Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

M (mitotic) phase See cell cycle.

macromolecule Any large, usually polymeric molecule (e.g., a protein, nucleic acid, polysaccharide) with a molecular mass greater than a few thousand daltons.

macrophages Phagocytic leukocytes that can detect broad patterns of pathogen markers via Toll-like receptors. They function as professional antigen-presenting cells and are a major source of cytokines.

major histocompatibility complex (MHC) Set of adjacent genes that encode class I and class II MHC molecules and other proteins required for antigen presentation, as well as some complement proteins; called the H-2 complex in mice and the HLA complex in humans. (Figure 23-21)

malignant Referring to a tumor or tumor cells that can invade surrounding normal tissue and/or undergo metastasis. See also benign.

MAP kinase Any of a family of protein kinases that are activated in response to cell stimulation by many different growth factors and that mediate cellular responses by phosphorylating specific transcription factors and other target proteins. (Figures 16-25 and 16-26)

matrix The lumen of the innermost compartment of the mitochondrion; also the fibrous proteins and carbohydrates external to a cell (called the extracellular matrix).

matrix metalloproteases (MMPs) Matrix metalloproteases (MMPs) are proteolytic enzymes that employ the metal zinc in their active sites. They operate in the extracellular space, where they cut proteins in the extracellular matrix and sometimes other proteins (e.g., some cell-surface receptors).

maturation-promoting factor (MPF) Cyclin-CDK complex that has the ability to induce entry into meiosis when injected into G₂-resting oocytes.

maximal velocity See V_max.

mechanosensor Any of several types of sensory structures that are embedded in various tissues and respond to touch, the positions and movements of the limbs and head, pain, and temperature.

Mediator A very large multiprotein complex that forms a molecular bridge between transcriptional activators bound to an enhancer and to RNA polymerase II bound at a promoter; functions as a coactivator in stimulating transcription. (Figures 9-39 and 9-40)

meiosis In eukaryotes, a special type of cell division that occurs during maturation of germ cells; comprises two successive nuclear and cellular divisions with only one round of DNA replication. Results in production of four genetically nonequivalent haploid cells (gametes) from an initial diploid cell. (Figure 6-3)

melting See denaturation.

membrane potential Electric potential difference, expressed in volts, across a membrane due to the slight excess of positive ions (cations) on one side and negative ions (anions) on the other. (Figures 11-18 and 11-19)

membrane transport protein Collective term for any integral membrane protein that mediates movement of one or more specific ions or small molecules across a cellular membrane regardless of the transport mechanism. (Figure 11-2)

memory The ability of an antigen-experienced immune system to respond more rapidly to a reexposure to that same antigenic stimulus.

meristem Organized group of undifferentiated, dividing cells that are maintained at the tips of growing shoots and roots in plants. All the adult structures arise from meristems.

G-16

merotelic attachment Indicates that a single kinetochore attaches to microtubules emanating from two opposite spindle poles.

mesenchymal stem cell A class of stem cells in the bone marrow that can differentiate into fat cells, osteoblasts (bone-forming cells), and cartilage-producing cells; some may also produce muscle and other types of differentiated cells.

mesenchyme Immature embryonic connective tissue, composed of loosely organized and loosely attached cells, derived from either the mesoderm or ectoderm in animals.

mesoderm The middle of the three primary cell layers of the animal embryo, lying between the ectoderm and endoderm; gives rise to the notochord, connective tissue, muscle, blood, and other tissues.

messenger RNA See mRNA.

metaphase Stage of mitosis at which condensed chromosomes are aligned equidistant between the poles of the mitotic spindle but have not yet started to segregate toward the spindle poles. (Figure 18-37)

metastasis Spread of cancer cells from their site of origin and establishment of areas of secondary growth.

metazoans A subset of the animal kingdom that includes all multicellular animals with differentiated tissues, such as nerves and muscles.

MHC See major histocompatibility complex.

MHC molecules Glycoproteins that display peptides, derived from foreign (and self) proteins, on the surface of cells and are required for antigen presentation to T cells. Class I molecules are expressed constitutively by nearly all nucleated cells; class II molecules, by professional antigen-presenting cells. (Figures 23-21 and 23-22)

micelle A water-soluble spherical aggregate of phospholipids or other amphipathic molecules that form spontaneously in aqueous solution. (Figure 10-3c)

Michaelis constant See K_m.

microfilament Cytoskeletal fiber (≈7 nm in diameter) that is formed by polymerization of monomeric globular (G) actin; also called actin filament. Microfilaments play an important role in muscle contraction, cytokinesis, cell movement, and other cellular functions and structures. (Figure 17-4)

micro-RNA See miRNA.

microsatellites Simple-sequence repeated DNA sequences 1–13 bases long (most are <5 bp) that occur in the human genome in tandem repeats of 150 repeats or fewer.

microtubule Cytoskeletal fiber (≈25 nm in diameter) that is formed by polymerization of α, β-tubulin monomers and exhibits structural and functional polarity. Microtubules are important components of cilia, flagella, the mitotic spindle, and other cellular structures. (Figures 18-2 and 18-3)

microtubule-associated protein (MAP) Any protein that binds to microtubules and regulates their stability. (Figures 18-13, 18-14, and 18-15)

microtubule-organizing center See MTOC.

microvillus (pl. microvilli) Small, membrane-covered projection on the surface of an animal cell containing a core of actin filaments. Numerous microvilli are present on the absorptive surface of intestinal epithelial cells, increasing the surface area for transport of nutrients. (Figures 17-4 and 20-10)

miRNA (micro-RNA) Any of numerous small, endogenous cellular RNAs, 20–30 nucleotides long, that are processed from double-stranded regions of hairpin secondary structures in long precursor RNAs. A single strand of the mature miRNA associates with several proteins to form an RNA-induced silencing complex (RISC) that inhibits translation of a target mRNA to which the miRNA hybridizes imperfectly. Several miRNAs must hybridize to a single mRNA to inhibit its translation. See also siRNA. (Figures 10-28a and 10-29)

mitochondria-associated membranes (MAMs) Portions of the endoplasmic reticulum that closely contact regions of mitochondria, influencing mitochondrial shape, function, and fission.

mitochondrion (pl. mitochondria) Large organelle that is surrounded by two phospholipid bilayer membranes, contains DNA, and carries out oxidative phosphorylation, thereby producing most of the ATP in eukaryotic cells. (Figures 1-20 and 12-6)

mitogen Any extracellular molecule, such as a growth factor, that promotes cell proliferation.

mitosis In eukaryotic cells, the process whereby the nucleus divides, producing two genetically equivalent daughter nuclei with the diploid number of chromosomes. See also cytokinesis and meiosis. (Figure 18-36)

mitotic CDKs Cyclin-CDK complexes that promote entry into and progression through mitosis.

mitotic spindle A specialized temporary structure, present in eukaryotic cells during mitosis, that captures the chromosomes and then pushes and pulls them to opposite sides of the dividing cell; also called mitotic apparatus. (Figure 18-37)

mobile DNA element See transposable DNA element.

model organism A non-human species used to study genes, proteins, and cellular functions. Model organisms are chosen based on the expectation that discoveries made via the model organism will provide insight into other organisms.

molecular chaperone See chaperone.

molecular complementarity Lock-and-key kind of fit between the shapes, charges, hydrophobicity, and/or other physical properties of two molecules or portions thereof that allow formation of multiple oncovalent interactions between them at close range. (Figure 2-12)

molecular markers, DNA-based DNA sequences that vary among individuals (DNA polymorphisms) of the same species and are useful in genetic linkage studies; includes SNPs and SSRs.

monoclonal antibody Antibody produced by the progeny of a single B cell and thus a homogeneous protein that recognizes a single antigen (epitope). It can be produced experimentally by use of a hybridoma. (Figure 4-6)

monomer Any small molecule that can be linked chemically with others of the same type to form a polymer. Examples include amino acids, nucleotides, and monosaccharides.

monomeric (small) G protein  A monomeric GTPase with a structure similar to that of the Ras protein that changes conformation when a bound GTP is hydrolyzed to GDP and phosphate. (Figure 15-5)

G-17

monopolin complex Protein complex that promotes the co-orientation of sister chromatids during meiosis I in budding yeast.

monosaccharide Any simple sugar with the formula (CH₂O)_n where n = 3–7.

monotelic attachment Indicates that only one of the sister kinetochore pair attached to microtubules.

monoubiquitinylation The covalent addition of a single ubiquitin molecule to a target protein.

morphogen A signaling molecule whose concentration determines the fate of target-differentiating cells.

motor protein Any member of a special class of mechanochemical enzymes that use energy from ATP hydrolysis to generate either linear or rotary motion; also called molecular motor. See also dyneins, kinesins, and myosins.

mRNA (messenger RNA) Any RNA that specifies the order of amino acids in a protein (i.e., the primary structure). It is produced by transcription of DNA by RNA polymerase. In eukaryotes, the initial RNA product (primary transcript) undergoes processing to yield functional mRNA. See also translation. (Figure 5-15)

mRNP exporter A heterodimeric protein that binds to mRNA-containing ribonucleoprotein particles (mRNPs) and directs their export from the nucleus to cytoplasm by interacting transiently with nucleoporins in the nuclear pore complex. (Figure 10-24)

MTOC (microtubule-organizing center) General term for any structure (e.g., centrosome, spindle pole, basal body) that organizes microtubules in cells. (Figure 18-5)

mRNA surveillance The processes that lead to the degradation of a pre-mRNA or mRNA that has been improperly processed.

multi-adhesive matrix proteins Group of long flexible proteins that bind to other components of the extracellular matrix and to cell-surface receptors, thereby cross-linking matrix components to the cell membrane. Examples include laminin, a major component of the basal lamina, and fibronectin, present in many tissues.

multimeric For proteins, containing several polypeptide chains (or subunits).

multiubiquitinylation The covalent addition of several single ubiquitin molecules, each at a distinct site, on a single target protein.

MuSK A receptor tyrosine kinase localized in the myotube plasma membrane that both induces clustering of acetylcholine receptors and serves to attract the termini of growing motor neuron axons.

mutagen A chemical or physical agent that induces mutations.

mutation In genetics, a permanent, heritable change in the nucleotide sequence of a chromosome, usually in a single gene; commonly causes an alteration in the function of the gene product.

myelin sheath Stacked specialized cell membrane that forms an insulating layer around vertebrate axons and increases the speed of impulse conduction. (Figure 22-18)

myofibril Long, slender structures within cytoplasm of muscle cells consisting of a regular repeating array of sarcomeres composed of thick (myosin) filaments and thin (actin) filaments. (Figure 17-30)

myosins A class of motor proteins that have actin-stimulated ATPase activity. Myosins move along actin microfilaments during muscle contraction and cytokinesis and also mediate vesicle translocation. (Figure 17-22)