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tic salmon and S. gairdneri, the rainbow trout. See Appendix A, Chordata, Osteichythes, Neopterygii, Salmoniformes. Salmonella together with Escherichia and Shigella, one of the genera of enteric bacteria containing fa- vorite species for genetic study. Salmonella typhimu- rium, the mouse typhoid bacillus, has been the most studied, but considerable work has also been done with S. abony, S. flexneri, S. minnesota, S. montevi- deo, S. pullorum, and S. typhosa.

The first method for assaying bacteriophage titer was worked out for S. typhimurium, and the tranduction phenomenon was also discovered in this species. See Appendix A, Bac- teria, Proteobacteria; Appendix C, 1917, d’Herelle; 1952, Zinder and Lederberg; histidine operon, viru- lence plasmid. saltation 1. a theory that new species originate suddenly from one or more mutations with large phenotypic effects (“macromutations”); referred to by R. Goldschmidt as “hopeful monsters.” 2. quan- tum speciation (q.v.). See evolution. saltatory replication lateral amplification of a chromosome segment to produce a large number of copies of a specific DNA sequence. See gene ampli- fication, rDNA amplification. salt linkage See electrostatic bond. salvage pathways metabolic pathways for the synthesis of nucleosides and nucleotides utilizing preformed purines and pyrimidines or nucleosides.

Examples include the conversion of a base to a nu- cleoside, interconversion of bases and nucleosides, conversion of nucleosides to nucleotides, and inter- conversions by base alterations. Compare with de novo pathway. Salvarsan dihydroxy diamino arsenobenzene dihy- drochloride, a drug used for 30 years in the treat- ment of syphilis until the advent of penicillin. See Appendix C, 1909, Ehrlich; 1940, Florey et al. samesense mutation a point mutation (usually in the third position of a codon) that does not change the amino acid specificity of the codon so altered; a “silent” mutation. See degenerate code. sampling error variability due to the limited size of the samples. Sandhoff disease a lysosomal storage disease (q.v.) with symptoms similar to the Tay-Sachs dis- ease (q.v.). K. Sandhoff and three colleagues pub- lished the first description of the condition in 1968.

The Sandhoff and Tay-Sachs diseases are due to mu- tations involving the same enzyme, hexosaminidase (q.v.). The catalytic form of the enzyme is a hetero- dimer, and each gene encodes a different monomer. Sanger-Coulson method See DNA sequencing tech- niques. saprobe an organism, often a fungus, that lives on and derives its nutrition from dead organic matter. saprophyte a plant saprobe (q.v.). sarcoma a cancer of mesodermal origin (e.g., con- nective tissue). See Rous sarcoma virus, simian sar- coma virus. sarcomere the repeating unit, about 2.5 microme- ters long, within striated muscle fibers, containing a set of interacting actin and myosin filaments. sarcosomes the mitochondria of the flight mus- cles of insects. Sargasso Sea an elliptical region of the northwest Atlantic Ocean covered with huge mats of floating sargassum seaweed. The central area is usually calm with little wind or currents, and it rotates clockwise from the West Indies past Bermuda to the Azores.

SARS severe acute respiratory syndrome. See coro- navirus. sat DNA or RNA satellite DNA or satellite RNA (q.v.). satellite a distal chromosomal segment separated from the rest of the chromosome by a thin chro- matic filament or stalk called the secondary constric- tion. satellite (sat) DNA any fraction of the DNA of an eukaryotic species that differs sufficiently in its base composition from that of the majority of the DNA fragments to separate as one or more bands distinct from the bands containing the majority of the DNA during isopycnic CsCl gradient centrifugation. Satel- lite DNAs obtained from chromosomes are either lighter (A + T rich) or heavier (G + C rich) than the majority of the DNA. Satellite DNAs are usually highly repetitious. See Alu family, mouse satellite DNA, reassociation kinetics, repetitious DNA. satellite (sat) RNA a small, linear, single-stranded RNA molecule that is encapsulated within certain specific plant RNA viruses. An example is the sat RNA of the cucumber mosaic virus, which in its mo- nomeric form is 335 nucleotides long.

The satellite RNA depends on the supporting helper virus to pro- vide a protective coat protein and some of the en- zymes necessary for its replication. The satellite RNA is not necessary for the replication of the

Schizosaccharomyces pombe

helper virus. The presence of the sat RNAs may at- tenuate the symptoms of the helper virus in the in- fected plant. See hairpin ribozyme, hammerhead ribo- zyme, viroid, virusoid. saturation density the maximum cell number at- tainable under specified culture conditions in a cul- ture vessel. This term is usually expressed as the number of cells per square centimeter in an anchor- age-dependent culture, or per cubic centimeters in a suspension culture. saturation hybridization an in vitro reaction in which one polynucleotide component is in great ex- cess, causing all complementary sequences in the other polynucleotide component to enter a duplex form. See DNA-driven hybridization reaction, RNA- driven hybridization. scaffold an ordered series of contigs (q.v.) sepa- rated by gaps whose approximate lengths are known.

See chromosome scaffold, shotgun se- quencing. scanning electron microscope See electron micro- scope. scanning hypothesis a theory to explain initiation of translation in eukaryotes according to which a 40S ribosomal subunit attaches at or near the mRNA cap and then drifts in the 3′ direction until it encounters an AUG start codon; at this point, an initiation complex is formed and the reading frame becomes established. scape a leafless flower-bearing stem arising from ground level (as in the dandelion and daffodil). scarce mRNA See complex mRNA. scatter diagram a diagram in which observations are plotted as points on a grid of x and y coordinates to see if there is any correlation (q.v.).

For example, one might plot for a number of species the LD 50 (q.v.) against the average DNA content per somatic cell nucleus. Finding a correlation suggests that the variables may be interrelated. No correlation sug- gests that the variables chosen have no bearing on one another. scattering the change of direction of particles or waves as a result of a collision or interaction. Scatter- ing of electrons by the specimen, for example, is re- sponsible for the electron microscopic image. SCE sister chromatid exchange (q.v.). Schiff reagent a compound developed about 1866 by Hugo Schiff. The reagent attaches to and colors aldehyde-containing compounds. Used in the PAS and Feulgen procedures (q.v.).

Schiff reagent

Schistocerca gregaria a locust whose population dynamics has been extensively studied. schistosomiasis also called bilharziasis or snail fe- ver; a helminthic infection of humans involving 200 million persons in Africa, the Middle East, the Ori- ent, and South America. The causative agent is the blood fluke Schistosoma mansoni, and its snail vector is Biomphalaria glabrata. The genetic control of sus- ceptibility of B. glabrata to S. mansoni is a subject of active investigation. schizogony a series of rapid mitoses without in- crease in cell size which gives rise to schizonts. schizont a sporozoan spore arising from schi- zogony.

schizophrenia the name given to a constellation of symptoms including hallucinations and delusions, disorders of thinking and concentration, and erratic behavior. Schizophrenia appears to be a family of diseases of high heritability that together afflict about 1 percent of all humans. Sequence variations in the neuregulin gene, Nrg1, have been associated with schizophrenia in Icelandic and Scottish popula- tions, and Nrg1 mutations in mice produce behav- ioral patterns consistent with mouse models of this disease. See neuregulins (NRGs).

Schizophyllum a genus of fungi that form fan- shaped, bracket-type fruiting bodies. S. commune has been the subject of considerable genetic research, and two linkage groups have been mapped. See Ap- pendix A, Fungi, Basidiomycota. Schizosaccharomyces pombe a fungus that can vary in shape from single globose to cylindrical cells that reproduce by septal fission (q.v.). A mitotic spindle forms during G2, and the chromosomes con- dense. However, the nuclear envelope remains in- tact. S. pombe can also form true hyphae (q.v.). Sex- ual reproduction results when haploid cells fuse to produce a zygote. This undergoes meiosis to form an ascus that contains four haploid ascospores. These develop into vegetative cells. Its genome contains 12.5 × 106 bp of DNA. Comparison of the genes of Saccharomyces cerevisiae (q.v.) and Schizosaccharo-

Schwann cell

myces pombe has revealed that genes of the latter are far more likely to contain introns than those from Saccharomyces. S. pombe is a favorite for the study of the genetic regulation of the progression through the cell cycle. See Appendix A, Fungi, Ascomycota; Ap- pendix C, 1976, Nurse, Thuriaux and Nasmyth; 1994, Chikashige et al. Schwann cell the cell that enfolds a myelinated nerve fiber and first described by the German histol- ogist Theodor Schwann. See myelin sheath. Sciara a genus of fungus gnats extensively studied in terms of the cytogenetics of chromosome diminu- tion (q.v.).

The giant polytene chromosomes of the larval salivary glands of S. coprophila have been mapped, and certain DNA puffs have been exten- sively studied. See chromosomal puff, Rhynchosciara. scintillation counter See liquid scintillation counter. scission a severance of both strands of a DNA molecule; a cut (q.v.). Compare with nick. scleroproteins the very stable fibrous proteins, present mainly as surface coverings of animals. Kera- tin and collagen are examples of scleroproteins. scrapie See prion. scurvy See ascorbic acid. scutellum 1. the single cotyledon of a grass em- bryo. 2. a shield-shaped metathoracic tergite of Dro- sophila. SD 1. standard deviation. 2. segregation distortion (q.v.). SD sequence See Shine-Dalgarno sequence. SDS sodium dodecyl sulfate (q.v.). SDS-PAGE technique See electrophoresis. SE, S.E. standard error (q.v.). Se Secretor gene (q.v.). sea floor spreading the horizontal spreading of the sea floor as tectonic plates separate.

A good ex- ample is the floor of the Atlantic Ocean where the mid-Atlantic ridge marks the line of separation be- tween the Eurasian plate, which is moving east, and the North American plate, which is moving west. The rending of the crust is accompanied by out- pouring of the volcanic materials that produce the ridge, while the void between the separating plates is filled with molten rock that rises from below and solidifies. Thus new sea floor is added to the trailing edges of the separating plates as they move laterally,

and consequently the sea floor widens. See Appendix C, 1960, Hess; 1963, Vine and Matthews; continen- tal drift, plate tectonics. Searle translocation a reciprocal X-autosome translocation in the mouse which exhibits paternal X inactivation in the somatic cells of female hetero- zygotes. All other X-autosome translocations stud- ied in the mouse show random inactivation of either normal or rearranged chromosome. See Lyon hy- pothesis.

seasonal isolation a type of ecological reproduc- tive isolation in which different species become re- productively active at different times; temporal iso- lation. sea squirt See Ciona intestinalis. sea urchin See echinoderm. secondary constriction a thin chromatic filament connecting a chromosomal satellite to the rest of the chromosome. secondary DNA See skeletal DNA hypothesis. secondary gametocytes See meiosis. secondary immune response See immune re- sponse. secondary nondisjunction sex chromosomal non- disjunction in an XXY individual resulting in ga- metes containing either two X chromosomes, one X, one Y, or an X and a Y. secondary protein structure See protein structure.

secondary sex ratio the ratio of males to females at birth; in contrast to the primary sex ratio at con- ception. secondary sexual character a characteristic of an- imals other than the organs producing gametes that differs between the two sexes (e.g., mammary glands, antlers, and external genitalia). See primary sexual character, sex combs. secondary speciation the fusion through hybrid- ization of two species that were formerly geographi- cally isolated, followed by establishment of a new adaptive norm through natural selection. second cousin See cousin. second degree relative when referring to a spe- cific individual in a pedigree (q.v.), any individual who is two meioses away from that individual (a grandparent, a grandchild, an uncle, an aunt, a niece, a nephew, or a half-sibling).

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