scaffold protein   Protein that binds groups of intracellular signaling proteins into a signaling complex, often anchoring the complex at a specific location in the cell. (Figure 15–10) 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 G1 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 G1 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, IP3, Ca2+, 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 antigenand 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.