Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

S (synthesis) phase See cell cycle.

S-adenosylmethionine (S-Ado-Met) A methyl donor derived from ATP and methionine, used in the synthesis of multiple metabolic intermediates. It is the methyl donor for methylation of DNA and RNA, including 5’ cap synthesis and internal m6A in mRNAs, methylation at multiple sites in rRNAs and tRNAs, and methylation of cytosines in DNA.

sarcomere Repeating structural unit of striated (skeletal) muscle composed of organized, overlapping thin (actin) filaments and thick (myosin) filaments and extending from one Z disk to an adjacent one; shortens during contraction. (Figures 17-30 and 17-31)

sarcoplasmic reticulum Network of membranes in cytoplasm of a muscle cell that sequesters Ca²⁺ ions; release of stored Ca²⁺ induced by muscle stimulation triggers contraction. (Figure 17-33)

satellite DNA See simple-sequence DNA.

saturated Referring to a compound (e.g., fatty acid) in which all the carbon-carbon bonds are single bonds.

SC-β cells Insulin-secreting β-islet cells, derived in the laboratory from human iPS and ES cells, which have the potential to treat patients with diabetes.

SCF (Skp1, Cullin, F-box proteins) Ubiquitin-protein ligase that ubiquitinylates inhibitors of S-phase CDKs and many other proteins, marking them for degradation by proteasomes.

second messenger A small intracellular molecule (e.g., cAMP, cGMP, Ca²⁺, DAG, and IP₃) whose concentration increases (or decreases) in response to binding of an extracellular signal and that functions in signal transduction. (Figure 15-6)

G-24

secondary structure In proteins, local folding of a polypeptide chain into regular structures including the α helix, β sheet, and β turns.

secretory pathway Cellular pathway for synthesizing and sorting soluble and membrane proteins localized to the endoplasmic reticulum, Golgi, and lysosomes; plasma membrane proteins; and proteins eventually secreted from the cell. (Figure 14-1)

securin A protein that prevents the onset of chromosome segregation by inhibiting separase, the protease that cleaves cohesin.

segregation The process that distributes an equal complement of chromosomes to daughter cells during mitosis and meiosis.

selectins A family of cell-adhesion molecules that mediate Ca²⁺-dependent interactions with specific oligosaccharide moieties in glycoproteins and glycolipids on the surface of adjacent cells or in extracellular glycoproteins. (Figures 20-2 and 20-40)

self-renewal The ability of a stem cell to reproduce itself during cell division without differentiating.

sensor Measures an intra- or extracellular property and converts into a signal.

separase A protease that cleaves cohesin, thereby initiating chromosome segregation.

short interspersed elements (SINEs) In mammals, repeated sequences related to a 150 (in mice)–300 (in humans) base-pair sequence that has been transposed throughout the genome over evolutionary time by proteins encoded by a long interspersed element (LINE) in the same organism. (Figure 8-8; Table 8-1)

shuttle vector Plasmid vector capable of propagation in two different hosts. (Figure 6-15)

side chain In amino acids, the variable substituent group attached to the (α) carbon atom that largely determines the particular properties of each amino acid; also called R group. (Figure 2-14)

signaling cascade Pathway that is activated by an intracellular or extracellular event and then transmits the signal to an effector.

signaling molecule General term for any extracellular or intracellular molecule involved in mediating the response of a cell to its external environment or to other cells.

signal peptide See targeting sequence.

signal-recognition particle (SRP) A cytosolic ribonucleoprotein particle that binds to the ER signal sequence in a nascent secretory protein and delivers the nascent chain/ribosome complex to the ER membrane, where synthesis of the protein and translocation into the ER are completed. (Figure 13-5)

signal sequence See targeting sequence.

signal transduction Conversion of a signal from one physical or chemical form into another. In cell biology commonly refers to the sequential process initiated by binding of an extracellular signal to a receptor and culminating in one or more specific cellular responses.

signal transduction pathway The set of proteins or small molecules involved in relaying a chemical message from the receptor at the cell surface to the target molecule(s) inside the cell.

silencer A sequence in eukaryotic DNA that promotes formation of condensed chromatin structures in a localized region, thereby blocking access of proteins required for transcription of genes within several hundred base pairs of the silencer; also called silencer sequence.

simple diffusion Net movement of a molecule across a membrane down its concentration gradient at a rate proportional to the gradient and the permeability of the membrane; also called passive diffusion.

simple-sequence DNA Short, tandemly repeated sequences that are found at centromeres and telomeres as well as at other chromosomal locations and are not transcribed; also called satellite DNA.

siRNA A small, double-stranded RNA, 21–23 nucleotides long with two single-stranded nucleotides at each end. A single strand of the siRNA associates with several proteins to form an RNA-induced silencing complex (RISC) that cleaves target RNAs to which the siRNA base pairs perfectly; variously called short or small interfering RNA and small inhibitory RNA. siRNAs can be designed to experimentally inhibit expression of specific genes. See also miRNA. (Figure 10-28b)

siRNA knockdown Technique for experimentally inhibiting translation of a specific mRNA by use of siRNA; useful for reducing the activity of a protein, particularly in organisms that are not amenable to classical genetic methods for isolating loss-of-function mutants.

sister chromatid resolution The process of untangling of the intertwined sister chromatids during prophase.

sister chromatids The two identical DNA molecules created during DNA replication and the associated chromosomal proteins. After DNA replication, each chromosome is composed of two sister chromatids.

Smads Class of transcription factors that are activated by phosphorylation following binding of members of the transforming growth factor β (TGFβ) family of signaling molecules to their cell-surface receptors. (Figure 16-3)

SM proteins Sec1/Munc18-like proteins, which bind syntaxin and are required for SNARE-mediated fusion of synaptic vesicles to a cell membrane.

SMC proteins Structural maintenance of chromosome proteins; a small family of nonhistone chromatin proteins that are critical for maintaining the morphological structure of chromosomes and their proper segregation during mitosis. Members of this family include condensins, which help condense chromosomes during mitosis, and cohesins, which link sister chromatids until their separation in anaphase. Bacterial SMC proteins function in the proper segregation of bacterial chromosomes to daughter cells. (Figures 8-32 and 19-25)

SNAREs Cytosolic and integral membrane proteins that promote fusion of vesicles with target membranes. Interaction of v-SNAREs on a vesicle with cognate t-SNAREs on a target membrane forms very stable complexes, bringing the vesicle and target membranes into close apposition. (Figure 14-10)

snoRNA (small nucleolar RNA) A type of small, stable RNA that functions in rRNA processing and base modification in the nucleolus.

snRNA (small nuclear RNA) One of several small, stable RNAs localized to the nucleus. Five snRNAs are components of the spliceosome and function in splicing of pre-mRNA. (Figures 10-9 and 10-11)

G-25

somatic cell Any plant or animal cell other than a germ cell.

somatic-cell nuclear transfer A procedure in which the nucleus of an adult somatic cell is transferred into an enucleated egg, a step in producing a cloned animal.

sorting signal A relatively short amino acid sequence within a protein that directs the protein to particular transport vesicles as they bud from a donor membrane in the secretory or endocytic pathway. (Table 14-2)

Southern blotting Technique for detecting specific DNA sequences separated by electrophoresis by hybridization to a labeled nucleic acid probe. (Figure 6-24)

specificity The ability of immune cells or their products to distinguish between structurally closely related molecules.

sperm The male gamete—a motile haploid cell that can bind to and fuse with an egg cell, forming a zygote.

S phase CDKs Cyclin-CDK complexes that promote the initiation of DNA replication.

sphingolipid Major group of membrane lipids, derived from sphingosine, that contain two long hydrocarbon chains and either a phosphorylated head group (sphingomyelin) or carbohydrate head group (cerebrosides, gangliosides). (Figure 7-8b)

spindle assembly checkpoint pathway Pathway that senses incorrect attachment of chromosomes to the mitotic spindle and induces cell cycle arrest in metaphase.

spindle midzone Middle portion of the mitotic spindle that plays an important role in cleavage furrow positioning in some organisms.

spindle pole bodies Functionally analogous structure of centrosomes in yeast.

spindle position checkpoint pathway Pathway that senses incorrect position of the mitotic spindle within the cell and induces cell cycle arrest in anaphase.

spliceosome Large ribonucleoprotein complex that assembles on a pre-mRNA and carries out RNA splicing. (Figure 10-11)

SRE-binding proteins (SREBPs) Cholesterol-dependent transcription factors, localized in the ER membrane, that are activated in response to low cellular cholesterol levels and then stimulate expression of genes encoding proteins involved in cholesterol synthesis and import as well as synthesis of other lipids. (Figure 16-38)

starch A very long, branched polysaccharide, composed exclusively of glucose units, that is the primary storage carbohydrate in plant cells. (Figure 12-36)

START Point in the cell cycle at which cells are irreversibly committed to cell division and can no longer return to the G₁ state.

steady state In cellular metabolic pathways, the condition when the rate of formation and rate of consumption of a substance are equal, so that its concentration remains constant. (Figure 2-23)

stem cell A self-renewing cell that can divide symmetrically to give rise to two daughter cells whose developmental potential is identical to that of the parental stem cell or asymmetrically to generate daughter cells with different developmental potentials. (Figure 21-10 and 21-11)

stem-cell niche A set of cells, extracellular matrices, and hormones that surrounds a stem cell and that maintains its stem-cell properties

stereoisomers Two compounds with identical molecular formulas whose atoms are linked in the same order but in different spatial arrangements. In optical isomers, designated D and L, the atoms bonded to an asymmetric carbon atom are arranged in a mirror-image fashion. Geometric isomers include the cis and trans forms of molecules containing a double bond.

steroids A group of four-ring hydrocarbons including cholesterol and related compounds. Many important hormones (e.g., estrogen and progesterone) are steroids. Sterols are steroids containing one or more hydroxyl groups. (Figure 10-8c)

strand invasion An early step in DNA recombination catalyzed by RecA in bacteria and Rad51 in eukaryotes in which a single-stranded region of DNA with a free 3' end hybridizes to its complementary strand in a second double-stranded DNA molecule. The complementary strand of this target double-stranded DNA is displaced as a single-stranded loop of DNA over the region of hybridization to the invading strand. (Figures 5-40 and 5-41)

structural motif A particular combination of two or more secondary structures that form a distinct three-dimensional structure that appears in multiple proteins and that often, but not always, is associated with a specific function.

substrate Molecule that undergoes a charge in a reaction catalyzed by an enzyme.

substrate-level phosphorylation Formation of ATP from ADP and P_i catalyzed by cytosolic enzymes in reactions that do not depend on a proton-motive force or molecular oxygen.

sulfhydryl group (—SH) A substituent group present in the amino acid cysteine and other molecules consisting of a hydrogen atom covalently bonded to a sulfur atom; also called a thiol group.

SWI/SNF chromatin-remodeling complex A multiprotein chromatin-remodeling co-activator complex with at least one subunit that is homologous to DNA helicases that facilitates nucleosomes sliding along DNA and decondensation of condensed chromatin structures.

symmetric cell division See cell division.

symport A type of cotransport in which a membrane protein (symporter) transports two different molecules or ions across a cell membrane in the same direction. See also antiport. (Figure 11-2, [2B])

synapse Specialized region between an axon terminal of a neuron and an adjacent neuron or other excitable cell (e.g., muscle cell) across which impulses are transmitted. At a chemical synapse, the impulse is conducted by a neurotransmitter; at an electric synapse, impulse transmission occurs via gap junctions connecting the pre- and postsynaptic cells. (Figure 22-3)

synaptic plasticity The modification of synaptic connectivity based on neuronal experience, which is a biological manifestation of forming and storing memories.

synaptic vesicles Small vesicles in axon termini that contain a neurotransmitter and that undergo exocytosis following arrival of an action potential.

G-26

synaptonemal complex (SC) Proteinaceous structure that mediates the association (synapsis) between homologous chromosomes during prophase of meiosis I.

syndecans A class of cell-surface proteoglycans that function in cell-matrix adhesion, interact with the cytoskeleton, and may bind external signals, thereby participating in cell-cell signaling.

syntelic attachment Indicates that kinetochores of a sister chromatid pair attach to microtubules emanating from the same pole.

synteny Occurrence of genes in the same order on a chromosome in two or more different species.