M phase • The portion of the cell cycle in which mitosis takes place.
M phase • The portion of the cell cycle in which mitosis takes place.
Macroevolution [Gr. makros: large, long] • Evolutionary changes occurring over long time spans and usually involving changes in many traits. (Contrast with microevolution.)
Macromolecule • A giant polymeric molecule. The macromolecules are proteins, polysaccharides, and nucleic acids.
Macronutrient • A mineral element required by plant tissues in concentrations of at least 1 milligram per gram of their dry matter.
Macrophage (mac´ roh faj) • A type of white blood cell that endocytoses bacteria and other cells.
Major histocompatibility complex (MHC) • A complex of linked genes, with multiple alleles, that control a number of cell surface antigens that identify self and can lead to graft rejection.
Malignant tumor • A tumor whose cells can invade surrounding tissues and spread to other organs.
Malleus (mal´ ee us) [L. malleus: hammer] • The first of the three bones that conduct movements of the eardrum to the oval window of the inner ear. (See incus, stapes.)
Malpighian tubule (mal pee´ gy un) • A type of protonephridium found in insects.
Mammal [L. mamma: breast, teat] • Any animal of the class Mammalia. Mammals are characterized by the production of milk by the female mammary glands and the possession of hair for body covering.
Mantle • A sheet of specialized tissues that covers most of the viscera of mollusks; provides protection to internal organs and secretes the shell.
Mapping • In genetics, determining the order of genes on a chromosome and the distances between them.
Marine [L. mare: sea, ocean] • Pertaining to or living in the ocean. (Contrast with aquatic, terrestrial.)
Marker • A gene of identifiable phenotype that indicates the presence on another gene, DNA segment, or chromosome fragment.
Marsupial (mar soo´ pee al) • A mammal belonging to the subclass Metatheria, such as opossums and kangaroos. Most have a pouch (marsupium) that contains the milk glands and serves as a receptacle for the young.
Mass extinctions • Geological periods during which rates of extinction were much higher than during intervening times.
Mass number • The sum of the number of protons and neutrons in an atom's nucleus.
Mast cells • Typically found in connective tissue, mast cells can be provoked by antigens or inflammation to release histamine.
Maternal effect genes • These genes code for morphogens that determine the polarity of the egg and larva in the fruit fly Drosophila melanogaster.
Mating types • A mating system in which the sexes are morphologically identical but carry different alleles and will mate.
Maternal inheritance • Inheritance in which the mother's phenotype is exclusively expressed. Mitochondria and chloroplasts are maternally inherited via egg cytoplasm. Also known as cytoplasmic inheritance.
Mechanoreceptor • A cell that is sensitive to physical movement and generates action potentials in response.
Medulla (meh dull´ luh) • (1) The inner, core region of an organ, as in the adrenal medulla (adrenal gland) or the renal medulla (kidneys). (2) The portion of the brain stem that connects to the spinal cord.
Megagametophyte • A female gametophyte that produces eggs only.
Megaspore [Gr. megas: large + spora: to sow] • In plants, a haploid spore that produces a female gametophyte.
Meiosis (my oh´ sis) [Gr. meiosis: diminution] • Division of a diploid nucleus to produce four haploid daughter cells. The process consists of two successive nuclear divisions with only one cycle of chromosome replication.
Membrane potential • The difference in electrical charge between the inside and the outside of a cell, caused by a difference in the distribution of ions.
Memory cells • Long-lived lymphocytes produced by exposure to antigen. They persist in the body and are able to mount a rapid response to subsequent exposures to the antigen.
Mendelian population • A local population of individuals belonging to the same species and exchanging genes with one another.
Mendel's first law • See Segregation.
Mendel's second law • See Independent assortment.
Menstrual cycle • The monthly sloughing off of the uterine lining if fertilization does not occur in the female. Occurs between puberty and menopause.
Meristem [Gr. meristos: divided] • Plant tissue made up of undifferentiated actively dividing cells.
Mesenchyme (mez´ en kyme) [Gr. mesos: middle + enchyma: infusion] • Embryonic or unspecialized cells derived from the mesoderm.
Mesoderm [Gr. mesos: middle + derma: skin] • The middle of the three embryonic tissue layers first delineated during gastrulation. Gives rise to skeleton, circulatory system, muscles, excretory system, and most of the reproductive system.
Mesophyll (mez´ uh fill) [Gr. mesos: middle + phyllon: leaf] • Chloroplast-containing, photosynthetic cells in the interior of leaves.
Mesosome (mez´ uh soam´) [Gr. mesos: middle + soma: body] • A localized infolding of the plasma membrane of a bacterium.
Messenger RNA (mRNA) • A transcript of one of the strands of DNA; carries information (as a sequence of codons) for the synthesis of one or more proteins.
Meta- [Gr.: between, along with, beyond] • A prefix used in biology to denote a change or a shift to a new form or level; for example, as used in metamorphosis.
Metabolic compensation • Changes in metabolic properties of an organism that render it less sensitive to temperature changes.
Metabolic pathway • A series of enzyme-catalyzed reactions so arranged that the product of one reaction is the substrate of the next.
Metabolism (meh tab´ a lizm) [Gr. metabole: to change] • The sum total of the chemical reactions that occur in an organism, or some subset of that total (as in respiratory metabolism).
Metamorphosis (met´ a mor´ fo sis) [Gr. meta: between + morphe: form, shape] • A change occurring between one developmental stage and another, as for example from a tadpole to a frog. (See complete metamorphosis, incomplete metamorphosis.)
Metaphase (met´ a phase) • The stage in nuclear division at which the centromeres of the highly supercoiled chromosomes are all lying on a plane (the metaphase plane or plate) perpendicular to a line connecting the division poles.
Metapopulation • A population divided into subpopulations, among which there are occasional exchanges of individuals.
Metastasis (meh tass´ tuh sis) • The spread of cancer cells from their original site to other parts of the body.
Methanogen • Any member of a group of archaea that release methane as a metabolic product. This group is considered to be an extremely ancient one.
Methylation • The addition of a methyl group (–CH3) to a molecule. Extensive methylation of cytosine in DNA is correlated with reduced transcription.
MHC • See Major histocompatibility complex.
Microbiology [Gr. mikros: small + bios: life + logos: discourse] • The scientific study of microscopic organisms, particularly bacteria, protists, and viruses.
Microevolution • The small evolutionary changes typically occurring over short time spans; generally involving a small number of traits and minor genetic changes. (Contrast with macroevolution.)
Microfilament • Minute fibrous structure generally composed of actin found in the cytoplasm of eukaryotic cells. They play a role in the motion of cells.
Microgametophyte • A male gametophyte that produces sperm only.
Micronutrient • A mineral element required by plant tissues in concentrations of less than 100 micrograms per gram of their dry matter.
Micropyle (mike´ roh pile) [Gr. mikros: small + pylon: gate] • Opening in the integument(s) of a seed plant ovule through which pollen grows to reach the female gametophyte within.
Microspore [Gr. mikros: small + spora: to sow] • In plants, a haploid spore that produces a male gametophyte.
Microtubules • Minute tubular structures found in centrioles, spindle apparatus, cilia, flagella, and cytoskeleton of eukaryotic cells. These tubules play roles in the motion and maintenance of shape of eukaryotic cells.
Microvilli (singular: microvillus) • The projections of epithelial cells, such as the cells lining the small intestine, that increase their surface area.
Middle lamella • A layer of polysaccharides that separates plant cells; a shared middle lamella lies outside the primary walls of the two cells.
Migration • The regular, seasonal movements of animals.
Mineral • An inorganic substance other than water.
Mineral nutrients • Inorganic ions required by organisms for normal growth and reproduction.
Mismatch repair • When a single base in DNA is changed into a different base, or the wrong base inserted during DNA replication, there is a mismatch in base pairing with the base on the opposite strand. A repair system removes the incorrect base and inserts the proper one for pairing with the opposite strand.
Missense mutation • A nonsynonymous mutation, or one that changes a codon for one amino acid to a codon for a different amino acid. (Contrast with frame-shift mutation, nonsense mutation, synonymous mutation.)
Mitochondrial matrix • The fluid interior of the mitochondrion, enclosed by the inner mitochondrial membrane.
Mitochondrion (my´ toe kon´ dree un) [Gr. mitos: thread + chondros: grain] • An organelle in eukaryotic cells that contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation.
Mitosis (my toe´ sis) [Gr. mitos: thread] • Nuclear division in eukaryotes leading to the formation of two daughter nuclei each with a chromosome complement identical to that of the original nucleus.
Mitotic center • Cellular region that organizes the microtubules for mitosis. In animals a centrosome serves as the mitotic center.
Moderately repetitive DNA • DNA sequences that appear hundreds to thousands of times in the genome. They include the DNA sequences coding for rRNAs and tRNAs, as well as the DNA at telomeres.
Modular organism • An organism which grows by producing additional units of body construction (modules) that are very similar to the units of which it is already composed.
Module • A self-contained unit that is part of a larger unit.
Mole • A quantity of a compound whose weight in grams is numerically equal to its molecular weight expressed in atomic mass units. Avogadro's number of molecules: 6.023 x 1023 molecules.
Molecular clock • The theory that macromolecules diverge from one another over evolutionary time at a constant rate; this rate may provide insight into the phylogenetic relationships among organisms.
Molecular weight • The sum of the atomic weights of the atoms in a molecule.
Molecule • A particle made up of two or more atoms joined by covalent bonds or ionic attractions.
Molting • The process of shedding part or all of an outer covering, as the shedding of feathers by birds or of the entire exoskeleton by arthropods.
Monoclonal antibody • Antibody produced in the laboratory from a clone of hybridoma cells, each of which produces the same specific antibody.
Monocot [Gr. mono: one + kotyledon: a cup-shaped hollow] • Any member of the angiosperm lineage in which the embryo produces a single cotyledon (seed leaf). Leaves of most monocots have their major veins arranged parallel to each other.
Monocytes • White blood cells that produce macrophages.
Monoecious (mo nee´ shus) [Gr. mono: one + oikos: house] • Describes organisms in which both sexes are "housed" in a single individual that produces both eggs and sperm. (In some plants, these are found in different flowers within the same plant.) Examples: corn, peas, earthworms, hydras. (Contrast with dioecious, perfect flower.)
Monohybrid cross • A mating in which the parents differ with respect to the alleles of only one locus of interest.
Monomer [Gr. mono: one + meros: • unit] • A small molecule, two or more of which can be combined to form oligomers (consisting of a few monomers) or polymers (consisting of many monomers).
Monophyletic (mon´ oh fih leht´ ik) [Gk. mono: one + phylon: tribe] • Descended from a single ancestral stock.
Monosaccharide • A simple sugar. Oligosaccharides and polysaccharides are made up of monosaccharides.
Monosynaptic reflex • A neural reflex that begins in a sensory neuron and makes a single synapse before activating a motor neuron.
Morphogen • A diffusible substances whose concentration gradients determine patterns of development in animals and plants.
Morphogenesis (more´ fo jen´ e sis) [Gr. morphe: form + genesis: origin] • The development of form; the overall consequence of determination, differentiation, and growth.
Morphology (more fol´ o jee) [Gr. morphe: form + logos: study, discourse] • The scientific study of organic form, including both its development and function.
Mosaic development • Pattern of animal embryonic development in which each blastomere contributes a specific part of the adult body. (Contrast with regulative development.)
Motor end plate • The modified area on a muscle cell membrane where a synapse is formed with a motor neuron.
Motor neuron • A neuron carrying information from the central nervous system to an effector such as a muscle fiber.
Motor proteins • Specialized proteins that use energy to change shape and move cells or structures within cells. See dynein, kinesin.
Motor unit • A motor neuron and the set of muscle fibers it controls.
mRNA • (See messenger RNA.)
Mucosa (mew koh´ sah) • An epithelial membrane containing cells that secrete mucus. The inner cell layers of the digestive and respiratory tracts.
Muscle • Contractile tissue containing actin and myosin organized into polymeric chains called microfilaments. Muscle fiber A single muscle cell. In the case of striated muscle, a syncitial, multinucleate cell.
Muscle spindle • Modified muscle fibers encased in a connective sheat and functioning as stretch receptors.
Mutagen (mute´ ah jen) [L. mutare: change + Gr. genesis: source] • Any agent (e.g., chemicals, radiation) that increases the mutation rate.
Mutation • A detectable, heritable change in the genetic material not caused by recombination.
Mutation pressure • Evolution (change in gene proportions) by different mutation rates alone (i.e., without the influence of natural selection).
Mutualism • The type of symbiosis, such as that exhibited by fungi and algae or cyanobacteria in forming lichens, in which both species profit from the association.
Mycelium (my seel´ ee yum) [Gr. mykes: fungus] • In the fungi, a mass of hyphae.
Mycorrhiza (my´ ko rye´ za) [Gr. mykes: fungus + rhiza: root] • An association of the root of a plant with the mycelium of a fungus.
Myelin (my´ a lin) • A material forming a sheath around some axons. Formed by Schwann cells that wrap themselves about the axon, myelin insulates the axon electrically and increases the rate of transmission of a nervous impulse.
Myofibril (my´ oh fy´ bril) [Gr. mys: muscle + L. fibrilla: small fiber] • A polymeric unit of actin or myosin in a muscle.
Myogenic (my oh jen´ ik) [Gr. mys: muscle + genesis: source] • Originating in muscle.
Myoglobin (my´ oh globe´ in) [Gr. mys: muscle + L. globus: sphere] • An oxygen-binding molecule found in muscle. Consists of a heme unit and a single globiin chain, and carrys less oxygen than hemoglobin.
Myosin • One of the two major proteins of muscle, it makes up the thick filaments. (See actin.)