a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z

S (Svedberg unit) A unit of sedimentation velocity, commonly used to describe molecular units of various sizes (because sedimentation velocity is related to size). S-9 mix A liver-derived cell extract used in the Ames test to activate or inactivate chemicals in a test for mutagens and potential carcinogens. sampling distribution The distribution of frequencies with which various possible events could occur or a probability distribution defined by a particular mathematical expression . sarcoma tumour of mesodermal origin (eg muscle, bone, cartilage). SAR scaffold attachment region - the position along eukaryotic DNA where it is anchored to the central scaffold of the chromosome. satellite A terminal section of a chromosome, separated from the main body of the chromosome by a narrow constriction. satellite chromosome chromosome that seems to be an addition to the normal genome. satellite DNA DNA that forms a separate band in a bouyant density gradient because of its different nucleotide composition (A:T rich DNAs are less dense than G:C rich DNAs). Highly repetitive eukaryotic DNA primarily located around centromeres. Satellite DNA usually has a different buoyant density than the rest of the cell's DNA. saturation mutagenesis Induction and recovery of large numbers of mutations in one area of a genome, or in one biological function, in the hope of identifying all the genes in that area, or affecting that function. scaffold The eukaryotic chromosome structure remaining when DNA and histones have been removed; made from nonhistone proteins. The central framework of a chromosome to which the DNA solenoid is attached as loops; composed largely of topoisomerase. scanning hypothesis Proposed mechanism by which the eukaryotic ribosome recognizes the initiation region of an mRNA after binding the 5' capped end of it. The ribosome scans the mRNA for the initiation codon. scientific method A procedure used by scientists to test hypotheses by making predictions about the outcome of an experiment before the experiment is performed. The results provide support or refutation of the hypothesis. screening technique A technique to determine the genotype or phenotype of an organism. second division segregation (SDS) The allele arrangement (2+2+2+2) in the spores of Ascomycetes with ordered spores that indicates a crossover between a locus and its centromere. A pattern of ascospore genotypes for a gene pair showing that the two alleles segregate into different nuclei only at the second meiotic division, as a result of a single crossover between that gene pair and its centromere; can only be detected in a ordered ascus. See First division segregation second-site mutation The second mutation of a double mutation within a gene. In many cases, the second-site mutation suppresses the first mutation, so that the double mutant has the wild-type phenotype. secondary oocyte A cell formed by meiosis I in female animals. secondary sexual character The sex-associated phenotype of somatic tissues in sexually dimorphic animals. secondary spermatocyte The product of the first meiotic division in male animals. secondary structure The alpha-helical or beta-sheet configuration of the polypeptide backbone of a protein. sector An area of tissue or colony whose phenotype is detectably different from the surrounding tissue or colony phenotype. sedimentation The sinking of a molecule under the opposing forces of gravitation and buoyancy. segmentation The process by which the correct number of segments are established in a developing segmented animal. segregation (1) Cytologically, the separation of homologous chromosomes into different cells at cell division. (2) Genetically, the production of two separate phenotypes, corresponding to two alleles of a gene, either in different individuals (meiotic segregation) or in different tissues (mitotic segregation). segregation, rule of Mendel's first principle describing how genes are passed from one generation to the next. segregational load Genetic load caused when a population is segregating less fit homozygotes because of heterozygote advantage. segregational petite A petite that in a cross with wild-type produces 50% petite and 50% wild-type progeny; caused by a nuclear mutation. selection (1) Genetic methods for obtaining desired strains often employ selective procedures based on differential survival under particular nutritional or envirnomental conditions. See conditional mutation. (2) Genetic breeding methods start by selecting particular desirable phenotypes as parents for the next generation. (3) See also natural selection. selection coefficient (s) The proportional excess or deficiency of fitness of one genotype in relation to another genotype. selection differential The difference between the mean of a population and the mean of the individuals selected to be parents of the next generation. selection progress The difference between the mean of a population and the mean of the offspring in the next generation born to selected parents. selection-mutation equilibrium An equilibrium allele frequency resulting from the balance between selection against the allele and mutation recreating this allele. selective medium A culture medium that is enriched with a particular substance to allow the growth of particular strains of organisms. selective neutrality A situation in which different alleles of a certain gene confer equal fitness. selective system An experimental technique that enhances the recovery of specific (usually rare) genotypes. self The fusion of male and female gametes from the same individual. See self-fertilization. self-assembly The ability of certain multimeric biological structures to assemble from their component parts through random movements of the molecules and formation of weak chemical bonds between surfaces with complementary shapes. self-fertilization Fertilization in which the two gametes are from the same individual. selfish DNA A segment of the genome with no apparent function other than to ensure its own replication. semiconservative replication The mode by which DNA replicates. Each strand acts as a template for a new double helix. The established model of DNA replication in which each double-stranded molecule is composed of one parental strand and one newly polymerized strand. See template. semisterility (half sterility) The phenotype of individuals heterozygotic for certain types of chromosome aberration; expressed as a reduced number of viable gametes and hence reduced fertility. Nonviability of a proportion of gametes or zygotes. Sequence tagged site (STS) Any site in a chromosome or genome that is identified by a known unique DNA sequence. STSs can be used to form genetic maps by standard mapping procedures. Ser Serine (an amino acid). sex chromosome A chromosome whose presence or absence is correlated with the sex of the bearer; a chromosome that plays a role in sex determination. Heteromorphic (different shaped, eg X and Y) chromosomes whose distribution in a zygote determines the sex of the organism. sex determination The genetic or environmental process by which the sex of an individual is established. sex linkage The location of a gene on a sex chromosome. sex linked The inheritance pattern of loci located on the sex chromosomes (usually the X chromosome in XY species); also refers to the loci themselves. sex reversal A syndrome known in humans and mice in which chromosomally XX individuals develop as males. In some cases, sex reversal is now known to be due to the translocation of the testis-determining region of the Y chromosome to the tip of the X chromosome in such individuals. sex switch A gene, normally found on the Y chromosome in mammals, that directs the indeterminate gonads toward development as testes. See testis-determining factor. sex-controlled trait Traits that appear more of time in one sex than in the other. sex-determining region Y (SRY) The sex switch or testis-determining factor in human beings located on the Y chromosome. (Sry in mice.) sex-lethal A gene in Drosophila, located on the X chromosome, that is a sex switch directing development toward femaleness when it is in the on state. It is regulated by numerator and denominator elements that act to influence the genic balance equation (X/A). sex-limited trait Trait expressed in only one sex. It may be controlled by sex linked or autosomal loci. sex-ratio phenotype A trait in Drosophila whereby females produce mostly if not only daughters. sexduction Sexual transmission of donor Escherichia coli chromosomal genes on the fertility factor. A process whereby a bacterium gains access to and incorporates foreign DNA brought in by a modified F factor during conjugation. sexual selection The forces determined by mate choice acting to cause one genotype to mate more frequently than another genotype. Shine-Delgarno hypothesis A proposal that prokaryotic mRNA is aligned at the ribosome by complementarity between the mRNA upstream from the initiation codon and the 3' end of the 16S rRNA. shotgun technique Cloning a large population of different DNA fragments, known to contain a fragment of interest, as a prelude to selecting or screening for that one particular clone containing the fragment of interest for intensive study. shuttle vector A vector (e.g. a plasmid) constructed in such a way that it can replicate in at least two different host species (eg a prokaryote and a eukaryote). A DNA recombined into such a vector can be tested or manipulated in several cell types. siblings(sibs) Brothers and sisters. sickle-cell anemia Potentially lethal human disease, inherited as an autosomal recessive, caused by a mutation in a gene coding for the beta subunit of the oxygen-transporting protein haemoglobin. Under conditions of low oxygen tension, the altered beta-globin molecule causes haemoglobin to aggregate forming rod-like arrays that distort the cell membrane forcing red blood cells to become sickle-shaped. The sickled red blood cells are damaged and rapidly removed from the circulation causing anaemia. sigma factor The protein that gives promoter-recognition specificity to the RNA polymerase core enzyme of bacteria. signal hypothesis The major mechanism whereby proteins that insert into or cross a membrane are synthesized by a membrane-bound ribosome. The first thirteen to thirty-six amino acids synthesized, termed a signal peptide, are recognized by a signal recognition particle that draws the ribosome to the membrane surface by interaction with a docking protein. The signal peptide may later be removed from the protein. signal sequence The N-terminal sequence of a secreted protein, which is required for transport through the cell membrane. signal transduction cascade Refers to a series of sequential events, such as protein phosphorylations, consequent upon binding of ligand by a transmembrane receptor, that transfer a signal through a series of intermediate molecules until final regulatory molecules, such as transcription factors, are modified in response to the signal. silent mutation mutation in which the function of the protein product of the gene is unaltered. SINE Short interspersed element. A type of small dispersed repetitive DNA sequence (eg Alu family in the human genome) found throughout a eukaryotic genome. single-strand binding protein Protein that binds to single-stranded DNA usually near the replication fork to stabilize the single strands. sister-chromatid exchange (SCE) An event similar to crossing over that can occur between sister chromatids at mitosis or at meiosis. site-specific recombination A crossover event, such as the integration of phage lambda, that requires homology of only a very short region and uses an enzyme specific for that recombination. Recombination occurring between two specific sequences that need not be homologous; mediated by a specific recombination system. skew A distortion of the shape of the normal distribution toward one side or the other. snRNP Small nuclear ribonucleoprotein (RNA plus protein) particle. Component of the spliceosome, the intron-removing apparatus in eukaryotic nuclei . sociobiology The study of the evolution of social behavior in animals. solenoid structure The supercoiled arrangement of DNA in eukaryotic nuclear chromosomes produced by coiling the continuous string of nucleosomes (about 7 nucleosomes per turn). somatic cell A cell that is not destined to become a gamete; a cell whose genes will cannot be passed on to future generations. somatic doubling A disruption of the mitotic process that produces a cell with twice the normal chromosome number. somatic hypermutation The occurrence of a high level of mutation in the variable regions of immunoglobulin genes. somatic mutation A mutation occurring in a somatic cell. somatic-cell genetics Asexual genetics, involving study of somatic mutation, mitotic crossing-overand segregation. somatic-mutation theory A theory to account for the high degree of antibody variability. It suggests that mutation of a basic immunoglobulin gene accounts for all the different types of immunoglobulins produced by B lymphocytes. See germ-line theory. somatostatin A human growth hormone. SOS box The region in the promoter of various genes that is recognized by the LexA repressor. Release of repression results in the induction of the SOS response. SOS repair The error-prone process whereby gross structural DNA damage is circumvented by allowing replication to proceed past the damage through imprecise polymerization. SOS response Repair systems (recA; uvr) induced by the presence of single-stranded DNA that usually occurs from postreplicative gaps caused by various types of DNA damage. The RecA protein, stimulated by single-stranded DNA, is involved in the inactivation of the LexA repressor thereby inducing the response. Southern blot Transfer of electrophoretically separated fragments of DNA, after denaturation, from the gel to an absorbent sheet of material, such as nitrocellulose, to which the DNA binds. The sheet is immersed in a solution containing a labeled probe that will hybridize to fragment(s) of interest. The method was first devised by E. M. Southern to transfer DNA fragments from an agarose gel to a nitrocellulose paper for hybridization, but similar transfer methods are now also used for transfering RNA or protein to papers of a variety of types followed by hybridization (RNA) or labeled antibody treatment (protein) to identify specific molecules. spacer DNA Regions of non-transcribed DNA between transcribed repeated genes such as ribosomal RNA genes in eukaryotes. Its function is probably to do with ensuring the high rates of transcription associated with these genes. specialized (restricted) transduction The situation in which a particular phage will transduce only specific regions of the bacterial chromosome. Form of transduction based on faulty looping out by a temperate phage. Only loci neighbouring the attachment site can be transduced. See generalized transduction. speciation A process whereby over time one species evolves into a different species (anagenesis) or whereby one species diverges to become two or more species (cladogenesis). species A group of organisms belong to the same biological species if they are capable of interbreeding to produce fertile offspring. However the biological test of a species is not always available, and so there is also a morphological species concept based on anatomical similarities. specific-locus test A system for detecting recessive mutations in diploids. Normal individuals treated with mutagen are mated to testers that are homozygous for the recessive alleles at a number of specific loci; the progeny are then screened for recessive phenotypes. sperm cell The male gamete. spermatid The four products of meiosis in males that develop into sperm. spermatocyte (1) (Zoological) A cell which undergoes meiosis and thereby produces spermatids. See primary spermatocyte and secondary spermatocyte. (2) (Botanical) A cell that becomes converted to a spermatozoid (a small motile male gamete with flagella) without intervention of cell division. spermatogenesis The process of sperm production. spermatogonium A cell type in the testes of male vertebrates that gives rise to primary spermatocytes by mitosis. spermiogenesis The process by which spermatids mature into sperm cells. spindle The set of tubulin fibers that appear to move eukaryotic chromosomes during division. The microtubule apparatus that controls chromosome movement during mitosis and meiosis. spiral cleavage The cleavage process in molluscs and some invertebrates whereby orientation of the spindle at mitosis is at an angle to the original egg axis. spirillum A spiral bacterium. spliceosome Protein-RNA complex that removes introns from eukaryotic nuclear RNAs. splicing The reaction that removes introns and joins together exons in eukaryotic nuclear primary RNA transcripts. spontaneous mutation A mutation occurring in the absence of mutagens, usually due to errors in the normal functioning of cellular enzymes. spore (1) In plants and fungi, sexual spores are the haploid cells produced by meiosis. (2) In fungi, asexual spores are somatic cells that are cast off to act either as gametes or as the initial cells for new haploid individuals. sporophyte The diploid sexual-spore-producing generation in the life cycle of plants. The stage in which meiosis occurs. The stage of a plant life cycle that produces spores by meiosis and alternates with the gametophyte stage. stabilizing selection A type of selection that removes individuals from both ends of a phenotypic distribution thus maintaining the same distribution mean. stacking The packing of the flattish nitrogen base-pairs at the centre of the DNA double helix. staggered cuts The cleavage of two opposite strands of duplex DNA at points near one another. standard deviation The square root of the variance. standard error of the mean The standard deviation divided by the square root of the sample size. It is the standard deviation of a sample of means. stasipatric speciation Instantaneous speciation caused by polyploidy. statistic A computed quantity characteristic of a population, such as the mean. statistical distribution The array of frequencies of different quantitative or qualitative classes in a population. statistics Measurements of attributes of a sample from a population. See parameters. stem-loop structure A lollipop-shaped structure formed when a single-stranded nucleic acid molecule loops back on itself to form a complementary double helix (stem) topped by a loop. steroid receptor A family of related proteins that act as transcription factors when bound to their cognate hormone ligands. Not all members of this family actually bind to steroids; the name derives from the first family member that was discovered, which was a steroid hormone receptor. stochastic A process with an indeterminate or random element as opposed to a deterministic process that has no random element. strain A pure-breeding lineage, usually of haploid organisms, bacteria, or viruses. stringent factor A protein catalyzing the formation of an unusual nucleotide (guanosine tetraphosphate, ppGpp) during the stringent response under amino acid starvation conditions. stringent response A translational control mechanism of prokaryotes that represses tRNA and rRNA synthesis during amino acid starvation. structural gene A gene encoding the amino acid sequence of a protein. Non-regulatory gene. submetacentric chromosome A chromosome whose centromere lies between its middle and its end but closer to the middle. subtelocentric chromosome A chromosome whose centromere lies between its middle and its end but closer to the end. subvital gene A gene that causes the death of some proportion (but not all) of the individuals that express it. sum rule The probability that one or the other of two mutually exclusive events will occur is the sum of their individual probabilities. The rule that states that the probability of the occurrence of mutually exclusive events is the sum of the probabilities of the individual events. supercoil A closed double-stranded DNA molecule that is twisted on itself. supercoiling Negative (tending to unwind the helix) or positive (tending to wind the helix) coiling of double-stranded DNA that differs from the relaxed state. supergene Several loci, which usually control related aspects of the phenotype, in close physical association. superinfection Phage infection of a cell that already harbours a prophage. supersuppressor A mutation that can suppress a variety of other mutations; typically a nonsense suppressor. suppressive petite A petite that in a cross with wild-type produces progeny of which variable non-Mendelian proportions are petite. suppressor gene A gene that, when mutated, apparently restores the wild-type phenotype to a mutation at another locus. suppressor mutation A mutation that counteracts the effects of another mutation. A suppressor maps at a different site than the mutation it counteracts, either within the same gene or at a more distant locus. Different suppressors act in different ways. survival of the fittest In evolutionary theory the survival of only those organisms best able (fittest) to obtain and utilize resources. This phenomenon is the cornerstone of Darwin's theory of evolution by natural selection. Svedberg unit A unit of sedimentation rate during centrifugation. Abbreviation is S as in 50S. swivelase speciation in which the evolution of reproductive isolating mechanisms occurs within the range and habitat of the parent species. This form of speciation may be common in parasites. synapsis The point-by-point pairing of homologous chromosomes during zygotene or in certain Dipteran tissues (eg Drosophila salivary glands) that undergo endomitosis. Close pairing of homologs at meiosis. synaptonemal complex A proteinaceous complex that apparently mediates synapsis during zygotene stage and then disintegrates. A complex structure that unites homologous chromosomes during the prophase of meiosis. syncytial blastoderm In insects, the syncytial stage of blastoderm preceding the formation of cell membranes around the individual nuclei of the early embryo. syncytium A single cell with many nuclei. synteny All loci on one chromosome are said to be syntenic (literally on the same ribbon). Loci may appear to be unlinked by conventional genetic tests for linkage but still be syntenic. synteny test A test that determines whether two loci belong to the same linkage group (ie are syntenic) by observing concordance (occurence of markers together) in hybrid cell lines. synthetic medium A chemically defined substrate upon which organisms are grown.