Essential Cell Biology 5th edition

Glossary G:11
membrane of a cell or organelle; typically characterized by
the presence of specific proteins.
membrane potential
Voltage difference across a membrane due to a slight
excess of positive ions on one side and of negative ions on
the other.
membrane protein
A protein associated with the lipid bilayer of a cell
membrane.
membrane transport protein
Any transmembrane protein that provides a passageway for
the movement of select substances across a cell membrane.
membrane-enclosed organelle
Any organelle in a eukaryotic cell that is surrounded by a
lipid bilayer—for example, the endoplasmic reticulum, Golgi
apparatus, and lysosome.
messenger RNA (mRNA)
RNA molecule that specifies the amino acid sequence of a
protein.
metabolism
The sum total of the chemical reactions that take place in
the cells of a living organism.
metaphase
Stage of mitosis in which chromosomes are properly
attached to the mitotic spindle at its equator but have not
yet segregated toward opposite poles.
metastasis
The spread of cancer cells from the initial site of the tumor
to form secondary tumors at other sites in the body.
Michaelis constant (K M )
The concentration of substrate at which an enzyme works
at half its maximum velocity; serves as a measure of how
tightly the substrate is bound.
micrometer
Unit of length equal to one millionth (10 –6 ) of a meter or
10 –4 centimeter.
microRNA (miRNA)
Small noncoding RNA that controls gene expression by
base-pairing with a specific mRNA to regulate its stability
and its translation.
microscope
Instrument for viewing extremely small objects. Some use a
focused beam of visible light and are used to examine cells
and organelles. Others use a beam of electrons and can be
used to examine objects as small as individual molecules.
microtubule
Long, stiff, cylindrical
structure composed of
the protein tubulin. Used
by eukaryotic cells to organize their cytoplasm and guide the
intracellular transport of macromolecules and organelles.
microtubule-associated protein
Accessory protein that binds to microtubules; can stabilize
microtubule filaments, link them to other cell structures, or
transport various components along their length.
mismatch repair
Mechanism for recognizing and correcting incorrectly
paired nucleotides—those that are noncomplementary.
mitochondrion
Membrane-enclosed organelle, about the size of a
bacterium, that carries out oxidative phosphorylation and
produces most of the ATP in eukaryotic cells.
mitogen
An extracellular signal molecule that stimulates cell
proliferation.
mitosis
Division of the nucleus of a eukaryotic cell.
mitotic spindle
Array of microtubules and associated molecules that forms
between the opposite poles of a eukaryotic cell during
mitosis and pulls duplicated chromosome sets apart.
mobile genetic element
Short segment of DNA that can move, sometimes through
an RNA intermediate, from one location in a genome to
another; an important source of genetic variation in most
genomes. Also called a transposon.
model organism
A living thing selected for intensive study as a representative
of a large group of species. Examples include the mouse
(representing mammals), the yeast Saccharomyces cerevisiae
(representing a unicellular eukaryote), and Escherichia coli
(representing bacteria).
molecular switch
Intracellular signaling protein that toggles between an
active and inactive state in response to receiving a signal.
molecular weight
Sum of the atomic weights of the atoms in a molecule; as
a ratio of molecular masses, it is a number without units.
molecule
Group of atoms joined together by covalent bonds.
monomer
Small molecule that can be linked to others of a similar type
to form a larger molecule (polymer).
monomeric GTPase
Small, single-subunit GTP-binding protein. Proteins of this
family, such as Ras and Rho, are part of many different
signaling pathways.
motor protein
Protein such as myosin or kinesin that uses energy derived
from the hydrolysis of a tightly bound ATP molecule to
propel itself along a protein filament or polymeric molecule.
mutation
A randomly produced, permanent change in the nucleotide
sequence of DNA.
myofibril
Long, cylindrical structure that constitutes the contractile
element of a muscle cell; constructed of arrays of highly
organized bundles of actin, myosin, and other accessory
proteins.
myosin
Type of motor protein that uses ATP to drive movements
along actin filaments. One subtype interacts with actin to
form the thick contractile bundles of skeletal muscle.
myosin filament
Polymer composed of interacting molecules of myosin-II;
interaction with actin promotes contraction in muscle and
nonmuscle cells.
myosin-I
Simplest type of myosin, present in all cells; consists of a
single actin-binding head and a tail that can attach to other
molecules or organelles.
myosin-II
Type of myosin that exists as a
dimer with two actin-binding
heads and a coiled-coil tail;
can associate to form long myosin filaments.