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, Ca2+ 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.