microRNAs (miRNAs) Short (~21 nucleotide) eukaryotic RNAs, produced by the processing of specialized RNA transcripts coded in the genome, that regulate gene expression through base-pairing with mRNA. (Figure 7–75) microsome   Small vesicle derived from endoplasmic reticulum that is produced by fragmentation when cells are homogenized. (Figure 12–34) microtubule flux Movement of individual tubulin molecules in the microtubules of the spindle toward the poles by loss of tubulin at their minus ends. Helps to generate the poleward movement of sister chromatids after they separate in anaphase. (Figure 17–35) microtubule-associated protein (MAP) Any protein that binds to microtubules and modifies their properties. Many different kinds have been found, including structural proteins, such as MAP2, and motor proteins, such as dynein. [Not to be confused with the “MAP” (mitogen-activated protein kinase) of “MAP kinase.”] microtubule-organizing center (MTOC) Region in a cell, such as a centrosome or a basal body, from which microtubules grow.
midbody   Structure formed at the end of cleavage that can persist for some time as a tether between the two daughter cells in animals. (Figure 17–43) mitochondrial hsp70   Part of a multisubunit protein assembly bound to the matrix side of the TIM23 complex that acts as a motor to pull mitochondrial precursor proteins into the matrix space.
mitochondrial matrix   Large internal compartment of the mitochondrion. The corresponding compartment in a chloroplast is known as the stroma.
mitochondrial precursor proteins   Proteins first fully synthesized in the cytosol and then translocated into mitochondrial subcompartments as directed by one or more signal sequences.
mitochondrion (plural mitochondria) Membrane-bounded organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eukaryotic cells. (Figure 1–28) mitogen Extracellular signal molecule that stimulates cells to proliferate.
mitotic chromosome Highly condensed duplicated chromosome as seen at mitosis, consisting of two sister chromatids held together at the centromere.
mitotic spindle   Bipolar array of microtubules and associated molecules that forms in a eukaryotic cell during mitosis and serves to move the duplicated chromosomes apart.  (Figure 17–23 and Panel 17–1, pp. 980–981) model organism A species that has been studied intensively over a long period and thus serves as a “model” for deriving fundamental biological principles.
molecular chaperone (chaperone) Protein that helps guide the proper folding of other proteins, or helps them avoid misfolding. Includes heat-shock proteins (hsp).
monoallelic gene expression   Expression of only one of the two copies of a gene in a diploid genome, occurring, for example, as a result of imprinting or X-chromosome inactivation.
monoclonal antibody   Antibody secreted by a hybridoma cell line. Because the hybridoma is generated by the fusion of a single B cell with a single tumor cell, each hybridoma produces antibodies that are all identical. (Page 444) monocyte   Type of white blood cell that leaves the bloodstream and matures into a macrophage in tissues.  (Figure 22–27) monomeric GTPase A single-subunit enzyme that converts GTP to GDP (also called small monomeric GTP-binding proteins). Cycles between an active GTP-bound form and an inactive GDP-bound form and frequently acts as a molecular switch in intracellular signaling pathways.