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:29
scanning electron microscope Type of electron microscope
that produces an image of the surface of an object.
(Figure 9–50)
S-Cdk Cyclin–Cdk complex formed in vertebrate cells by an
S-cyclin and the corresponding cyclin-dependent kinase (Cdk).
(Figure 17–11 and Table 17–1, p. 969)
SCF Family of ubiquitin ligases formed as a complex of
several different proteins. One is involved in regulating the
eukaryotic cell cycle, directing the destruction of inhibitors of
S-Cdks in late G 1 and thus promoting the activation of S-Cdks
and DNA replication. (Figures 3–71 and 17–15)
Schwann cell Glial cell responsible for forming myelin
sheaths in the peripheral nervous system. Compare
oligodendrocyte. (Figure 11–33)
S-cyclin Member of a class of cyclins that accumulate during
late G 1 phase and bind Cdks soon after progression through
Start; they help stimulate DNA replication and chromosome
duplication. Levels remain high until late mitosis, after which
these cyclins are destroyed. (Figure 17–11)
Sec61 complex Three-subunit core of the protein
translocator that transfers polypeptide chains across the
endoplasmic reticulum membrane.
second messenger (small intracellular mediator) Small
intracellular signaling molecule that is formed or released for
action in response to an extracellular signal and helps to relay
the signal within the cell. Examples include cyclic AMP, cyclic
GMP, IP 3 , Ca 2+ , and diacylglycerol.
secondary cell wall Permanent rigid cell wall that is laid
down underneath the thin primary cell wall in certain plant cells
that have completed their growth.
secondary Ig repertoire Immunoglobulins produced
by B cells after antigen- and helper-T-cell-induced somatic
hypermutation and class switching. Compared to the primary Ig
repertoire, these Igs have a greatly increased diversity of both
Ig classes and antigen-binding sites and have increased affinity
for antigen.
secondary immune response The adaptive immune
response that occurs in response to a second or subsequent
exposure to an antigen. The response is more rapid in onset
and stronger than the primary immune response. (Figure 24–16)
secondary structure Regular local folding pattern of a
polymeric molecule; in proteins, α helices and
β sheets.
secretion system Specialized bacterial systems that secrete
effector proteins that interact with host cells.
secretory vesicle Membrane-enclosed organelle in which
molecules destined for secretion are stored prior to release.
Sometimes called secretory granule because darkly staining
contents make the organelle visible as a small solid object.
(Figures 13–65)
securin Protein that binds to the protease separase and
thereby prevents its cleavage of the protein linkages that hold
sister chromatids together in early mitosis. Securin is destroyed
at the metaphase-to-anaphase transition. (Figure 17–38)
segment Divisions of an insect body along its anteroposterior
axis, each forming highly specialized structures, but all built
according to a similar fundamental plan.
segment-polarity gene In Drosophila development, a gene
involved in specifying the anteroposterior organization of each
body segment. (Figure 21–19)
segmentation clock The gene-expression oscillator
controlling regular segmentation during vertebrate embryonic
development.
segmentation genes Genes expressed by subsets of cells
in the embryo that refine the pattern of gene expression so as
to define the boundaries and ground plan of the individual body
segments.
selectin Member of a family of cell-surface carbohydratebinding
proteins that mediate transient, Ca 2+ -dependent
cell–cell adhesion in the bloodstream—for example between
white blood cells and the endothelium of the blood vessel wall.
(Figure 19–28)
selectivity filter The part of an ion channel structure that
determines which ions it can transport. (Figures 11–24 and
11–25)
sensory bristles Miniature sense organs present on most
exposed surfaces of Drosophila, consisting of a sensory neuron
and supporting cells and responding to chemical or mechanical
stimuli.
separase Protease that cleaves the cohesin protein linkages
that hold sister chromatids together. Acts at anaphase,
enabling chromatid separation and segregation. (Figure 17–38)
septum Structure formed during bacterial cell division by the
inward growth of the cell wall and plasma membrane and that
divides the cell into two.
sequential induction Development process that generates
a progressively more complicated pattern. A series of
local inductions whereby one of two cell types present in a
developing tissue can produce a signal to induce neighboring
cells to specialize in a third way; the third cell type can then
signal back to the other two cell types nearby to generate a
fourth and a fifth cell type, and so on.
serine protease Type of protease that has a reactive serine in
the active site. (Figures 3–12 and 3–39)
serine/threonine kinase Enzyme that phosphorylates
specific proteins on serine or threonines.
SH2 domain Src homology region 2, a protein domain
present in many signaling proteins. Binds a short amino acid
sequence containing a phosphotyrosine. (Panel 3–2,
pp. 142–143)
side chain The part of an amino acid that differs between
amino acid types. The side chains give each type of amino acid
its unique physical and chemical properties. (Panel 3–1,
pp. 112–113)
signal patch Protein-sorting signal that consists of a specific
three-dimensional arrangement of atoms on the folded protein’s
surface. (Figure 13–46)
signal peptidase Enzyme that removes a terminal signal
sequence from a protein once the sorting process is complete.
(Figure 12–35)
signal sequence Short continuous sequence of amino acids
that determines the eventual location of a protein in the cell. An
example is the N-terminal sequence of 20 or so amino acids
that directs nascent secretory and transmembrane proteins to
the endoplasmic reticulum. (Table 12–3, p. 648)
signal-recognition particle (SRP) Ribonucleoprotein particle
that binds an ER signal sequence on a partially synthesized
polypeptide chain and directs the polypeptide and its attached
ribosome to the endoplasmic reticulum. (Figure 12–36)