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

Glossary G:31
mammalian cell cycle was called the restriction point. In this
book we use Start for both. (Figure 17–9)
start-transfer signal Short amino acid sequence that
enables a polypeptide chain to start being translocated across
the endoplasmic reticulum membrane through a protein
translocator. Multipass membrane proteins sometimes have
both N-terminal (signal sequence) and internal start-transfer
signals. (Figure 12–42)
STAT (signal transducer and activator of transcription)
Latent transcription regulator that is activated by
phosphorylation by Janus kinases (JAKs) and enters the
nucleus in response to signaling from receptors of the cytokine
receptor family. (Figure 15–56)
stem cell Undifferentiated cell that can continue dividing
indefinitely, throwing off daughter cells that can either commit
to differentiation or remain a stem cell (in the process of selfrenewal).
(Figure 22–3)
stem-cell niche The specialized microenvironment in a tissue
in which self-renewing stem cells can be maintained.
steroid hormones Hormones, including cortisol, estrogen,
and testosterone, that are hydrophobic lipid molecules derived
from cholesterol that activate intracellular nuclear receptors.
stimulatory G protein (G s ) G protein that, when activated,
activates the enzyme adenylyl cyclase and thus stimulates
the production of cyclic AMP. See also G protein.
(Table 15–3, p. 846)
stochastic Random. Involving chance, probability, or random
variables.
stop-transfer signal Hydrophobic amino acid sequence
that halts translocation of a polypeptide chain through the
endoplasmic reticulum membrane, thus anchoring the protein
chain in the membrane. (Figure 12–42)
strand exchange Reaction in which one of the single-strand
3′ ends from one duplex DNA molecule penetrates another
duplex and searches it for homologous sequences through
base-pairing. Also called strand invasion.
strand-directed mismatch repair A proofreading system
that removes DNA replication errors missed by the DNA
polymerase proofreading exonuclease. Detects the potential for
DNA helix distortion from noncomplementary base pairs then
recognizes and excises the mismatch in the newly synthesized
strand and resynthesizes the excised segment using the old
strand as a template.
stress fibers Cortical fibers of contractile actin-myosin II
bundles that connect the cell to the extracellular matrix or
adjacent cells through focal adhesions or a circumferential belt
and adherens junctions.
stroma (1) “Bedding”: the connective tissue in which a
glandular or other epithelium is embedded. Stromal cells
provide the environment necessary for the development of
other cells within the tissue. (2) The large interior space of a
chloroplast, containing enzymes that incorporate CO 2 into
sugars. (Figure 14–38)
substrate Molecule on which an enzyme acts.
superresolution Describes several approaches in light
microscopy that bypass the limit imposed by the diffraction of
light and successfully allow objects as small as 20 nm to be
imaged and clearly resolved.
survival factor Extracellular signal that promotes cell survival
by inhibiting apoptosis. (Figure 18–12)
symporter Carrier protein that transports two types of solute
across the membrane in the same direction. (Figure 11–8)
synapse Communicating cell–cell junction that allows signals
to pass from a nerve cell to another cell. In a chemical synapse,
the signal is carried by a diffusible neurotransmitter. (Figure
19–22) In an electrical synapse, a direct connection is made
between the cytoplasms of the two cells via gap junctions.
(Figure 11–34 and 19–23)
synapse elimination Process by which each muscle cell
at first receives synapses from several motor neurons, but is
ultimately left innervated by only one.
synaptic plasticity Changes in the strength with which
a chemical synapse transmits a signal. It is thought to be
important in memory formation, where concentrations of
postsynaptic AMPA receptor are modulated in response to a
synapse’s activity.
synaptic signaling Intercellular signaling performed by
neurons that transmit signals electrically along their axons and
release neurotransmitters at synapses, which are often located
far away from the neuronal cell body.
synaptic vesicle Small neurotransmitter-filled secretory
vesicle found at the axon terminals of nerve cells. Its contents
are released into the synaptic cleft by exocytosis when an
action potential reaches the axon terminal.
synaptonemal complex Structure that holds paired
homologous chromosomes tightly together in pachytene of
prophase I in meiosis and promotes the final steps of crossingover.
(Figures 17–55 and 17–56)
syncytium Mass of cytoplasm containing many nuclei
enclosed by a single plasma membrane. Typically the result
either of cell fusion or of a series of incomplete division cycles in
which the nuclei divide but the cell does not.
TATA box Sequence in the promoter region of many
eukaryotic genes that binds a general transcription factor
(TFIID) and hence specifies the position at which transcription is
initiated. (Figure 6–14)
T cell receptor (TCR) Transmembrane receptor for
antigen on the surface of T lymphocytes, consisting of an
immunoglobulin-like heterodimer. (Figure 24–32)
T-cell-mediated immune response Any adaptive immune
response mediated by antigen-specific T cells.
telomerase Enzyme that elongates telomere sequences in
DNA, which occur at the ends of eukaryotic chromosomes.
telomere End of a chromosome, associated with a
characteristic DNA sequence that is replicated in a special way.
Counteracts the tendency of the chromosome otherwise to
shorten with each round of replication. From Greek telos, end,
and meros, portion.
telomere End of a chromosome, associated with a
characteristic DNA sequence that is replicated in a special way.
Counteracts the tendency of the chromosome otherwise to
shorten with each round of replication. From Greek telos, end.
telophase Final stage of mitosis in which the two sets of
separated chromosomes decondense and become enclosed
by nuclear envelopes. (Panel 17–1, pp. 980–981)
template Single strand of DNA or RNA whose nucleotide
sequence acts as a guide for the synthesis of a complementary
strand. (Figure 1–3)