stimulatory G protein (Gs) G protein that, when activated, activates the enzyme adenylyl cyclase and thus stimulates the production of cyclic AMP. See also G protein. (Table 15–3, p. 846) stochastic   Random. Involving chance, probability, or random variables.
stop-transfer signal   Hydrophobic amino acid sequence that halts translocation of a polypeptide chain through the endoplasmic reticulum membrane, thus anchoring the protein chain in the membrane. (Figure 12–42) strand exchange Reaction in which one of the single-strand 3′ ends from one duplex DNA molecule penetrates another duplex and searches it for homologous sequences through base-pairing. Also called strand invasion.
strand-directed mismatch repair   A proofreading system that removes DNA replication errors missed by the DNA polymerase proofreading exonuclease. Detects the potential for DNA helix distortion from noncomplementary base pairs then recognizes and excises the mismatch in the newly synthesized strand and resynthesizes the excised segment using the old strand as a template.
stress fibers Cortical fibers of contractile actin-myosin II bundles that connect the cell to the extracellular matrix or adjacent cells through focal adhesions or a circumferential belt and adherens junctions.
stroma (1) “Bedding”: the connective tissue in which a glandular or other epithelium is embedded. Stromal cells provide the environment necessary for the development of other cells within the tissue. (2) The large interior space of a chloroplast, containing enzymes that incorporate CO2 into sugars. (Figure 14–38) substrate Molecule on which an enzyme acts.
superresolution   Describes several approaches in light microscopy that bypass the limit imposed by the diffraction of light and successfully allow objects as small as 20 nm to be imaged and clearly resolved.
survival factor   Extracellular signal that promotes cell survival by inhibiting apoptosis. (Figure 18–12) symporter   Carrier protein that transports two types of solute across the membrane in the same direction. (Figure 11–8) synapse Communicating cell–cell junction that allows signals to pass from a nerve cell to another cell. In a chemical synapse, the signal is carried by a diffusible neurotransmitter. (Figure 19–22) In an electrical synapse, a direct connection is made between the cytoplasms of the two cells via gap junctions. (Figure 11–34 and 19–23) synapse elimination   Process by which each muscle cell at first receives synapses from several motor neurons, but is ultimately left innervated by only one.
synaptic plasticity   Changes in the strength with which a chemical synapse transmits a signal. It is thought to be important in memory formation, where concentrations of postsynaptic AMPA receptor are modulated in response to a synapse’s activity.
synaptic signaling   Intercellular signaling performed by neurons that transmit signals electrically along their axons and release neurotransmitters at synapses, which are often located far away from the neuronal cell body.
synaptic vesicle   Small neurotransmitter-filled secretory vesicle found at the axon terminals of nerve cells. Its contents are released into the synaptic cleft by exocytosis when an action potential reaches the axon terminal.
synaptonemal complex   Structure that holds paired homologous chromosomes tightly together in pachytene of prophase I in meiosis and promotes the final steps of crossing-over. (Figures 17–55 and 17–56) syncytium Mass of cytoplasm containing many nuclei enclosed by a single plasma membrane. Typically the result either of cell fusion or of a series of incomplete division cycles in which the nuclei divide but the cell does not.