selectin Member of a family of cell-surface carbohydrate-binding proteins that mediate transient, Ca2+-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) 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.