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Modern Genetic Analysis Glossary
S
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 phase The part of interphase of the cell cycle in which DNA synthesis occurs.
S-9 mix A liver-derived supernatant used in the Ames test to activate or inactive mutagens.
SARs Scaffold attachment regions; the positions along 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 A chromosome that seems to be an addition to the normal genome.
satellite DNA Any type of highly repetitive DNA; formerly defined as DNA forming a satellite band after cesium chloride density gradient centrifugation.
saturation mutagenesis Induction and recovery of large numbers of mutations in one area of the genome or in one function, in the hope of identifying all the genes in that area or affecting that function.
scaffold The central framework of a chromosome to which the DNA solenoid is attached as loops; composed largely of topoisomerase.
SCE See sister-chromatid exchange.
second-division segregation pattern A pattern of ascospore genotypes for a gene pair showing that the two al-leles separate into different nuclei only at the second meiotic division, as a result of a crossover between that gene pair and its centromere; can be detected only in a linear ascus.
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 the double mutant has the wild-type phenotype.
secondary sexual characteristics The sex-associated phenotypes of somatic tissues in sexually dimorphic animals.
secondary structure of a protein A spiral or zigzag arrangement of the polypeptide chain.
sector An area of tissue whose phenotype is detectably different from the surrounding tissue phenotype.
sedimentation The sinking of a molecule under the opposing forces of gravitation and buoyancy.
segment-polarity gene In Drosophila, a member of the class of genes that contribute to the final aspects of establishing the correct number of segments. Segment-polarity mutations cause a loss of a comparable part of each of the body segments.
segmental identity The process by which the A/P fates of the segments are established.
segmentation The process by which the correct number of segments is established in a developing segmented animal.
segregation (1) Cytologically, the separation of homologous structures. (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).
segregational petite A petite that, in a cross with wild type, produces 1/2 petite and 1/2 wild-type progeny; caused by a nuclear mutation.
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.
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 To fertilize eggs with sperms from the same individual.
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.
semiconservative replication The established model of DNA replication in which each double-stranded molecule is composed of one parental strand and one newly polymerized strand.
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.
sequence-tagged site A relatively small sequenced region of a cloned genomic fragment that can be used by a computer to align the cloned fragment into a contig.
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.
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 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.
sexduction Sexual transmission of donor E. coli chromosomal genes on the fertility factor.
sexual spore. See spore.
shotgun technique Cloning a large number of different DNA fragments as a prelude to selecting one particular clone type for intensive study.
shuttle vector A vector (for example, a plasmid) constructed in such a way that it can replicate in at least two different host species, allowing a DNA segment to be tested or manipulated in several cellular settings.
sickle-cell anemia Potentially lethal human disease caused by a mutation in a gene encoding the oxygen-transporting molecule hemoglobin. The altered molecule causes red blood cells to be sickle shaped. Inherited as an autosomal recessive.
signal sequence The N-terminal sequence of a secreted protein, which is required for transport through the cell membrane.
signal transduction cascade A series of sequential events, such as protein phosphorylations, that pass a signal received by a transmembrane receptor through a series of intermediate molecules until final regulatory molecules, such as transcription factors, are modified in response to the signal.
silencer A cis-regulatory sequence that can reduce levels of transcription from an adjacent promoter.
silent mutation Mutation in which the function of the protein product of the gene is unaltered.
SINE (short interspersed element) A type of small repetitive DNA sequence found throughout a eukaryotic genome.
sister-chromatid exchange (SCE) An event similar to crossing-over that can take place between sister chromatids at mitosis or at meiosis; detected in harlequin chromosomes.
site-specific recombination Recombination between two specific sequences that need not be homologous; mediated by a specific recombination system.
solenoid structure The supercoiled arrangement of DNA in eukaryotic nuclear chromosomes produced by coiling the continuous string of nucleosomes.
somatic cell A cell that is not destined to become a gamete; a "body cell," whose genes will not be passed on to future generations.
somatic-cell genetics Asexual genetics, including the study of somatic mutation, assortment, crossing-over, and cell fusion.
somatic mutation A mutation in a somatic cell.
somatostatin A human growth hormone.
SOS repair The error-prone process whereby gross structural DNA damage is circumvented by allowing replication to proceed past the damage through imprecise polymerization.
Southern blot Transfer of electrophoretically separated fragments of DNA from a gel to an absorbent sheet such as paper. This sheet is then immersed in a solution containing a labeled probe that will bind to a fragment of interest.
spacer DNA DNA found between genes; its function is unknown.
specialized (restricted) transduction The situation in which a particular phage will transduce only specific regions of the bacterial chromosome.
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.
spindle The set of microtubular fibers that appear to move eukaryotic chromosomes during division.
splicing The reaction that removes introns and joins together exons in RNA.
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 plantsthat is, the stage in which meiosis occurs.
SSLP (short sequence length polymorphisms) The presence of different numbers of short repetitive elements (mini- and microsatellite DNA) at one particular locus in different homologous chromosomes; heterozygotes represent useful markers for genome mapping.
stacking The packing of the flattish nitrogen bases at the center 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.
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.
stem cell See blast cell.
steroid hormone A class of hormones synthesized by glands of the endocrine system that, by virtue of their nonpolar nature, are able to pass directly through the plasma membrane of cells. Steroid hormones act by binding to and activating transcription factors called steroid hormone receptors.
steroid hormone receptor A family of related proteins that act as transcription factors when bound to their cognate hormones. Not all members of this family actually bind to steroids; the name derives from the first family member to have been discovered, which was indeed a steroid hormone receptor.
strain A pure-breeding lineage, usually of haploid organisms, bacteria, or viruses.
structural gene A gene encoding the amino acid sequence of a protein.
structural genomics characterizing and locating the entire set of genes in a genome.
STS See sequence-tagged site.
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.
supercoil A closed, double-stranded DNA molecule that is twisted on itself.
superinfection Phage infection of a cell that already harbors 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 A secondary mutation that can cancel the effect of a primary mutation, resulting in wild-type phenotype.
suppressor mutation A mutation that counteracts the effects of another mutation. A suppressor maps at a different site from the mutation that it counteracts, either within the same gene or at a more distant locus. Different suppressors act in different ways.
survival factor A ligand that, when bound to a receptor, blocks the activation of the apoptosis pathway.
synapsis Close pairing of homologs at meiosis.
synaptonemal complex A complex structure that unites homologs during prophase of meiosis.
syncytial blastoderm In insects, the stage of blastoderm preceding the formation of cell membranes around the individual nuclei of the early embryo.
syncytium A single cell with many nuclei.
synonymous substitution Mutational replacement of an amino acid with one of similar chemical properties.
syntenic Description of DNA segments in which the gene order is identical in different related species.

© 1999 by W. H. Freeman and Company. All rights reserved.