flagella, and the cytoskeleton.
Among other functions, it is the primary structural component of the eukaryotic flagellum. In cells they play a very important role in speeding up the transport of substances along the cell. Here they act like miniature “conveyor belts” so that transported molecules in special transport “vesicles” can be moved much more quickly along the “belt” than they would move down the cell by passive bulk flow or by diffusion. Individual microtubules cannot be seen by conventional light-microscopy, but aggregates of microtubules can.

Microtubule motor proteins

Proteins that bind to and move on microtubules.
The mechanism by which molecular motor proteins convert energy from ATP hydrolysis into mechanical force is not known. A problem related to their mechanism of function is the molecular basis of polarity of translocation along the microtubule: some kinesin motors move toward microtubule minus, instead of plus ends like Kinesin-1. The coupling of the ATPase cycle to force generation and the determinants of motor polarity are actively being investigated using biochemical, biophysical, and molecular approaches.

Microvillus (Plural = Microvilli)

Thin fingerlike protrusions from the surface of a cell, often used to increase absorptive capacity or to trap food particles.
One of many fine, fingerlike projections of the epithelial cells in the lumen of the small intestine.
The "collar" of choanoflagellates is actually composed of closely spaced microvilli
These projections increase the surface are of the small intestine cells, allowing for a greater area over which digestion can take place.


A point, M, on a line segment, AB, such that AM = MC.

Mild learning and behavior disorders

A generic classification of disorders involving academic and/or social-interpersonal performance deficits that generally become evident in a school-related setting and make it necessary for the individual to receive additional support services beyond those typically offered in a regular education setting.
However, it is assumed mildly disordered students remain in the regular education setting for the majority of the school day. The severity of the performance deficits for this population ranges from one to two standard deviations below the interindividual and/or intraindividual mean on the measure(s) being recorded.

milligram (mg)

One-thousandth of a gram.

Milligram per litre(mg/l)

a unit of the concentration of a constituent in water or wastewater. It represents 0.001 gram of a constituent in 1 liter of water. It is approximately equal to one part per million (PPM).

million gallons per day (Mgd)

a rate of flow of water equal to 133,680.56 cubic feet per day, or 1.5472 cubic feet per second, or 3.0689 acre-feet per day. A flow of one million gallons per day for one year equals 1,120 acre-feet (365 million gallons).


A phenomenon in which one species (the mimic) benefits by a superficial resemblance in appearance and/or behavior to another species, organism or object (the model).
A predator or species of prey may gain a significant advantage through mimicry.


Inorganic substances such as iron and calcium.
In nutrition, one of many chemical elements, other than carbon, hydrogen, oxygen, and nitrogen, that an organism requires for proper body functioning Many minerals are essential nutrients.


A corticosteroid hormone secreted by the adrenal cortex that regulates salt and water homeostasis.


The points on a curve where the value is less than that of the surrounding points.

Minimal brain dysfunction (MBD)

A medical term used to indicate a delay or
mild neurological disorder in the ability to perform sensory or motor functions appropriately.
Can be a result of brain injury, and is a common source of learning difficulties in the child with near-average intelligence.

Mining water use

water use during quarrying rocks and extracting minerals from the land.

Minor arc

An arc on a circle that is less than 180 degrees.

Minor axis

The shortest distance across an ellipse through the center.


The unit of measure of an angle that is 1/60 of a degree.

Minute Volume

The amount of air which moves in and out of the lungs over a period of one minute.
This is usually calculated by multiplying the tidal volume by the respiratory rate.
Tidal Volume x Respiratory Rate = Minute Volume

Minnesota Multiphasic Personality Inventory (MMPI or MMPI-2)

A personality assessment tool widely used in making psychological evaluations.
Normally given to persons 16-18 years of age and older.

Minor motor seizures

Seizures that have been identified as myoclonic (shocklike contractions in muscles or muscle groups), akinetic (sudden loss of muscle tone), and infantile spasms (jackknife seizures).

Missense mutation

The most common type of mutation involving a base-pair substitution within a gene that changes a codon, but the new codon makes sense in that it still codes for an amino acid.

Mitochondrion (Plural = Mitochondria)

Complex organelle found in most eukaryotic cells, mitochondria are the site of most of the energy production in most eukaryotes; Site of aerobic respiration (energy production that is dependent on oxygen) in eukaryotic cells.
Believed to be descended from free-living bacteria that established a symbiotic relationship with a primitive eukaryote. It is referred to as the powerhouse of the cell because it responsible for production of the “energy” molecule (ATP) in cells. The process of ATP production occurs through cellular “respiration”.

Mitochondrial disease

Mitochondrial disease is a recently recognised category of human disease with the first human case being described in 1962. Initially thought to be a rare group of neurologic disorders predominantly affecting children, it is now known that affected patients can present with a wide range of symptoms at any age from infancy to well into adulthood. The pathological hallmark of mitochondrial disease in muscle is known as the “ragged-red fibre”, but other morphological changes such as cytochrome c oxidase (COX) deficiency and “ragged-blue” fibres as identified by succinate dehydrogenase (SDH) staining can also be observed. Although any or all of these histological features may be found in the muscle of patients with mitochondrial disease, it is now known that not all patients with mitochondrial disease will have these abnormalities. This lack of ‘gold standard’ for the diagnosis of mitochondrial disease in addition to the heterogeneity in clinical manifestations has provided a diagnostic challenge to clinicians in this field of medicine. Whilst various diagnostic criteria have been proposed, none are entirely satisfactory and diagnosis rests on the synthesis of a complex array of clinical, pathological, biochemical and genetic data.
Genetic abnormalities in mitochondrial disease:
Mitochondria play a key role in cellular energy production, organ function and health. They produce energy via the respiratory chain, a series of multi-enzyme complexes that are encoded by both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA). Thus, mutations in both mtDNA and nDNA can cause mitochondrial disease. Given the presence of these pathogenic genetic mutations, mitochondrial disease can now be diagnosed by identifying genetic mutations in affected patients. Pathogenic mutations in mtDNA were first identified in 1988 and since then over 100 pathogenic mtDNA mutations have been described in the literature. A database of reported mtDNA mutations can be found at www.mitomap.org. In addition to mtDNA mutations, mutations in the nuclear genome (nDNA) have been reported to cause mitochondrial disease. Affected adults are more likely to have mutations in mtDNA, whereas affected children may have mutations in either genome. Mutations in POLG, Thymidine phosphorylase, ANT1 and TWINKLE are the commonest genes that cause mitochondrial disease (particularly CPEO phenotypes) in adultsCS.
Prevalence of mitochondrial DNA mutations in Australia:
Early epidemiologic studies estimated a minimum population prevalence of mitochondrial disease between 9.2-16.5 in 100 000. However, these prevalence estimates probably underestimated the true prevalence of the mitochondrial disease because they relied on diagnosed case referrals and calculations based on census data. Given that individuals with mitochondrial disease may not be accurately diagnosed (because they may have diverse or mild clinical manifestations), this approach is unreliable, as it assumes that all affected individuals will present for diagnosis and be accurately diagnosed.

Mitochondrial matrix

The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the Krebs cycle.


A process of nuclear division in eukaryotic cells by which a cell separates its duplicated genome into two identical halves.
It is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis conserves chromosome number by equally allocating replicated chromosomes to each of the daughter nuclei.
It is generally followed immediately by cytokinesis (cell division) which divides the cytoplasm and cell membrane. This results in two identical daughter cells with a roughly equal distribution of organelles and other cellular components.

Mitral prolapse, stenosis, regurgitation

Damage to the mitral valve, the major valve separating the upper and lower chambers on the left side of the heart.
Blood flows through four chambers in the heart separated by one-way valves. The mitral (or atrioventricular) valve separates the upper and lower chambers on the left side of the heart. The mitral valve is so-called because it is shaped like an upside down Bishop's hat, a miter. If the flaps of this valve tear away due to disease, the process is called prolapse, "a falling forward". This results in leakage and backward flow called "regurgitation". Sometimes a valve is abnormally narrow causing partial obstruction constricting flow. Stenosis means "a narrowing".


Used in reference to organisms which use a mixture of nutritional strategies, such as organisms with chloroplasts and carry out photosynthesis but are also able to feed by phagocytosis.

MMR (Measles, Mumps, Rubella vaccine)

A vaccine which protects children against the measles, mumps, and rubella diseases.
Recommended administration is at 15 months and 4-6 years of age.


The pathways through which an individual receives information and thereby learns.
The "modality concept" postulates that some individuals learn better through one modality than through another. For example, the child may receive data better through the visual modality (by seeing it) than through the auditory modality (by hearing it).


The number that occurs most frequently in a set of data.

Model Based Curve Fitting

A model based standard curve is a method of data reduction which combines all standard determinations to derive one mathematical equation to describe every point on the curve.
Model based methods do not make the assumption that each standard response mean is the true response of that standard. By combining all standard determinations, model based methods have more degrees of freedom and therefore a high level of statistical reliability. Model based methods allow each standard point to be weighted according to the reliability of the point at that region of the curve. Model based methods reduce the distortion caused by outliers. Model based standard curves can be compared statistically to the reference assay standard curves.

Moderate learning and behavior disorders

A generic classification of disorders involving intellectual, academic, and/or social-interpersonal performance deficits that range between two and three standard deviations below the interindividual and/or intraindividual mean on the measure(s) being recorded. These performance deficits are not limited to any given setting, but are typically evident in the broad spectrum of environmental settings.
Etiology of the problem(s) may be identified in some causes, but typically cannot be precisely pinpointed. Individuals with functional disorders at this level require substantially altered patterns of service and treatment and may need modified environmental accommodations.

Modern synthesis

A comprehensive theory of evolution emphasizing natural selection, gradualism, and populations as the fundamental units of evolutionary change; also called neo-Darwinism.


Composed of interchangeable parts. With respect to prostheses, femoral hip prostheses are most commonly modular in design. Different types of femoral implants have interchangeable heads, necks, and/or stems. See monoblock.


The absolute value of a complex number.

Moisture and Volatile Matter

Weight loss of a fat or food material after heating for a prescribed time under controlled conditions.
The weight loss is accounted for by the loss of water and other materials which escape in the vapor state.

Moisture/Protein Ratio (MPR)

Percent moisture of a product divided by the percent protein of a product.
Most often used in meat product analysis to determine product safety and shelf-stability.

Molar solution

A solution that contains 1 mole of solute in l liter of solution.
One mole is equal to the molecular weight (MW) of the solute in grams, and contains 6.024 × 1023 molecules (Avogadro's number). Thus, solutions of equal molarity have the same number of molecules in solution, even though their molecular weights may be different.


A common measure of solute concentration, referring to the number of moles of solute in 1 L of solution. A measure of the concentration of a solution. A 1M solution contains one mole of the substance per litre of solution.
No. of grams
Molarity = _________________________

Mol. wt. x volume (litres)


A rapidly growing, asexually reproducing fungus.


The number of grams of a substance that equals its molecular weight in daltons and contains Avogadro's number of molecules. The amount of a substance in grams which contains Avogadro's number (6.022 x 10(23)) of atoms or molecules of it.


The smallest particle into which a substance can be divided and still have the chemical identity of the original substance.
Two or more atoms held together by covalent bonds.

Molecular beacon

Term used to refer to specific oligonucleotides possessing a "hairpin loop" and bearing a fluorescent dye.of a second protein.
A "quencher dye" located on a nearby portion of the hairpin loop prevents fluorescence until the hairpin loop is opened-up.

Molecular beacons (sometimes called fluorogenic probes) are utilized [e.g., in high-throughput screening or high-throughput identification] to detect the presence of a desired "target" molecule. When the "target" (i.e., a molecule possessing the desired functional group or desired property) is present within a given sample being evaluated, the "hairpin loop" opens-up because a portion of it forms a stronger bond to the "target" (than to the rest of the loop); thereby allowing the fluorescent dye to emit light.

Molecular Biology

The science of studying the genetic composition and mechanism of living organisms at the molecular level.
It historically refers to the understanding and manipulation of genes (DNA). The molecular studies of all other organic molecules like proteins, fats, and carbohydrates is called biochemistry.

Molecular chaperone

A protein that aids in the folding of a second protein.
The chaperone prevents proteins from taking conformations that would be inactive.

Molecular formula

A type of molecular notation indicating only the quantity of the constituent atoms.
The number and types of atom in a molecule.  For example the molecular formula of methane is CH4, one atom of carbon and four atoms of hydrogen.

Molecular Operational Taxonomic Unit (MOTU)

taxonomic unit defined using the DNA of an organism rather than it’s morphology.
Ideally it is at the species level.

Molecular weight

The sum of the atomic weights of the constituent atoms in a molecule. The sum of the atomic weights of the atoms comprising a molecule; alternatively, the number of grams of a compound which contains a mole of it.


A group of atoms bonded together; A chemically unique aggregate of at least two atoms. 
It is the smallest part of a substance that retains the chemical properties of the whole. The atoms are linked to each other by chemical (covalent) bonds. All matter is made up of molecules. Free atoms are rarely found but are important in the form of salts or metal ions in water.


A rotating effect.


The speed or force of something that is moving.
The product of mass times velocity. Momentum is conserved in any system of particles.

Monadic tests

Evaluation of a sample without direct comparison to other samples.

Monitor Point

A monitor point is a mathematical determination of a physical property of an assay run which can be used as a gauge to measure the performance of some aspect of that assay run.Baseline monitor points such as %NSB/TA (the amount of nonspecific binding as a percentage of the total tracer activity) and %MB/TA (the maximum amount of binding as a percentage of the total tracer activity) provide fundamental information about two absolute binding conditions. Estimated doses such as ED20, ED50, and ED80 measure the location of the standard curve at specific percentages (20%, 50%, 80%, respectively) of the maximum binding obtained that assay run. By using a value normalized to an internal point, binding condition variations which affect the whole assay run are factored out of the standard curve measurements and individual performance at specified points on the standard curve can be monitored.


A single structure without interchangeable parts. See modular.

Monoclonal antibody

Monoclonal antibodies are immunoglobulins which arise from a single clone of B-lymphocyte cells; Highly specific, purified antibody that is derived from only one clone of cells and recognizes only one antigen.
A defensive protein produced by cells descended from a single cell; an antibody that is secreted by a clone of cells and, consequently, is specific for a single antigenic determinant.
Antibody produced by a single population of identical cells that recognizes and binds to one specific region (epitope) of an antigen.
Discovered and developed in the 1970s by Cesar Milstein and Georges Kohler.

Monoclonal antiobodies are made by fusing myeloma cancer cells (which multiply very fast) with antibody-producing cells, then spreading the resulting conjugate colony so thin that each cell can be grown into a whole, separate colony (i.e., cloning). In this way, one gets whole batches of the same (monoclonal) antibody, which are all specific to the same antigen. Monoclonal antibodies have found markets in diagnostic kits, pharmaceuticals (e.g., trastuzumab, rituximab, bevacizumab, adalimumab, infliximab, etc.), imaging agents, and in purification processes.

One example of a diagnostic use is the invention in 1997 by Bruno Oesch of a monoclonal antibody-based rapid test to detect the prion (PrP 5c ) that causes bovine spongiform encephalopathy (BSE) in cattle.


Also called monocyte macrophages. The round-nucleated cells that circulate in the blood.
In summary they engulf and kill microorganisms, present antigen to the lymphocytes, kill certain tumor cells, and are involved in the regulation of inflammation.
These cells are often the first to encounter a foreign substance or pathogen or normal cell debris in the body. When they do, the material is taken up and degraded by means of oxidative and hydrolytic enzymatic attack. Peptides that result from the degradation of foreign protein are then bound to a monocyte protein called class II MHC (major histocompatibility complex) and this self-foreign complex then migrates to the surface of the cell where it is embedded into the cell membrane in such a way as to present the peptide to the outside of the cell.
This positioning allows T lymphocytes to recognize the peptide. Whereas self-peptides derived from normal cellular debris are ignored, foreign peptides activate precursors of helper T cells to further mature into active, lymphokine-secreting helper T lymphocytes, also known as TH cells. When monocytes move out of the bloodstream and into the tissues they are then called macrophages.


A hybrid individual that is heterozygous for one gene or a single character.

Monohybrid cross

A breeding experiment that uses parental varieties differing in a single character.


An algebraic expression that does not involve any additions or subtractions.


The subunit that serves as the building block of a polymer;  Small molecules that link together to form a polymer.

Monophyletic group (clade)

Set of species/organism containing a common ancestor and all its decendants; Pertaining to a taxon derived from a single ancestral species that gave rise to no species in any other taxa.
Term applied to a group of organisms which includes the most recent common ancestor of all of its members and all of the descendants of that most recent common ancestor. A monophyletic group is called a clade.


Paralysis that involves one limb.


Chemical compound formed by a combination of one fatty acid unit with one glycerine unit.
The addition of monoglycerides to an oil or shortening tends to lower the smoke point of the oil.


Monosaccharide is a carbohydrate that cannot be split into smaller units by the action of dilute acids; The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides.
Also known as simple sugars, the molecular formulas of monosaccharides are generally some multiple of CH2O.

The largest group of monosaccharides are the hexoses with six carbon atoms in the molecule (e.g glucose, fructose, mannose, galactose). Other monosaccharide categories are the heptoses with seven carbon atoms (e.g. xylose), the pentoses with five carbon atoms, and tetroses with four carbon atoms.

Monosodium glutamate (MSG)

Sodium salt of glutamic acid. Glutamic acid, or glutamate, is one of the most common amino acids found in nature.
In the early part of the century, MSG was extracted from seaweed and other plant sources. Today, MSG is produced in many countries around the world through a fermentation process of molasses from sugar cane or sugar beets, as well as starch and corn sugar.


An egg-laying mammal, represented by the platypus and echidna.

Mood-congruent psychotic features

Delusions or hallucinations whose content is entirely consistent with either a depressed or a manic mood.
If the mood is depressed, the content of the delusion or hallucinations would involve themes of either personal inadequacy, guilt, disease, death or nihilism or deserved punishment. If the mood is manic, the content of the delusions or hallucinations would involve themes of inflated worth, power, knowledge or identity or special relationship to a deity or a famous person.

Mood-incongruent psychotic features

Delusions or hallucinations whose content is not consistent with either a depressed or a manic mood.
Examples of such symptoms are persecutory delusions, thought insertion, thought broadcasting and delusions of being controlled, whose content has no apparent relationship to those seen in the mood-congruent psychotic features. (Note: The catatonic symptoms of stupor, mutism, negativism and posturing in manic episodes are also considered mood-incongruent psychotic features.)


Gram-negative rod is parasitic on the mucous membranes of humans and other warm-blooded animals.

Moro reflex

The "startle" reflex seen in infants.
The reaction of infants to a variety of stimuli (i.e., when a table that an infant is lying on is bumped), characterized by a sudden extension and abduction of arms, hands and fingers from their usual fixed posture; the legs may follow the same movement pattern. Present at birth and is strongest during the first 3 months of life.


A substance, such as bicoid protein, that provides positional information in the form of a concentration gradient along an embryonic axis.


The development of body shape and organization during ontogeny.

Morphological species concept

The idea that species are defined by measurable anatomical criteria.


Form and structural features such as the form and structure of an organism and its parts. Can be used to describe organisms and geological features amongst other things.
The word is sometimes used to also mean the form or structure itself.
Hence: Morphological = Pertaining to form and structure, at any level of organization.


A species defined by its anatomical features.

Mortality rate

Death rate.

Mosaic development

A pattern of development, such as that of a mollusk, in which the early blastomeres each give rise to a specific part of the embryo.
In some animals, the fate of the blastomeres is established in the zygote.

Mosaic evolution

The evolution of different features of an organism at different rates.


A type of Down syndrome in which the chromosomal accident occurs after fertilization.


Able to move oneself about, capable of self-locomotion.
Moving, for example by swimming, gliding, crawling, jumping, or kicking. Part of the body (e.g., cilia) may be motile in a cell which is not motile and is fixed in one position.
Hence: Motility = The ability to move
Compare to Sessile.


Refers to a type of speech training used with hearing-disordered people, involving the feeling of a peron’s face and reproducing breath and voice patterns.

Motor end plate

The specialized region on a muscle where a nerve terminal makes synaptic contact with the muscle.
At the motor end plate there are junctional folds of the membrane on which are located receptors for the transmitter that is released from the terminal of the presynaptic motor neuron on the opposite side of the synaptic cleft that separates the motor neuron from the muscle.

Motor end plate potential (EPP)

The change in membrane potential of the motor end plate.
The high safety factor for transmission between a motor neuron and skeletal muscle means that each Action Potential (AP) in the motor neuron produces an Action Potential in the skeletal muscle. One major contributor to this high safety factor is the very large amount of neurotransmitter, Acetylcholine (ACh), released by a pre-synaptic motor nerve AP from the terminal.
While an EPP is produced by controlled release of Ach by a pre-synaptic AP, unregulated spontaneous leakage of ACh from the pre-synaptic terminal results in miniature EPPs (mEPPs). The study of mEPPs was fruitful in leading to the concept of quantal release of neurotransmitter.

Motor neuron

A nerve cell that transmits signals from the brain or spinal cord to muscles or glands.

Motor unit

A single motor neuron and all the muscle fibers (muscle cells) it controls.
Each muscle receives input from a number of motor neurons but the motor neurons do not control equal numbers of muscle cells. Large motor units consist of a motor neuron and a large number of muscle cells whereas small motor units consist of a motor neuron and a small number of muscle cells.

This gradation in size of motor units means that the activity produced by a muscle can be graded to the effort required for the specific task that is being attempted with that muscle. Lifting a feather requires only a small amount of effort and so only small motor units need to be activated to the muscles of the finger lifting the feather. In contrast, lifting a heavy weight requires a large amount of effort and so both small and large motor units need to be activated to the muscles of the hand lifting the weight. This gradation in activation of motor units is called the “size principle” for motor unit activiation.

Motor control

Regulation of the timing and amount of contraction of muscles of the body to produce smooth and coordinated movement.
The regulation is carried out by operation of the nervous system.

Motor planning

Action formulated in the mind before attempting to perform.


Group of multi-cellular fungi which grow in thread-like strands called hyphae. A fungus-type microorganism whose growth on food is usually visible and colourful.
Molds may grow on many foods, including acid foods like jams and jellies and canned fruits. They usually are not a cause of food borne illness, but in the right environment, cause food spoilage. Recommended heat processing and sealing practices prevent their growth on these foods. Some are introduced into foods to give added flavour, e.g. in some cheeses.


A mouse is a computer input device that controls the movement of the cursor on the screen to make selections and to perform operations.

Mowat sensor

A hand-held travel aid approximately the size of a flashlight, used by people who are blind.
An alternative to the cane for warning of obstacles in front of the person.

M phase

The mitosis phase of the cell cycle, which includes mitosis and cytokinesis, and during which a cell divides into two.
This precedes GO/G1 and follows G2. M phase and G2 cells contain the same amount of DNA and the same number of chromosomes. In normal cells, this is the tetraploid number (4N) of chromosomes.

MPF (M-phase prmoting factor)

A protein complex required for a cell to progress from late interphase to mitosis; the active form consists of cyclin and cdc2, a protein kinase.

MTOC (microtubule organizing center)

MTOCs are bundles of protein tubes which may be found at the base of a eukaryotic flagellum. In animals, they also function in creating the arrays of microtubules that pull the chromosomes apart during mitosis.


Made of, or with the texture of, mucus.


A gelly-like substance produced by organisms, texture may vary from virtually fluid to stiff and rubber-like.

Mullerian mimicry

A mutual mimicry by two unpalatable species.
Resemblance among several unpalatable/vigilant species. The Müllerian mimicry theory postulates that several species mimic each others' warning signal in order to more effectively train predators to avoid eating individuals displaying the shared signal


Any organism composed of more than one cell.

Multigene family

A collection of genes with similar or identical sequences, presumably of common origin.


Involving many genes in the expression of a trait.


Having the ability to develop into more than one cell type of the body.
Most adult stem cells are not pluripotent but are multipotent and are restricted in their developmental ability to differentiate to a cell type associated with their tissue origin.

Multiple sclerosis

A disease in which there are multiple patches of demyelination in the white matter of the central nervous system. These are repaired and new patches appear in different places in subsequent attack

Multiplexed sequencing

A sequencing approach that uses several pooled samples simultaneously, greatly increasing sequencing speed.

Multiplying factor

The multiplying factor is a number which, when multiplied times the raw response, yields the adjusted response.
The multiplying factor is used to factor out the signal variation from assay to assay from the binding measurements recorded for each sample. The multiplying factor is determined by dividing the average of the tracer activity's raw response by the relative activity.

Multipoint analysis

Multipoint analysis is the measurement of a specimen at various dilutions of the specimen.


The numbers being multiplied.
In the equation ab = c, a and b are multiplicands.


The operation of repeated addition of the same number

Multiplicative identitiy

The number 1 is the multiplicative identity because 1 * a = a for all a.

Multiplicative inverse

The number, b, that when multiplied by a number, a, gives a result of 1.   Reciprocal.   b = 1/a.

Multivariate processes

Statistical analyses which can help explain large amounts of data, such as cluster analysis and factor analysis.


An acute viral disease with painful swelling around the jaw.
Can cause central nervous system damage, deafness, painful inflammation of the male sex glands, kidney inflammation, and infections in other major organs. Immunizations with the MMR vaccine protect children from this disease.

Municipal water use

a water system that has at least five service connections or which regularly serves 25 individuals for 60 days; also called a public water system

Muscle fibre

Muscle cell.
A long, cylindrical, multinucleated cell containing numerous myofibrils, which is capable of contraction when stimulated

Muscles of the eye

CILIARY MUSCLE — controls the diameter of the pupil (how much light passes
through the lens); ciliary muscle adjusts the shape of the lens (and
hence focal length) by varying the tension of the muscle
LATERAL RECTUS MUSCLE — contraction of this muscle rotates the eye in
the orbit toward the ear
MEDIAL RECTUS MUSCLE — contraction of this muscle rotates the eye in the
orbit toward the nose
ORBICULARIS OCULI MUSCLE — controls opening and closing of eyelid

Muscle Fiber Configurations 
or Fascicular arrangement

The arrangements of the muscle fibers that make up a muscle give the muscle its characteristic appearance.
The arrangement of fascicles determines range and power;
example: parallel muscle - longer fascicles produce greater range of movement;
example: multipennate muscle - more fascicles produce more power
Muscles are described by the directions of the fibers compared to the long axis of the muscle, or the direction of the action it produces: 
Parallel - all muscle fibers are parallel to the long axis of the muscle. Because all the fibers go in the same direction and are parallel, when these muscles contract, they allow a greater movement of the insertion point.
Convergent - In these muscles the origin is wider than the insertion. The direction of the fibers is toward the middle of the muscle, which narrows toward the insertion. When these muscles contract they concentrate a greater force on the insertion.
Divergent - In these muscles the insertion is wider than the origin. The direction of the fibers is away from the center of the muscle. The contraction of these muscles allows for the greatest range of motion.
Circular - In these muscles, the fibers go around in circles. They function to seal off bodily openings, and contract with a great deal of force. 
Pennate muscle - Fascicle forms an oblique angle to the tendon (like a feather).
Unipennate muscle - short pennate fascicles run down one side of the tendon.
Bipennate muscle - short pennate fascicles run down both sides of the tendon.
Multipennate - muscle where the tendon branches within the muscle and short pennate fascicles are attached to each tendon branch.

Muscle structure

A hierarchical organization from the level of myofilaments through to the whole muscle itself.
Starting from the macroscopic level of organization: Each muscle consists of bundles of fasciculi, which are groups of muscle fibres. Each fasciculus is surrounded by a connective tissue layer called the perimysium. Each muscle fibre contains bundles of myofibroils that are made up of many individual sarcomeres made up of the thick and thin filaments that make up the basic structural and contractile unit of muscle, together with a number of regulatory protein molecules and some elastic elements that hold sarcomeres together and give it extra properties.

Muscle tone

The baseline level of muscle contraction (and force) produced even when a muscle is quiescent; Used in clinical practice to describe the resistance of a muscle to being stretched.
Even when a whole muscle is not contracting, a small number of its motor units are involuntarily activated to produce a sustained contraction of the muscle fibers. The process gives rise to muscle tone.
To sustain muscle tone, small groups of motor units are alternately active and inactive in a constantly shifting pattern. Muscle tone keeps skeletal muscles firm, but it does not result in a contraction strong enough to produce movement.
When the peripheral nerve to a muscle is severed, the muscle becomes flaccid (limp). When nerve fibers in the brain or spinal cord are damaged, the balance between facilitation and inhibition of muscle tone is disturbed. The tone of some muscles may become increased and they resist being stretched - a condition called hypertonicity or spasticity.

Muscular dystrophy (MD)

A group of inherited, chronic disorders that are characterized by gradual wasting and weakening of the voluntary skeletal muscles.


A substance that induces mutations.
A chemical or physical agent that interacts with DNA and causes a mutation.


The cretion of mutations.


A cell microorganism that manifests new characteristics due to a change in its genetic material.
An organism carrying a gene that has undergone a mutation


Any change in the genotype (the nucleotide sequence) of an organism occurring at the gene, chromosome or genome level.
Mutations are modifications of a DNA sequence as a result of natural processes or exposure to chemical substances, ultraviolet radiation or X rays. Mutations are changes in the nucleotide sequence of the genome (affecting genes) and the resulting changes in the amino acid sequence of proteins. The amino acid sequence of proteins determines their structure and function, the latter being subject to natural selection. Mutation occur in different forms, from single nucleotide mutations to insertions and deletions of longer sequences as well as gene duplications, deletions and chromosomal rearrangements in higher organisms. Mutations ultimately create genetic diversity.


A symbiotic inter-specifies relationship in which both organisms benefit.
Example: flower pollination by insects.


Toxins produced by fungi.
More than 350 different mycotoxins are known to man. Almost all mycotoxins possess the capacity to harmfully alter the immune systems of animals. Consumption by humans and animals of certain mycotoxins (e.g., via eating infected corn, nuts, peanuts cottonseed products, etc.) can result in liver toxicity, gastrointestinal lesions, cancer and muscle necrosis.

Myelin sheath

In a neuron, an insulating coat of cell membrane produced by Schwann cells or Olgiodendrocytes. It covers the axon of the neuron, as a series of discontinuous sheaths along the length of the axon, interrupted between successive sheaths by unmyelinated regions called nodes of Ranvier.
Not all neurons have this insulating myelin sheath and neurons possessing this sheath are called “myelinated” neurons.
The myelin sheath is not a continuous structure over the entire axon of the neuron but is a series of discontinuous sheaths, each of which covers a segment of the axon.

Each segment is produced by one Schwann cell or Olgiodendrocyte (the former in the Peripheral Nervous System and the latter in the Central Nervous System) that wraps itself around the neuron’s axon over that segment and progressively squeezes itself tight around that segment of the axon membrane. As it does so, it squeezes all of the cytoplasm of that Schwann cell/Oligodendrocyte to the outer part of the cell so that the axon membrane is now covered with whorls of the cell membrane of the insulating Schwann cell/Oligodendrocyte.

These whorls act to insulate the axon membrane from loss of current flow during transmission of an Action Potential, the mechanism whereby neurons transmit information.

Between adjacent segments of this myelin sheath the axon is not myelinated and this uninsulated region is known as the node of Ranvier. The node is critical in regenerating the information flow during transmission of an Action Potential.

The process of transmission of an Action Potential in such myelinated neurons is known as “saltatory conduction”, coming from the Latin word “saltare” which means “to jump”. This refers to the fact that the Action Potential appears to jump from one node to the next node along the length of the axon as the neuron transmits the information (in the form of Action Potentials) down the length of the axon.

Saltatory conduction allows for faster transmission of information along the neuron since the Action Potential has to be regenerated less often (only at the nodes of ranvier) in a myelinated neuron than in an unmyelinated neuron. In the latter type of neuron it has to be regenerated very frequently because the absence of an insulating myelin sheath means that current leaks out as it flows along the neurons axon. Then, without regeneration, the current would be rapidly lost – i.e., the information that the neuron is trying to transmit would be lost very soon along it’s length.


The most severe form of spina bifida, in which a portion of the
spinal cord itself protrudes through the back. In some cases, sacs are covered with skin;
in others, tissue and nerves are exposed.
Generally, people use the terms "spina bifida" and "myelomeningocele" interchangeably.
(see also Meningocele and Meningomyelocele)

Myocardial infarction

Necrosis (death) of heart tissue due to ischemia.
Prolonged ischemic conditions (a decrease in blood supply) can lead to death, or necrosis, of the affected tissue; in the heart it is called myocardial infarction, or heart attack. Whereas angina is a result of a reduced blood flow to the heart muscle, myocardial infarction occurs as a result of a sudden and complete interruption of blood supply to a portion of the heart.

Myoclonic seizure

A type of seizure that is characterized by short, isolated shocklike jerks (contractions) involving parts of a muscle, an entire muscle, or groups of related muscles.


A fibril collectively arranged in longitudinal bundles in muscle cells (fibers).
Myofibriks are composed of thin filaments of actin and a regulatory protein, and thick filaments of myosin.


An oxygen-storing, pigmented protein in muscle cells.


A type of protein filament that interacts with actin filaments to cause cell contraction.


A surgical reconstruction of a perforated eardrum.


An incision made in the eardrum to drain fluid from an infected ear.
In a myringotomy and tubes, a small "bobbin" of plastic is inserted in the incision to allow air to enter the middle ear and assist in the drainage of fluid.