Dr. Paul's Virtually Biology Show.

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Macro-evolution. The processes related to the patterns observed in biogeography and in the fossil record related to patterns of evolution in groups of related species. A familiar example is adaptive radiation. Note that macroevolution is not strictly the same thing as speciation and even many texts are confused on this point.
Macrophage. Literally a "big eater". A white blood cell involved in the release of histamine, eating anything that it detects as being foreign, and finally in presenting antigens, or pieces of antigens called epitopes, to other cells in the immune system.

Marsupials. A group of mammals with young that leave the mother's uterus at a very early stage and finish their development in a pouch. Kangaroo and possum are two examples.

Mass number. The mass number of an atom is the mass of the atom's protons plus neutrons plus electrons.

Matter. Anything that takes up space and has mass. Electrons, protons, and anchovies are all matter. Light is not.

Meiosis. A type of cell division in eukaryotes characterized by the reduction of chromosome number from the diploid to haploid number and by several processes of recombination so that the daughter cells have different combinations of genes. In animals meiosis leads to the production of gametes. In plants the products of meiosis are called spores. Contrast mitosis.
Mesoderm. The middle of the three embryonic tissue layers formed during an embryo's gastrulation.
Mesophyll. The ground tissue found in the middle of the leaf, between the upper and the lower epidermis. Typically the mesophyll is divided between the palisade mesophyll, specialized for the light reactions of photosynthesis and the spongy mesophyll specialized more for gas exchange and the dark reactions of photosynthesis. In plants with fleshy(succulent) leaves, other mesophyll is specialized for water storage.
Messenger RNA(mRNA). RNA produced by transcription of DNA and that is destined to leave the nucleus to be translated as part of protein synthesis.

Metabolism. The set of all energy using and energy producing reactions within an organism; or the sum of all reactions that break down chemical compounds(for example digestion)and reactions that build complex compounds from simple compounds(for example protein synthesis).

Metabolic pathway. A sequence of chemical reactions involved in metabolism. Examples of metabolic pathways include glycolysis, the Kreb's cycle and the Calvin cycle.
MHC marker. A protein found on the surface of the cells of our bodied that enables or cells to be recognized by the immune system as belonging to the body....or at least ignored by the immune system. Sometimes the immune system doesn't properly ignore these markers.
Micro-evolution. A change in the genetic make-up of a population through time due to one of the agents of evolution. These are: natural selection, nonrandom mating, genetic drift, immigration, emigration and mutation.
Microfilament. One of a number of cytoskeleton components made from the protein actin When associated with filaments of myosin these can contract and form the basis for cell movement including our muscle cells.
Microtubule. A hollow tube made from a helical formation of a protein called tubulin. Microtubules are important in cell division, as components of centrioles, cilia and flagella.
Mimicry. The tendency of two species to closely resemble each other because such resemblance confers an advantage with respect to predators of the species. There are two main types of mimicry. In Batesian mimicry, a harmless creature has evolved to resemble a harmful or noxious creature. In Mullerian mimicry, two or more noxious(to the predators!) species have evolved to resemble each other.
Mitochondrion(a). A DNA containing organelle found in most eukaryotic cells. Mitochondria are the sites of the Kreb's cycle and electron transport phosphorylation.
Mitosis. A type of cell(nuclear) division in eukaryotes characterized by the production of identical daughter nuclei. Typically the cytoplasm of the parent cell divides, a process called cytoplasmic division but this need not be the case. Mitosis is used for growth and repair of multicellular organisms and for asexual reproduction. Contrast meiosis.
Molecule. A chemical entity consisting of two or more atoms of the same or different elements chemically bonded together.

Monera. One of the five currently recognized Kingdoms of life. These organisms have a cell wall, but lack a nucleus or other membrane bound organelles. The cells are called prokaryotic (pro = before; karyotic= seed(nucleus). Representative prokaryotes are the true bacteria, cyanobacteria and archebacteria. This kingdom, based on current research is likely to be broken up into several large kingdoms.

Monocot. Any of a group of angiosperms with a single seed leaf, leaf veins parallel, vascular bundles scattered throughout the stem and flower parts in multiples of three. Well known monocots include grasses, lilies, bamboo and orchids.
Monohybrid cross. A genetic cross where only one trait is under investigation. Usually these crosses start our with each parent being homozygous for different alleles of the same gene.

Monotremes. A group of primitive mammals restricted to the Australian region that are characterized by laying eggs. The Platypus and the Echidna are the living representatives of this group.

Morphogenesis. Literally the origin of form. The process by which an embryo develops from a single cell into a organized system of differentiated tissues. This involves a complex combination of genetic and environmental influences.
Morphological convergence. Structures in unrelated organisms that have a similar appearance due to common evolutionary pressures. Analogous structures are evidence of morphological convergence.
Motor neuron(effector neuron). A neuron that sends information from the central nervous system to a muscle, gland or both. Hence since they may enervate other effectors than muscle these neurons are more properly called effector neurons. Contrast with interneuron, sensory neuron.

Mutation. A change in the make-up of the genetic material of an organism. In multicellular organisms mutations may affect somatic(body) cells or sexual( the gametes) cells. The former may give rise to cancer while the latter may be passed on from one generation to the next providing the raw material for evolution.

Mutualism A type of symbiotic relationship where both partners benefit. An important mutualistic relationship involves the roots of trees and specialized fungi. This relationship which improves the absorption of nutrients by roots and provides food for the fungus is called a mychorrhiza.
Mycelium. In fungi, the mass of food absorbing cellular fibers(Hyphae) that make up the main part of the fungus. The fuzzy bread mold you might be familiar with is an example.

Myelin Sheath. A protein wrapped around the axons of many neurons and produced by one of a number of specialized cells. For motor neurons, the myelin sheath is produced by Schawnn cells. The myelin sheath speeds up the transmission of nerve impulses down the length of the axon.
Myosin. A protein found in muscle cells that interacts with actin to bring about muscle contraction.