S phase Period of a eucaryotic cell cycle in which DNA is synthesized. Saccharomyces Genus of yeasts that reproduce asexually by budding or
sexually by conjugation. Economically important in brewing and baking, they
are also widely used in genetic engineering and as simple model organisms in
the study of eucaryotic cell biology. sarcoma Cancer of connective tissue. sarcomere Repeating unit of a myofibril in a muscle cell, composed of an
array of overlapping thick (myosin) and thin (actin) filaments between two
adjacent Z discs. sarcoplasmic reticulum Network of internal membranes in the cytoplasm of a
muscle cell that contains high concentrations of sequestered Ca2+ which is
released into the cytosol during muscle excitation. satellite DNA Regions of highly repetitive DNA from a eucaryotic
chromosome, usually identifiable by its unusual nucleotide composition.
Satellite DNA is not transcribed and has no known function. saturated Describes a molecule containing carbon carbon bonds that has only
single covalent bonds. scaffold protein Protein that organizes groups of interacting intracellular
signaling proteins into signaling complexes. scanning electron microscope Type of electron microscope that produces an
image of the surface of an object. S-Cdk Complex formed in vertebrate cells by an S-cyclin and the
corresponding cyclin-dependent kinase (Cdk). Schwann cell Glial cell responsible for forming myelin sheaths in the
peripheral nervous system. SDS polyacrylamide gel electrophoresis (SDS-PAGE) Type of
electrophoresis in which the protein mixture to be separated is run through a
gel containing the detergent sodium dodecyl sulfate (SDS) which unfolds the
proteins and frees them from association with other molecules. second messenger Small molecule that is formed in or released into the
cytosol in response to an extracellular signal and helps to relay the signal to the
interior of the cell. Examples include cAMP, IP3, and Ca2+. secondary immune response Adaptive immune response to an antigen that is
made on a second or subsequent encounter with a given antigen. It is more
rapid in onset, stronger, and more specific than the primary immune response. secondary structure Regular local folding pattern of a polymeric molecule. In
proteins, a helices and b sheets. secretory vesicle Membrane-bounded organelle in which molecules destined
for secretion are stored prior to release. Sometimes called secretory granule
because darkly staining contents make the organelle visible as a small solid
object. section A very thin slice of tissue, suitable for viewing under the microscope. 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. selectivity filter That part of an ion channel structure that determines which
ions it can transport. septate junction Main type of occluding cell junction in invertebrates; their
structure is distinct from that of vertebrate tight junctions. serine protease Type of protease that has a reactive serine in the active site. sex chromosome Chromosome that may be present or absent, or present in a
variable number of copies, according to the sex of the individual. In mammals,
the X and Y chromosomes. sexual reproduction Type of reproduction in which the genomes of two
individuals are mixed in the formation of a new organism. Individuals
produced by sexual reproduction differ from either of their parents and from
each other. SH2 domain Src homology region 2, a protein domain present in many
signaling proteins; it binds a short amino acid sequence containing a
phosphotyrosine. side chain The part of an amino acid that differs between different amino
acids, giving the amino acid its unique physical and chemical properties. signal molecule Extracellular or intracellular molecule that cues the response
of a cell to the behavior of other cells or objects in the environment. signal patch Protein sorting signal that consists of a specific three-dimensional
arrangement of atoms on the folded protein's surface. signal peptidase Enzyme that removes a terminal signal sequence from a
protein once the sorting process is complete. 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. 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. signal transduction Relaying of a signal by conversion from one physical or
chemical form to another. In cell biology, the process by which a cell converts
an extracellular signal into a response. single-nucleotide polymorphism (SNP) Variation between individuals at
certain nucleotide positions in the genome. single-pass transmembrane protein Membrane protein in which the
polypeptide chain crosses the lipid bilayer only once. single-strand DNA-binding 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. sister chromatid see chromatid site-directed mutagenesis Technique by which a mutation can be made at a
particular site in DNA. site-specific recombination Type of recombination that does not require
extensive similarity in the two DNA sequences undergoing recombination.
Can occur between two different DNA molecules or within a single DNA
molecule. small intracellular mediator see second messenger small nuclear RNA (snRNA) Small RNA molecules that are complexed with
proteins to form the ribonucleoprotein particles involved in RNA splicing. smooth endoplasmic reticulum (smooth ER) Region of the endoplasmic
reticulum not associated with ribosomes. It is involved in lipid synthesis. smooth muscle cell Type of long, spindle-shaped mononucleate muscle cell
making up the muscular tissue found in the walls of arteries and of the
intestine and other viscera, and in some other locations of the vertebrate body.
Called "smooth" because it lacks the striated myofibrils of skeletal and cardiac
muscle cells. SNAREs Large family of transmembrane proteins present in organelle
membranes and the vesicles derived from them. They are involved in guiding
vesicles to their correct destinations. They exist in pairs a v-SNARE in the
vesicle membrane that binds specifically to a complementary t-SNARE in the
target membrane. SNP see single-nucleotide polymorphism snRNA see small nuclear RNA solute Any molecule that is dissolved in a liquid. The liquid is called a solvent. somatic cell Any cell of a plant or animal other than a germ cell or germ-cell
precursor. (From Greek soma, body.) somite One of a series of paired blocks of mesoderm that form during early
development and lie on either side of the notochord in a vertebrate embryo.
They give rise to the vertebral column, muscles and associated connective
tissue. Each somite produces the musculature of one vertebral segment, plus
associated connective tissue. sorting signal Amino acid sequence that directs the delivery of a protein to a
specific location outside the cytosol. Southern blotting Technique in which DNA fragments separated by
electrophoresis are immobilized on a paper sheet. Specific fragments are then
detected with a labeled nucleic acid probe. (Named after E.M. Southern,
inventor of the technique.) spectrin Abundant protein associated with the cytosolic side of the plasma
membrane in red blood cells, forming a rigid network that supports the
membrane. Spemann's Organizer Specialized tissue at the dorsal lip of the blastopore in
an amphibian embryo; a source of signals that help to orchestrate formation of
the embryonic body axis. (After H. Spemann and H. Mangold, co-discoverers.) sperm (spermatozoon,spermatozoa) The mature male gamete in animals. It
is motile and usually small compared with the egg. spermatogenesis Development of sperm. spindle-attachment checkpoint Checkpoint that operates during mitosis to
ensure that all chromosomes are properly attached to the spindle before sisterchromatid
separation starts. spliceosome Large assembly of RNA and protein molecules that performs premRNA
splicing in eucaryotic cells. Src family Family of cytoplasmic tyrosine kinases (pronounced "sark") that
associate with the cytoplasmic domains of some enzyme-linked receptors (for
example, the T cell antigen receptor) that lack intrinsic tyrosine kinase activity.
They transmit a signal onwards by phosphorylating the receptor itself and
other signaling proteins. SRP see signal-recognition particle standard free-energy change (DG°) Free-energy change of two reacting
molecules at standard temperature and pressure when all components are
present at a concentration of 1 mole per liter. starch Polysaccharide composed exclusively of glucose units, used as an
energy storage material in plant cells. start-transfer signal Short amino-acid sequence that enables a polypeptide
chain to start being translocated across the endoplasmic reticulum membrane
through a protein translocator. Multipass membrane proteins have both Nterminal
(signal sequence) and internal start-transfer signals. stem cell Relatively undifferentiated cell that can continue dividing
indefinitely, throwing off daughter cells that can undergo terminal
differentiation into particular cell types. stereocilium A large, rigid microvillus found in "organ pipe" arrays on the
apical surface of hair cells in the ear. A stereocilium contains a bundle of actin
filaments, rather than microtubules, and is thus not a true cilium. steroid Hydrophobic lipid molecule with a characteristic four-ringed structure.
Many important hormones such as estrogen and testosterone are steroids. (See
Panel 2 5, pp. 118 119.) stimulatory G protein (Gs) G protein that, when activated, activates the
enzyme adenylyl cyclase and thus stimulates the production of cyclic AMP. 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 (12 49). strand-directed mismatch repair see mismatch repair striated muscle Muscle composed of transversely striped (striated) myofibrils.
Skeletal and heart muscle of vertebrates are the best-known examples. stroma (1) The connective tissue in which a glandular or other epithelium is
embedded. (2) The large interior space of a chloroplast, containing enzymes
that incorporate CO2 into sugars. structural gene Region of DNA that codes for a protein or for an RNA
molecule that forms part of a structure or has an enzymatic function.
Distinguished from regions of DNA that regulate gene expression. substrate Molecule on which an enzyme acts. substratum Solid surface to which a cell adheres. subunit Component of a multicomponent complex for example, one protein
component of a protein complex or one polypeptide chain of a multichain protein. sucrose Disaccharide composed of one glucose unit and one fructose unit. The
major form in which glucose is transported between plant cells. sugar Small carbohydrates with a monomer unit of general formula (CH2O)n.
Examples are the monosaccharides glucose, fructose and mannose, and the
disacharide sucrose (composed of a molecule of glucose and one of fructose
linked together). sulfhydryl (thiol, SH) Chemical group containing sulfur and hydrogen
found in the amino acid cysteine and other molecules. Two sulfhydryls can
join to produce a disulfide bond. supercoiled DNA Region of DNA in which the double helix is further twisted
on itself. (6 20.) survival factor Extracellular signal required for a cell to survive; in its
absence the cell will undergo apoptosis and die. symbiosis Intimate association between two organisms of different species
from which both derive a long-term selective advantage. symporter Carrier protein that transports two types of solute across the
membrane in the same direction. 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; in an electrical synapse a direct connection is made
between the cytoplasms of the two cells via gap junctions. synapsis (1) In genetic recombination, the initial formation of base pairs
between complementary DNA strands in different DNA molecules that occurs
at sites of crossing-over between chromosomes. (2) In meiosis, the pairing of
maternal and paternal copies of a chromosome as they become attached to
each other along their length. synaptic signaling Type of cell cell communication that occurs across
chemical synapses in the nervous system. synaptic vesicle Small neurotransmitter-filled secretory vesicle formed at the
axon terminals of nerve cells and whose contents are released into the synaptic
cleft by exocytosis when an action potential reaches the axon terminal. synaptonemal complex Structure that holds paired chromosomes together
during prophase I of meiosis and promotes genetic recombination. 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. synteny The presence in different species of regions of chromosomes with the
same genes in the same order.
