Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

G:20 Glossary
metaphase plate Imaginary plane at right angles to the
mitotic spindle and midway between the spindle poles; the
plane in which chromosomes are positioned at metaphase.
(Panel 17–1, pp. 980–981)
metaphase-to-anaphase transition Transition in the
eukaryotic cell cycle preceding sister-chromatid separation at
anaphase. If the cell is not ready to proceed to anaphase, the
cell cycle is halted at this point. (Figure 17–9, and Panel 17–1,
pp. 980–981)
metastases Secondary tumors, at sites in the body additional
to that of the primary tumor, resulting from cancer cells breaking
loose, entering blood or lymphatic vessels, and colonizing
separate environments.
metastasis The spread of cancer cells from their site of origin
to other sites in the body. (Figures 20–1 and 20–16)
MHC complex (major histocompatibility complex) Cluster
of genes in one vertebrate chromosome (chromosome 6 in
humans) that code for a set of highly polymorphic cell-surface
glycoproteins (MHC proteins). (Figure 24–37)
microbiome The combined genomes of the various species
of a defined microbiota.
microbiota The collective of microorganisms that reside in or
on an organism.
microelectrode A piece of fine glass tubing, pulled to an
even finer tip, that is used to inject electric current into cells or
to study the intracellular concentrations of common inorganic
ions (such as H + , Na + , K + , Cl – , and Ca 2+ ) in a single living cell
by insertion of its tip directly into the cell interior through the
plasma membrane.
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.
morphogen Diffusible signal molecule that can impose a
pattern on a field of cells by causing cells in different places to
adopt different fates. (Figure 21–8)
morphogenesis Developmental process in which cells
undergo movements and deformations in order to assemble
into tissues and organs with specific shapes and sizes.
motor protein Protein that uses energy derived from
nucleoside triphosphate hydrolysis to propel itself along a linear
track (protein filament or other polymeric molecule).
mRNA degradation control Regulation by a cell of gene
expression by selectively preserving or destroying certain
mRNA molecules in the cytoplasm.
mTOR The mammalian version of the large protein kinase
called TOR, involved in cell signaling; mTOR exists in two
functionally distinct multiprotein complexes.
multidrug resistance An observed phenomenon in which
cells exposed to one anticancer drug evolve a resistance not
only to that drug, but also to other drugs to which they have
never been exposed.
multidrug resistance (MDR) protein Type of ABC
transporter protein that can pump hydrophobic drugs (such as
some anticancer drugs) out of the cytoplasm of eukaryotic cells.