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:30 Glossary
signaling center Cluster of specialized cells in developing
tissues that serves as a source of developmental signals—for
example, the generation of a morphogen gradient.
single-nucleotide polymorphism (SNP) A variation between
individuals in a population due to a relatively common difference
in a specific nucleotide at a defined point in the DNA sequence.
single-particle reconstruction Computational procedure
in electron microscopy in which images of many identical
molecules are obtained and digitally combined to produce an
averaged three-dimensional image, thereby revealing structural
details that are hidden by noise in the original images.
(Figures 9–54 and 9–55)
single-pass transmembrane protein Membrane protein in
which the polypeptide chain crosses the lipid bilayer only once.
(Figure 10–24)
single-strand DNA-binding (SSB) protein Protein that
binds to the single strands of the opened-up DNA double helix,
preventing helical structures from reforming while the DNA is
being replicated. (Figure 5–15)
sister chromatids Tightly linked pair of chromosomes that
arise from chromosome duplication during S phase. They
separate during M phase and segregate into different daughter
cells. (Figure 17–21)
sliding clamp Protein complex that holds the DNA
polymerase on DNA during DNA replication. (Figure 5–17)
Slit Signal protein, secreted by cells of the neural tube floor
plate, responsible for repelling the growth cones of commissural
axons after they have crossed the midline, thereby ensuring
these neurons do not re-cross the midline.
Smad family Latent transcription regulators that are
phosphorylated and activated by receptor serine/threonine
kinases and carry the signal from the cell surface to the
nucleus. (Figure 15–57)
small interfering RNAs (siRNAs) Short (21–26 nucleotide)
double-stranded RNAs that inhibit gene expression by directing
destruction of complementary mRNAs. Production of siRNAs is
usually triggered by exogenously introduced double-stranded
RNA. (Figure 7–77)
small nuclear RNA (snRNA) Small RNA molecules that are
complexed with proteins to form the ribonucleoprotein particles
(snRNPs) involved in RNA splicing. (Figures 6–28 and 6–29)
small nucleolar RNA (snoRNA) Small RNAs found in
the nucleolus, with various functions, including guiding the
modifications of precursor rRNA. (Table 6–1, p. 305, and
Figure 6–41)
smooth endoplasmic reticulum (smooth ER) Region of the
endoplasmic reticulum not associated with ribosomes. Involved
in detoxification reactions, Ca 2+ storage, and lipid synthesis.
(Figure 12–33)
Smoothened Seven-pass transmembrane protein with a
structure very similar to a GPCR but does not seem to act
as a Hedgehog receptor or as an activator of G proteins; it is
controlled by the Patched and iHog proteins.
SNARE proteins (SNAREs) Members of a large family of
transmembrane proteins present in organelle membranes
and the vesicles derived from them. SNAREs catalyze the
many membrane fusion events in cells. They exist in pairs—a
v-SNARE in the vesicle membrane that binds specifically to a
complementary t-SNARE in the target membrane.
sodium dodecyl sulfate polyacrylamide-gel
electrophoresis (SDS-PAGE) Type of electrophoresis
used to separate proteins by size. The protein mixture
to be separated is first treated with a powerful negatively
charged detergent (SDS) and with a reducing agent
(β-mercaptoethanol), before being run through a polyacrylamide
gel. The detergent and reducing agent unfold the proteins, free
them from association with other molecules, and separate the
polypeptide subunits.
somatic cell Any cell of a plant or animal other than cells of
the germ line. From Greek soma, body.
somatic hypermutation In immunology: accumulation
of point mutations in the assembled variable-region-coding
sequences of immunoglobulin genes that occurs when B cells
are activated to form memory cells. Results in the production
of antibodies with altered antigen-binding sites, some of which
bind antigen with increased affinity; it is responsible for affinity
maturation in antibody responses.
somatic mutations In cancer, one or more detectable
abnormalities in the DNA sequence of tumor cells that
distinguish them from the normal somatic cells surrounding the
tumor.
somite One of a series of paired blocks of mesoderm
that form during early development and lie on either side of
the notochord in a vertebrate embryo. They give rise to the
segments of the body axis, including the vertebrae, muscles,
and associated connective tissue. (Figure 21–38)
sorting signal Signal sequence or signal patch that directs
the delivery of a protein to a specific location, such as a
particular intracellular compartment.
spectrin Abundant protein associated with the cytosolic side
of the plasma membrane in red blood cells, forming a network
that supports the membrane. Also present in other cells.
(Figure 10–38)
S phase Period of a eukaryotic cell cycle in which DNA is
synthesized. (Figure 17–4)
spinal cord Bundle of neurons and support cells that extends
from the brain.
spindle assembly checkpoint Regulatory system that
operates during mitosis to ensure that all chromosomes
are properly attached to the spindle before sister-chromatid
separation starts. (Figure 17–9 and Panel 17–1, pp. 980–981)
spliceosome Large assembly of RNA and protein molecules
that performs pre-mRNA splicing in eukaryotic cells.
Src (Src protein family) Family of cytoplasmic tyrosine
kinases (pronounced “sark”) that associate with the cytoplasmic
domains of some enzyme-linked cell-surface receptors
(for example, the T cell antigen receptor) that lack intrinsic
tyrosine kinase activity. They transmit a signal onward by
phosphorylating the receptor itself and specific intracellular
signaling proteins on tyrosines. (Figure 3–10)
SRP (signal-recognition particle) receptor Component
in the endoplasmic reticulum (ER) membrane that guides the
signal recognition particle to the ER membrane.
starch Polysaccharide composed exclusively of glucose units,
used as an energy-storage material in plant cells. (Figure 2–51)
Start (restriction point) Important transition at the end of
G 1 in the eukaryotic cell cycle. Passage through Start commits
the cell to enter S phase. The term was originally used for this
point in the yeast cell cycle only; the equivalent point in the