Sarcoma A tumor derived from mesodermally derived cells, e.g., connective tissue, muscle (myosarcoma), or bone (osteosarcoma).

Sarcomere  Repeating structural unit of myofibril composed of thick and thin filaments that extends between two adjacent Z lines.

Sarcoplasmic reticulum  Network of membranes, derived from the smooth endoplasmic reticulum, that surrounds each myofibril in a muscle cell and sequesters Ca²⁺ ions. Depolarization (stimulation) of a muscle cell induces release of Ca²⁺ ions into the cytosol, triggering coordinated contraction along the length of the cell. Endoplasmic reticulum in muscle fiber ; site of storage and release of calcium ions.

Saturation density  Maximum number of cells attainable per cm² (monolayer culture) or per ml (suspension culture) under specified culture conditions.

schizophrenia  Disease, or family of diseases, characterized by altered motor behavior, distorted perceptions, disturbed thinking, altered mood, and abnormal interpersonal behavior.

Scrotum  sac that contains testes and epididymides.

Second messenger  An intracellular signaling molecule whose concentration increases (or decreases) in response to binding of an extracellular ligand to a cell-surface receptor and participates in mediating the cellular response to the ligand. Examples include cAMP, Ca²⁺, diacylglycerol, and inositol 1,4,5-trisphosphate.

Secondary structure  In proteins, local folding of a polypeptide chain into regular structures including the α helix, β pleated sheet, and U-shaped turns and loops.

Secretin  Peptide hormone secreted by upper small intestine ; stimulates pancreas to secrete bicarbonate intoo small intestine.

Secretory vesicle  membrane-bound vesicle produced by Golgi apparatus ; contains protein to be secreted by cell.

Seeding efficiency The percentage of the inoculum that attaches to the substrate within a stated period of time (implying viability, or survival, but not necessarily proliferative capacity).

Selectable marker A gene carried by a vector and conferring a recognizable characteristic on a cell containing the vector or a recombinant DNA molecule derived from it.

Selectins  Three adhesion molecules, P-selectin (CD62P), E-selectin (CD62E) and L-selectin (CD62L) involoved in slowing leucocytes during their transit through venules.

Selection A means of obtaining a clone containing a desired recombinant DNA molecule.

Semen  Sperm-containing fluid of male ejaculate.

Serotonin  Biogenic amine neurotransmitter ; paracrine agent in blood platelets and digestive tract ; also called 5-hydroxytryptamine, or 5-HT.

Serum  Blood plasma from which fibrinogen and other clotting proteins have been removed as result of clotting.

Sex hormone  Estrogen, progesterone, testosterone, or related hormones.

Shotgun cloning A cloning strategy that involves the insertion of random fragments of a large DNA molecule into a vector, resulting in a large number of different recombinant DNA molecules.

Shuttle vector A vector that can replicate in the cells of more than one organism (e.g. in E.coil and in yeast).

Sickle-cell anemia  Disease in which anamino acid in hemoglobin is abnormal, and at low oxygen concentrations erythrocytes assume sickle shapes or other bizarre forms that block capillaries.

Signal sequence  A relatively short amino acid sequence that directs a protein to a specific location within the cell; also called signal peptide. One example is the N-terminal sequence of a nascent secretory or membrane protein that causes the growing polypeptide to cross the membrane of the endoplasmic reticulum into the lumen; the signal sequence is cleaved in the process. Initial portion of newly synthesized protein( if protein is destined for secretion).

Signal-transduction pathway  A series of coupled intracellular events, triggered by binding of a signaling molecule to a receptor, that occur in a sequential fashion to convert an extracellular signal into a cellular response. Examples include pathways initiated by binding of ligand to G protein-linked receptors, receptor tyrosine kinases, and tyrosine kinase-linked receptors.

Simian virus 40 (SV40) A mammalian virus used as the basis for a series of cloning vectors.

SLE  = Systemic lupus erythematosus.

Somatic cell genetics The study of cell genetics by recombination and segregation of genes in somatic cells. Usually by cell fusion.

Somatic mutation  A process occurring during B-cell maturation and affecting the antibody gene region, which permits refinement of antibody specificity.

Southern transfer A technique for transferring bands of DNA from an agarose gel to a nitrocellulose or similar membrane.

Sphincter  Smooth-muscle ring that surrounds a tube, closing tube as muscle contracts.

Spinal nerve  One of 86 peripheral nerves(43 pairs) that join spinal cord.

Spleen  A major secondary lymphoid organ lying in the peritoneal cavity next to the stomach. Largest lymphoid organ ; located between stomach and diaphragm.

Split ratio The divisor of the dilution ratio of a cell culture at subculture, e.g., one flask divided into four or 100 ml up to 400 ml would be a split ratio of 4.

Starling force  Factor that determines derection and magnitude of fluid movement across capillary wall.

Starling’s law of the heart  within limits, increased end-diastolic volume of heart(increased muscle-fiber length) increases force of cardiac contraction.

Stem-loop A hairpin structure, consisting of a base-paired stem and a non-base-paired loop, that may form in a polynucleotide.

Stereoisomers (D and L)  Two compounds that have identical molecular formulas and atoms linked in the same order, but which have a different arrangement of atoms about an asymmetric carbon atom. D and L refer to particular configurations related to the configurations of D- and L-glyceraldehyde. All naturally occurring amino acids are L-isomers, and naturally occurring sugars generally are D-isomers.

Sticky end An end of a double-stranded DNA molecule where there is a single-stranded extension.

Strong promoter An efficient promoter that can direct synthesis of RNA transcripts at a relatively fast rate.

Stuffer fragment  The part of a λ replacement vector that is removed during insertion of new DNA.

Subconfluent Less than confluent. All of the available substrate is not covered.

Subculture  = Passage

Substrate The matrix or solid underlay upon which a monolayer culture grows. Reactant in enzyme-mediated reaction.

Substate-level phosphorylation  Formation of ATP from ADP and Pi catalyzed by cytosolic enzymes in reactions that do not depend on a proton-motive force. During glycolysis, two molecules of ATP are produced by this process.

Supercoiled The conformation of a covalently closed-circular DNA molecule, which is coiled by torsional strain into the shape taken by a wound-up elastic band.

Superconfluent When a monolayer culture progresses beyond the state where all the cells are attached to the substrate and multilayering occurs.

Suppressor gene A gene that exhibits the transformed (malignant) phenotype, usually associated with dominant negative regulation of cell proliferation or cell migration. Often it is mutated or deleted in transformed cells and cancer.

Suppressor mutation  A mutation that reverses the phenotypic effect of a second mutation. Suppressor mutations are frequently used to identify genes encoding interacting proteins.

Suppressor T (T_S) cell  Functionally defined populations of T cells, or switch the response into a different pathway from that under investigation. T cell that inhibits antibody production and cytotoxic T-cell function.

Surfactant  Detergentlike phospholipid produced by pulmonary type II alveolar cells ; reduces surface tension of fluid film lining alveoli.

Suspension culture Where cells will multiply suspended in medium.

Symport  A type of cotransport in which two different molecules or ions move across a membrane in the same direction. Transport is mediated by specific membrane-bound proteins called symporters. Opp. Antiport.

Synapse  Region between an axon terminus 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 conduction occurs via gap junctions connecting the cytoplasms of the pre- and post-synaptic cells.

Synergism  Cooperative interaction.

Syngeneic animals  Strains of animals produced by repeated inbreeding so that each pair of autosomes within an individual is identical.

Synkaryon A hybrid cell that results from the fusion of the nuclei it carries.

Systemic lupus erythematosus (SLE)  An autoimmune disease of humans usually involving anti-nuclear antibodies.

Systole  Period of ventricular contraction.