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somatic recombination 415 Sogin’s first symbiont ber of chromosomes; in tetraploid organisms, so- somatic doubling the doubling of the diploid chro- mosome set. Such doubling may be induced experi-matic cells contain the 4N number, etc. mentally by applying the alkaloid colchicine in a lano- somatic cell genetic engineering correction of lin paste to somatic tissues that are undergoing mitosis. genetic defects in somatic cells by genetic engineer- ing: e.g., insertion of genes for insulin production somatic mutation a mutation occurring in any cell into defective pancreatic cells. Such correction that is not destined to become a germ cell. If the would not be hereditary. mutant cell continues to divide, the individual will come to contain a patch of tissue of genotype differ- somatic cell genetics the genetic study of asexu- ent from the cells of the rest of the body. Compare ally reproducing body cells, utilizing cell fusion tech- with gametic mutation. niques, somatic assortment, and somatic crossing over. See Appendix C, 1964, Littlefield; 1965, Harris somatic pairing the conjoining of the homologous and Watkins; 1967, Weiss and Green; 1969, Boon chromosomes in somatic cells, a phenomenon seen and Ruddle; 1985, Smithies et al. in dipterans. The fact that the polytene chromo- somes of Drosophila undergo somatic pairing makes somatic cell hybrid a hybrid cell resulting from possible the identification of chromosomal rearrange- cell fusion (q.v.). ments, the mapping of deficiencies, and, as a result, the cytological localization of genes. See Diptera, somatic cell nuclear transfer therapeutic cloning transvection. (q.v.). somatic crossing over the exchange of DNA be- somatic recombination genetic recombination that does not involve germ cells but rather somatictween non-sister chromatids in a somatic cell. Also called mitotic crossing over. See mitotic recombina- cells, usually of a specific type and at a particular developmental stage. For example, somatic recombi-tion, site-specific recombination. 416 somatic sex determination nation occurs in developing B lymphocytes (q.v.). V positively charged residues and usually one or more prolines. The targets can occur at a variety of places(D) J recombination (q.v.) results in the joining of any one of many variable Ig gene segments to one of in different nuclear proteins. The peroxisomal-target- ing signal is usually located near the carboxy termi-a few constant segments. The arrangement that re- sults is different in the cells that produce the anti- nus of the protein, and it consists of three amino acids (serine, lysine, and leucine). There are also tar-body from all other somatic cells and germ cells. Compare with somatic crossing over. geting signals that cause specific proteins to be re- tained in the ER or Golgi apparatus or to be targeted somatic sex determination the genetic and devel- to lysosomes. In animals like Caenorhabditis or Dro- opmental process that specifies sexual identity and sophila, about 5% of the proteins contain sorting sig- sex-specific development of the somatic cells of an nals that direct them to mitochondria. However, in organism. Compare with germ line sex determination. plants, like Arabidopsis, nearly 25% of the nuclear See sex determination. genes direct encoded proteins to either chloroplasts somatoclonal variation the appearance of new or mitochondria. See protein sorting. traits in plants that regenerate from a callus in tissue SOS boxes the operator sequences in E. coli DNA culture. Some of the variations represent single nu- that are recognized by a repressor called the LexA cleotide changes; others involve chromosomal trans- protein. This protein represses several loci involved locations, losses, or duplications. Much of the varia- in DNA repair functions. See regulon, SOS response. tion occurs during tissue culture, rather than as a result of unmasking the variation present in the par- SOS response a cellular response to extensive ent plant. See gametoclonal variation. DNA damage in which certain genes, called SOS genes, are sequentially activated in order to repairsomatocrinin growth hormone releasing hormone. See human growth hormone. the damaged DNA. In E. coli about 20 such genes have been identified, including lexA, whose product somatomammotropin See human gro wt h hor mo ne. normally represses the SOS genes. The remainder somatostatin a polypeptide hormone that stimu- include genes such as uvrA, uvrB, recA, sulA, and lates the release of growth hormone by the pituitary umuC. Among the functions assigned to these genes and of insulin and glucagon by the pancreas. The are recombinational repair, nucleotide excision re- gene for this 14 amino acid peptide was chemically pair, inhibition of cell division, and error-prone re- synthesized, spliced into a plasmid, and cloned in E. pair. Normally, SOS genes are repressed by the coli. The transformed bacteria secreted somatostatin, LexA protein, which binds to operator sequences, and this led to the first commercial production of called SOS boxes (q.v.), upstream of each of these a synthetic human protein. See Appendix C, 1977, genes. When DNA is damaged, single-stranded re- Itakura et al.; human growth hormone. gions become exposed, and these interact with the RecA protein (q.v.) to form a complex (RecA*), somatotropin See human growth hormone. which acquires protease activity and facilitates the sonicate subject (a biological sample) to ultrasonic cleavage of the LexA repressor (q.v.). The cleaved vibration so as to fragment the cells, macromole- LexA protein is unable to bind DNA, thus allowing cules, and membranes. A biological sample that has the SOS genes to be de-repressed. When the DNA been subjected to such treatment. has been repaired, RecA becomes inactivated, LexA is no longer cleaved and accumulates in the cell, and Sonic hedgehog (Shh) the vertebrate homolog of the SOS genes are shut down. In addition to accu- the Drosophila gene hedgehog (q.v.). In humans Shh rate, error-free repair, the SOS response also induces has been mapped to 7q36. Shh encodes a signal pro- DNA repair that leads to mutagenesis, i.e., error- tein that controls the patterning of the ventral neural prone or mutagenic repair, in which the DNA tem- tube, the anterior-posterior limb axis, and the ven- plate is read with reduced fidelity. Thus, in the pres- tral somites. ence of extensive DNA damage the cells survive, al- Sordaria fimicola an ascomycete fungus often beit at the cost of introducing some errors in their used in studies of gene conversion (q.v.). DNA. The acronym, SOS, is derived from “Save Our Souls,” the Morse code signal given by ships in sorting See protein sorting. danger and conveys that this is an emergency re- sorting signals segments several amino acids long sponse in cells that are in danger of dying. See Ap- in proteins that target them to their final destina- pendix C, 1967, Witkins. tions. For example, the nuclear-targeting signal is four to eight amino acids in length, and it contains several South African clawed frog See Xenopus. specificity 417 Southern blotting a technique, developed by E. an increase in species number at any time within the lineage; vertical evolution or speciation; phyleticM. Southern, for transferring electrophoretically re- solved DNA segments from an agarose gel to a nitro- evolution or speciation. See Appendix C, 1954, Mayr; 1975, King and Wilson; 1985, Carson; 2000, Singhcellulose filter paper sheet via capillary action. Sub- sequently, the DNA segment of interest is probed and Kulathinal; alloparapatric speciation, allopatric speciation, cichlid fishes, evolution, founder effect,with a radioactive, complementary nucleic acid, and its position is determined by autoradiography. A Hawaiian Drosophilidae, parapatricspeciation, peripa- tric speciation, punctuated equilibrium, selector genes,similar technique, referred to as northern blotting, is used to identify RNAs. For example, an electropher- sex gene pool theory of speciation, sexual selection, silversword alliance, sympatric speciation.ogram containing a multitude of different mRNAs could be probed with a radioactive cloned gene. In species 1. biological (genetic) species: reproduc- cases where proteins have been separated electro- tively isolated systems of breeding populations. 2. pa- phoretically, a specific protein on an electrophero- leospecies (successional species): distinctly different gram can be identified by the western blotting proce- appearing assemblages of organisms as a consequence dure. In this case, the probe is a radioactively labeled of species transformation (q. v.). 3. taxonomic (mor- antibody raised against the protein in question. See phological; phenetic) species: phenotypically distinc- Appendix C, 1975, Southern; 1977, Alwine et al.; tive groups of coexisting organisms. 4. microspecies probe. (agamospecies): asexually reproducing organisms (mainly bacteria) sharing a common morphology and physi- soybean See Glycine max. ology (biochemistry). 5. biosystematic species (eco- spaced training referring to experiments on mem- species) populations that are isolated by ecological ory during which repeated training sessions are given factors rather than ethological isolation (q.v.). with short rest intervals between the sessions. The species group superspecies (q.v.). term massed training refers to repeated training ses- sions with no such rest intervals. Comparisons of the species selection a form of group selection (q.v.) results from both types of experiments have shown in which certain species (produced by cladogenesis) that the memory generated immediately after train- continue the cladogenic process and others become ing is short lived and disruptable. During a rest pe- extinct. riod such short-term memory (STM) is consolidated species transformation the transformation of a into a longer-lasting, more stable, long-term memory species (A) into another (species B) during the pas- (LTM). For the consolidation of STM into LTM, the sage of time. Species transformation does not in- syntheses of the protein products of specific genes crease the number of species, since species A and B are required. See CREBs. do not coexist in time. See anagenesis, speciation, spacer DNA untranscribed segments of eukaryotic vertical evolution. and some viral genomes flanking functional genetic specific activity the ratio of radioactive to non- regions (cistrons). Spacer segments usually contain radioactive atoms or molecules of the same kind. repetitive DNA. The function of spacer DNA is not Sometimes given as the number of atoms of radio- presently known, but it may be important for synap- isotope per million atoms of stable element. Also ex- sis. See transcribed spacer. pressed in curies per mole. special creation a nonscientific philosophy assert- specific immune suppression an immune re- ing that each species has originated through a sepa- sponse in which the initial exposure to a particular rate act of divine creation by processes that are not antigen results in the loss of the ability of the organ- now in operation in the natural world. ism to respond to subsequent exposures of that anti- specialized 1. an organism having a narrow range gen, but not to different antigens. See immunological of tolerance for one or more ecological conditions. 2. tolerance. a species having a relatively low potential for further specific ionization the number of ion pairs per evolutionary change; the opposite of generalized. unit length of path of the ionizing radiation in a specialized transduction See transduction. given medium (per micron of tissue, for example). specificity selective reactivity between substances:speciation 1. the splitting of an ancestral species into daughter species that coexist in time; horizontal e.g., between an enzyme and its substrate, between a hormone and its cell-surface receptor, or betweenevolution or speciation; cladogenesis. 2. the gradual transformation of one species into another without an antigen and its corresponding antibody. 418 specificity factors specificity factors proteins that temporarily asso- transcribed from FOXP2 by alternative splicing (q.v.), and these transcripts are plentiful in the fetalciate with the core component of RNA polymerase and determine to which promoters the enzyme will brain. The most common splice form encodes a pro- tein 715 amino acids long. A segment of this proteinbind (e.g., the sigma factor, q.v.). See antispecificity factor. contains a DNA-binding site, suggesting that it func- tions as a regulator of transcription. Homologs of specimen screen the support for sections to be FOXP2 have been identified in the chimpanzee, the viewed under the electron microscope consisting of gorilla, and the orangutan. The FOXP2 proteins of a disc made of copper or gold mesh. the apes are all identical to each other, but the hu- man protein has different amino acids at two sites. spectrin a protein that is a major component of Perhaps these changes gave new properties to the the plasma membranes of animal cells. It is com- protein which influenced neural systems in ways posed of two different polypeptide chains, alpha and that eventually led to the acquisition of speech. See beta, which form heterodimers. Each polypeptide Appendix C, 2001, Lai et al. contains tandemly repeated sequences that can fold upon themselves and so give the spectrin filament spelt Triticum spelta (N = 21), the oldest of the great flexibility. In the cell membrane, spectrin fil- cultivated hexaploid wheats, grown since the latter aments form a pentagonal network in which their days of Roman Empire. See wheat. ends attach to junctions made of actin and other proteins. Spectrin has been identified as one of the Spemann-Mangold organizer named after Hans molecular components of the spectrosome (q.v.) and Spemann and Hilde Mangold who published the de- the fusome (q.v.)inDrosophila. See peripheral pro- tails of their tissue implantation experiments in tein. 1924. They showed that tissue from the dorsal blas- topore lip of the amphibian gastrula can induce a spectrophotometer an optical system used in biol- secondary body axis in another embryo. The second- ogy to compare the intensity of a beam of light of ary brain and spinal cord did not arise from the specified wave length before and after it passes transplanted cells, but from the presumptive ventral through a light-absorbing medium. See microspec- epidermis of the host. They concluded that the im- trophotometer. planted material contained diffusible “organizing factors” that determined the future differentiation of spectrosome a prominent spectrin-rich, spherical the adjacent host tissues. The Spemann-Mangold or- mass found in the cytoplasm of germ line stem cells ganizer is now known to play a vital role during de- (q.v.) and cytoblasts (q.v.) in the Drosophila ovary. velopment in all members of the Chordata (q.v. ). The spectrosome is rich in cytoskeletal proteins such See blastoporal lip, chordamesoderm, gastrulation, as actin (q.v.), α- and β-spectrin, the adducin-like goosecoid, Triton, xenograft. Hts protein, and ankyrin (q.v.). This organelle is though to be a precursor of the fusome (q.v.) and to S period See cell cycle. anchor the mitotic spindle during germ line stem cell and cystoblast divisions. hts and α-spectrin muta- sperm a single male gamete or spermatozoon. tions eliminate spectrosome and fusome formation Sperm can also refer to multiple male gametes or and result in aberrant mitotic spindle orientation spermatozoa. during germ line stem cell and cystoblast/cystocyte spermateleosis spermiogenesis (q.v.). divisions. See adducin, ankyrin, cystocyte divisions, hu-li tai shao (hts), spectrin. spermatheca the organ in a female or a hermaph- rodite which receives and stores the spermatozoa speech-language disorder 1 an extremely rare donated by the mate. condition, showing autosomal dominant inheritance, that affects a British family (the KE family) and spermatid one of four haploid cells formed during causes a severe language disorder. The afflicted indi- meiosis in the male. Spermatids without further di- viduals are unable to learn certain rules of grammar vision transform into spermatozoa, a process known and tense, and they cannot enunciate certain verbal as spermiogenesis (q.v.). patterns. The gene involved is FOXP2, located at 7q31, and it spans approximately 600 kb of DNA. spermatocyte a diploid cell that undergoes meio- sis and forms four spermatids. A primary spermato-Two functional copies of FOXP2 are required for the acquisition of normal spoken language. The gene cyte undergoes the first of the two meiotic divisions and gives rise to two secondary spermatocytes. Eachcontains over 20 exons, and it spans approximately 600 kb of DNA. There are at least four mRNAs of these divides to produce two haploid spermatids. spike 419 spermatogenesis the developmental process that sperm along with exogenous sperm from a previous mating. See hermaphrodite.results in the formation of mature sperm in an or- ganism. Spermatogenesis involves a series of events, Sphaerocarpus donellii a species of liverwort including mitotic divisions in spermatogonia (q.v.), used in the classic mutagenesis experiments which meiotic divisions in spermatocytes, and morphologi- showed that the wave length of UV specifically ab- cal changes in spermatids that lead to the formation sorbed by DNA, not protein, was the most effective of mature spermatozoa. Many of these events re- in producing mutations. See Appendix A, Plantae, quire interactions between the germ line (q.v.) and Bryophyta, Hapaticae; Appendix C, 1939, Knapp et the surrounding soma (q.v.). See spermatocyte, sper- al.; ultraviolet radiation. matid, spermatozoon, spermiogenesis. S phase See cell cycle. spermatogonia mitotically active cells in the go- nads of male animals that are the progenitors of pri- sphenophytes horse tails, a group of plants that mary spermatocytes. originated during the Devonian and is represented today by the rush Equisetum. In Carboniferous for- Spermatophyta in older taxonomies the division ests, sphenophytes grew to heights of 15 meters. of the plant kingdom containing the contemporary dominant flora. Spermatophytes are characterized spheroplast a protoplast (q.v.) to which some cell by the production of pollen tubes and seeds. All an- wall remnants are attached. For example, a rod- giosperms and gymnosperms are included in the shaped bacterium treated with lysozyme becomes Spermatophyta. See Appendix A. spherical because the enzyme removes peptidogly- can components that give rigidity to the cell wall.spermatozoon a single male gamete or sperm (plural, spermatozoa). When the word is used as an sphingomyelin a molecule belonging to a family advective the spelling is spermatozoan. of compounds that occur in the myelin sheath of sperm bank a depository where samples of human nerves. All sphingomyelins contain sphingosine, semen are stored in liquid nitrogen at −196°C; when phosphorylcholine, and a fatty acid. needed, perhaps years later, a sample can be thawed and used in artificial insemination. spermiogenesis the series of morphological and chemical changes that transform the spermatids re- sulting from the meiotic divisions of a spermatocyte into functional spermatozoa. In most animals, excess cytoplasm is expelled from the spermatid, and the acrosome (q.v.) and the flagellum (q.v.) are formed. An interesting exception is found in the Nematoda which have amoeboid sperm. sperm polymorphism the production of normal and aberrant sperm during spermatogenesis. The normal sperm are called eupyrene, those containing subnormal numbers of chromosomes are oligopyrene, and those lacking a nucleus altogether are apyrene. Apyrene and oligopyrene sperm are formed by cer- sphingosine an amino dialcohol component of tain snails (Viviparus malleatus is an example) and the sphingolipids, which are abundant in the brain. moths (Bombyx mori), but the function of these ab- normal gametes is unknown. sperm sharing a phenomenon occurring in Brazil- ian freshwater snails of the genus Bioaphalaria in which a simultaneous hermaphrodite (acting me- chanically as a male) transfers sperm to its partner that was collected when it functioned as a female in a previous mating. Sperm sharing may occur both spike an inflorescence, such as the catkin of the pussy willow, in which the flowers arise directlywithin and between species. The term sperm com- merce refers to the transfer of a sperm donor’s own from a central axis, the rachis. See raceme. 420 spikelet spikelet in grasses, a secondary spike bearing few vertical planes but at right angles to one another, producing a quartet of blastomeres. The next divi-flowers. sions are horizontal, cutting off successive quartets. spinal bulbar muscular atrophy one of several However, each quartet is slightly displaced from the neurological diseases due to an unstable trinucleotide one above, giving a spiral appearance to the embryo. repeat (q.v.). The X-linked gene involved encodes an The direction of the spiral is genetically determined. androgen receptor, and the trinucleotide repeat is lo- cated in the coding portion of the gene. spirochete (also spirochaete) bacteria that are nonflagellated, spiral, and move by flexions of the spindle a collection of microtubules responsible body. See Appendix A, Eubacteria, Spirochaetae; for the movement of eukaryotic chromosomes sub- Treponema pallidum. sequent to their replication. See centromere, centro- some, Fungi, meiosis, microtubule organizing centers spiroplasmas helical, motile bacteria that resem- (MTOCs), mitosis, spindle pole body. ble spirochaetes. Unlike spirochaetes, spiroplasmas lack a cell wall, and they are therefore included in spindle attachment region (also spindle fiber at- the Aphragmabacteria (see Appendix A). Spiroplas- tachment, spindle fiber locus) centromere (q.v.). mas are responsible for certain plant diseases and cause male-specific lethality among the progeny of spindle checkpoint a checkpoint (q.v.) that pro- female Drosophila carrying them. See sex ratio organ- tects the integrity of the genome by initiating a delay isms. in the cell cycle if all the chromosomes are not prop- erly attached to the spindle. See MAD mutations. splice junctions segments containing a few nucle- otides that reside at the ends of introns and function spindle fiber one of the microtubular filaments of in excision and splicing reactions during the process- a spindle. ing of transcripts from split genes. The sequence at spindle poison any compound that binds to cer- the 5′ end of any intron transcript is called the donor tain molecular components of spindles and causes junction and the sequence at the 3′ end the acceptor them to malfunction. A subset of the spindle poi- junction. U1 RNA (q.v.) contains a segment adjacent sons, notably paclitaxel, vinblastine, and vincristine to its 5′ cap that exhibits complementarity to the ( all of which see), have turned out to be potent anti- sequences at the donor and acceptor splice junctions cancer drugs. Others like colchicine and podophyllin of introns. U1 binds to such segments, causing in- (both of which see) have been clinical failures. See tu- trons to loop into a lariat (q.v.) that allows intron bulin. excision and exon splicing. See Usn RNAs. spindle pole body in yeast, the organelle that or- spliceosome the intranuclear organelle in which ganizes nuclear and cytoplasmic microtubules into a the excision and splicing reactions that remove in- mitotic spindle. The spindle pole body in fungi is trons from premessenger RNAs occur. See alterna- the functional equivalent of the centrosome (q.v.)in tive splicing, Cajal body, exon, intron, posttransla- animals. During mitosis in yeast, the nuclear enve- tional processing, RNA splicing, small nuclear RNAs, lope does not break down, and the spindle pole body splice junctions, Usn RNAs. remains embedded in the nucleus. See tubulin. splicing 1. RNA splicing: the removal of introns spineless-aristapedia one of the homeotic muta- and the joining of exons from eukaryotic primary tions (q.v.)ofDrosophila located at 3-58.5. The dis- RNA transcripts to create mature RNA molecules of tal portions of the antennae are transformed into leg- the cytoplasm. 2. DNA splicing. See recombinant like structures with claws. The homeotic mutations DNA research. illustration on page 210 allows a comparison of a splicing homeostasis a phenomenon in which a normal pair of antennae, each bearing a bristle-like maturase (q.v.) helps to catalyze the excision of an arista (Fig. A) with a mutant antenna which lacks an intron from its own primary RNA transcript. In so arista and has distal claws (Fig. B). Mutations of the doing, the maturase destroys its own mRNA and ssa gene demonstrate that legs and antennae are ho- thereby limits its own level of activity. mologous structures and suggest that antennae of ar- thropods evolved from an anterior pair of legs. See split genes genes containing coding regions (ex- metamerism. ons) that are interrupted by noncoding regions (in- trons). This type of genetic organization is typical ofspiral cleavage a type of embryonic development seen in invertebrates such as annelids and molluscs. most eukaryotic genes and some animal viral ge- nomes, but introns are not found in prokaryotic or-The first and second divisions of the zygote are in src 421 ganisms. See Appendix C, 1977, Roberts and Sharp; spore mother cell a diploid cell that by meiosis gives rise to four haploid spores.adenovirus, R-loop mapping. sp. n. new species. sporogenesis the production of spores. SPO 11 a gene which encodes a type 2 topoisom- sporophyte the spore-producing, 2N individual. erase (q.v.)inSaccharomyces cerevisiae. It was iso- In the higher plants the sporophyte is the conspicu- lated as a sporulation mutant, hence the spo symbol. ous plant. In lower plants like mosses, the gameto- No meiotic recombination was observed in the mu- phyte is the dominant and conspicuous generation. tant, although normal synaptonemal complexes See alternation of generations. (q.v.) appeared during meiotic prophase. The SPO 11 topoisomerase catalyzes meiosis-specific DNA Sporozoa a class of parasitic protoctists in the phy- double-strand breaks. The Drosophila mutant mei- lum Apicomplexa that reproduce sexually with an W68 is a homolog of SPO 11. See Appendix C, 1997, alternation of generations (q.v.). Both haploids and Keeney, Giroux, and Kleckner; double-strand break diploids undergo schizogony (q.v.) to produce small (DSB) formation, recombination nodules (RNs). infective spores. All species of Plasmodium belong to the Sporozoa. See Appendix A, malaria. sponge body a membrane-less, cytoplasmic struc- ture with a sponge-like appearance, found in female sporozoite the stage in the life cycle of the malaria germ line cells of Drosophila and thought to function parasite that infects humans. Lance-shaped sporozo- in the assembly and transport of materials required ites reside in the salivary gland of the Anopheles mos- for RNA localization in the oocyte (q.v.). Sponge quito and are delivered to the bloodstream of the bodies generally consist of endoplasmic reticulum- victim when the mosquito takes a meal. The major like cisternae and vesicles embedded in an electron- surface antigen of the sporozoite is the circumsporo- dense matrix that is devoid of ribosomes. They con- zoite (CS) protein. In Plasmodium knowlesi, the CS tain RNA and protein, and are often surrounded by protein contains a 12-amino-acid epitope that is re- mitochondria. They are first observed during early peated 12 times. When host antibodies bind to the oogenesis (q.v.) near the nurse cell nuclear mem- CS protein, it sloughs off and is renewed. Thus the brane, change in morphology as development prog- CS protein serves as an immune decoy. The nucleo- resses, migrate through the ring canals (q.v.), and tide sequence of the gene encoding the entire CS dissociate toward later oogenesis into smaller parti- protein has been determined. Unlike most eukaryo- cles that are incorporated into the ooplasm (q.v.). tic genes, it is not interrupted by introns. See Appen- Sponge bodies share morphological and functional dix C, 1983, Godson et al.; malaria, Plasmodium life characteristics with Balbiani bodies and mitochon- cycle. drial clouds. See Balbiani body, cytoplasmic localiza- tion, mitochondrial cloud, nurse cells. sporulation 1. the generation of a bacterial spore. 2. production of meiospores by fungi and many spontaneous generation the origin of a living sys- other eukaryotic organisms. tem from nonliving material. See Appendix C, 1668, Redi; 1769, Spallanzani; 1864, Pasteur. spreading position effect the situation in which a number of genes in the vicinity of a translocation orspontaneous mutation a naturally occurring mu- tation. inversion seem to be simultaneously inactivated. See Appendix C, 1963, Russell. spontaneous reaction exergonic reaction (q.v.). 38, 40, 45S preribosomal RNAs See preribosomal sporangium a structure housing asexual spores. RNA. spore 1. sexual spores of plants and fungi are hap- loid cells produced by meiosis. 2. asexual spores of src the oncogene of the Rous sarcoma virus (q.v.). The human SRC gene lies at 20 q12-q13, while thefungi are somatic cells that become detached from the parent and can either germinate into new hap- Drosophila Src gene is on the third chromosome at 64B. Src genes encode proteins that function as pro-loid individuals or can act as gametes. 3. certain bac- teria respond to adverse growth conditions by enter- tein tyrosine kinases and are characterized by SH2 and SH3 domains. These are important for intra-ing a spore stage until more favorable growth conditions return. Such spores are metabolically in- and intermolecular interactions that regulate both the catalytic activity of the molecules and their re-ert and exhibit a marked resistance to the lethal ef- fects of heat, drying, freezing, deleterious chemicals, cruitment of substrates. SH2 domain is about 100 amino acids long, while SH3 domain is about 50and radiation. 422 src tyrosine kinase amino acids long and is rich in proline. Src PTCs reg- vary gland chromosomes, ribosomal RNA genes, ribo- some, RNA polymerase, Xenopus.ulate the actin cytoskeleton, and they play a role in the morphogenesis of ring canals (q.v.) during oo- 5.8S rRNA a component of the large ribosomal genesis. See actin, c-src, domain, v-src. RNA molecule that is transcribed in the nucleolus. src tyrosine kinase See pp60v-src. 5.8S rRNA is the structural equivalent of the 5′-ter- minal 160 nucleotides of prokaryotic 23S rRNAs. 30S, 40S, 50S, 60S ribosomal subunits See ribo- Thus, in eukaryotes, the 5.8S and 28S coding se- some. quences are separated by an internal transcribed spacer that is absent from the rDNA unit that is60S, 70S, 73S, 78S, 80S ribosomes See ribosome, ribosomes of organelles. transcribed into the RNA of the large subunit of prokaryotic ribosomes. The 5.8S and 28S molecules 4S RNA transfer RNA (tRNA) (q.v.). are eventually separated by posttranscriptional exci- sion of the spacer. However, these molecules remain 7S RNA See signal recognition particle. associated by intermolecular base pairing interac- 5S rRNA a small RNA molecule that is a compo- tions as the large subunit of the ribosome matures. nent of most ribosomes. The 5S rRNA molecule See Miller trees, ribosomal RNA genes, ribosome. shown in the illustration is from E. coli. 5S rRNA occurs in the large ribosomal subunit in the cytosol 16S rRNA the RNA molecule found in the small ribosomal subunits of prokaryotes. This RNA is of-of all prokaryotes and eukaryotes. While the mole- cule stabilizes the structure of the large ribosomal ten abbreviated SSU rRNA (small subunit rRNA). The secondary structure of the 16S rRNA of E. colisubunit, 5S rRNA does not contribute directly to any of the active sites in the subunit. 5S rRNA oc- is shown on page 423. This 30S subunit also con- tains 20 specific proteins. The folding pattern resultscurs in the ribosomes of the mitochondria of plants and in the ribosomes of their chloroplasts. However, from hydrogen bonding of C to G and A to U mole- cules. The nucleotides are numbered starting with 1the ribosomes of the mitochondria of fungi and ani- mals lack 5S rRNAs. In humans, the 5S rRNA locus at the 5′ end and ending with 1,542 at the 3′ end. Comparison of the nucleotide sequence of 16Sis near the telomere of the short arm of chromosome 1. In Drosop hi la melanogast er , it is on 2R at 56 E-F. rRNAs from widely diverse species has allowed the construction of a “universal tree of life” (q.v.). SeeSee Appendix C, 1963, Rosset and Monier; 1970, Wimber and Steffensen; 1973, Ford and Southern; Appendix C, 1977, Woese and Fox; 1980, Woese et al.; ribosome, Shine-Dalgarno (S-D) sequence.1985, Miller, McLachlan, and Klug; Drosophila sali- 5S rRNA stacking 423 5′ 1 3′ 1542 16S rRNA 16S, 18S, 23S, 28S rRNAs the RNA molecules with rudimentary ovaries. See Appendix C, 1987, Page et al. ; 2003, Skaletsky et al.; human Y chromosome,that reside in the subunits of ribosomes. Prokaryotes have 16S and 23S RNAs in their small and large sub- selector genes, sex determination, Y chromosome. units, respectively. Eukaryotes have 18S and 28S SSC sister-strand crossover. See sister chromatid ex- RNAs in their small and large units, respectively. change. 5S rRNA genes genes that are transcribed into 5S rRNAs. Such genes occur in tandemly linked clus- ssDNA single-stranded DNA. ters in all eukaryotes. In Xenopus laevis, 5S rRNA genes account for 0.5% of the entire genome. There SSU rRNA small subunit rRNA. See 16S rRNA. are three separate 5S rRNA multigene families. Two stabilizing selection normalizing selection (q.v.). of these, the major oocyte and trace oocyte families, are expressed only in oocytes, while a third, somatic stable equilibrium an equilibrium state of alleles 5S rDNA, is expressed in all types of somatic cells. at a genetic locus to which the population returns The major oocyte, trace oocyte, and somatic 5S following temporary disturbances of the equilibrium rDNAs are present in 20,000, 1,300, and 400 cop- frequencies. For example, a locus with overdomi- ies, respectively, per haploid genome. nance should form a stable equilibrium as long as SRY sex-determining region Y, the gene at p11.3 selection favoring heterozygotes remains constant. on the Y chromosome that is both required and suf- ficient to initiate testis development in human em- stable isotope a nonradioactive isotope of an ele- bryos. SRY is an intronless gene that spans 3.8 kb. It ment. encodes a 204 amino acid protein which regulates the transcription of the genes that function in sexual dif- stacking 1. the planar alignment of adjacent flat- tish nitrogen bases in a DNA double helix. 2. stack-ferentiation by binding to target sequences in their DNAs. XY individuals with loss-of-function muta- ing of dye molecules on RNA to yield metachromasy (q.v.).tions in the SRY gene are phenotypically female, but 424 staggered cuts staggered cuts the result of breaking two strands mbp and an estimated gene number of 2,600. These microbes cause disease or damage tissue when theyof duplex DNA at different positions near one an- other, as occurs by action of many restriction endo- move away from their normal habitats, particularly in individuals with weakened immune systems. As anucleases (q.v.). human pathogen, S. aureus produces toxins that can stamen the pollen-bearing organ of the angio- cause a wide array of infections and toxic effects, sperm flower. It consists of a filament bearing a ter- such as boils, pneumonia, meningitis, urinary tract minal anther. See flower. infections, bone infections, food poisoning, and toxic shock syndrome (q.v.). Pathogenesis by S. epidermis standard deviation (s) a measure of the variability is relatively less understood. Ninety percent of in a population of items. The standard deviation of Staphylococcus strains are resistant to penicillin and a sample is given by the equation penicillin-derived antibiotics, presenting a challenge for doctors to treat Staphylococcus-derived ailments. s = √ Σ(x − x ) 2 /N − 1 Many genes encoding virulence factors have been characterized, proteins involved in pathogenesis where N is the number of items in the sample and identified, and factors associated with drug resis- Σ(x − x ) 2 is the sum of the squared deviations of tance detected. See Appendix A, Bacteria, Deinocci; each measurement from the mean (x ). Appendix E, Species Web Sites; Gram-staining proce- standard error (SE) a measure of variation of a dure. population of means. starch the storage polysaccharide of most plants. It is a polymer made up of α- D -glucose molecules. See SE = s √ N − 1 formula below. start codon a group of three adjacent ribonucleo- where N = the number of items in the population tides (AUG) in an mRNA coding for the methionine and s = standard deviation. in eukaryotes (formylated methionine in bacteria) that initiates polypeptide formation; also called an standard type the most common form of an or- initiation codon. See genetic code, initiator tRNA. ganism. start kinase See cyclins. Stanford-Binet test used to gauge intelligence, it consists of a series of questions and problems startpoint in molecular genetics, the base pair on grouped for applicability to ages up to 16 years. DNA that corresponds to the first nucleotide incor- Some questions require verbal recognition and oth- porated into the primary RNA transcript by RNA ers recognition of form and manual skills. The sub- polymerase. ject’s performance is expressed in terms of his men- tal age. See intelligence quotient. startsite synonym for startpoint (q.v.). Staphylococcus a genus of spherical, Gram-posi- stasigenesis referring to a period during the pale- tive bacteria, belonging to the family Staphylococca- ontological history of a lineage during which little or ceae. Of the 19 species identified, only two—S. no significant evolutionary change occurred. aureus and S. epidermis—are considered relevant to human health. S. aureus is found predominantly in stasipatric speciation speciation resulting from the dispersion of a favorable chromosomal re-the nasal passages and S. epidermis on the skin of normal humans. S. aureus has a genome size of 2.80 arrangement that yields homozygotes that are adap- glucose Starch [...]... surface upon which organisms walk, crawl, or are attached subtertian malaria See malaria subtractive hybridization ference analysis (RDA) See representational dif- subvital mutation a gene that significantly lowers viability, but causes the death before maturity of less than 50% of those individuals carrying it Contrast with semilethal mutation sucrose the sugar of commerce, a disaccharide composed of. .. possible: AB, AB, ab, ab (referred to as the parental ditype); AB, Ab, aB, ab, where two chromatids are recombinant (the tetratype); and Ab, Ab, aB, aB, where all chromatids are recombinant (the nonparental ditype) tetrahydrofolate See folic acid Tetrahymena a genus containing T pyriformis, the species for which the most genetic information is available, and T thermophila, the species in which UAA and UAG... pronounced “tah- TATA box-binding protein (TBP) an essential transcription factor for RNA polymerases I, II, and III of eukaryotes TBP does not occur in eubacteria, but archaebacteria (q.v.) contain a TBP that has amino acid sequence similarities to the eukaryotic TBP These and other data suggest that archaebacteria and eukaryotes are more closely related than either is to eubacteria tautomeric shift a reversible... produces hydrops fetalis (q.v.) With only one alpha gene, excess beta chains form a tetramer (β4), resulting in hemoglobin H disease Individuals with two or three alpha genes are almost indistinguishable from normal Epidemiological studies have shown that individuals with alpha thalassemia trait ( /aa or -a/ -a) are more resistant to malaria than aa/aa individuals Incomplete beta chains can be produced by... Tay-Sachs disease Both HEXA and HEXB contain 14 exons and are believed to have arisen from a single ancestral gene The disease gets its name from Warren Tay and Bernard Sachs, who published accounts of its pathologies in 188 1 and 188 7, respectively See Appendix C, 1935, Klenk; Ashkenazi, ganglioside, hexosaminidase, lysosomal storage diseases T bacteriophages See T phages T box genes any gene containing... products are held together, as for example, in the case of meiospores confined in an ascus sac Genera suitable for such analyses include Ascobolus, Aspergillus, Bombardia, Neurospora, Podospora, Saccharomyces, Schizosaccharomyces, Sordaria, and Sphaerocarpus tetrad segregation types For a bivalent containing the genes A and B on one homolog and a and b on the other, three patterns of chromatid segregation are... circular DNA molecule that contains 1 ,86 0,725 base pairs There are 1 ,87 7 ORFs, each with an average size of 947 bps These coding sequences cover 95% of the chromosome The largest gene family encodes ABC transporters (q.v.) The organization of the majority of the genes of T maritima places it in the bacteria However, a quarter of the genome is archaeal in nature The mosaic nature of the T maritima genome... micronucleus A similar sort of chromatin elimination occurs in ciliates of the related genus Oxytricha In Stylonychia lemnae, UAA and UAG encode the amino acid glutamine rather than serving as termination codons See Appendix A, Protoctista, Ciliophora; Appendix C, 1969, Ammermann; genetic code, nuclear dimorphism subculture a culture made from a sample of a stock culture of an organism transferred into a fresh... in a particular part of the geographical range of the ancestral species stasis in evolutionary studies, the persistence of a species over a span of geological time without significant change stationary phase a period of little or no growth that follows the exponential growth phase (q.v.) in a culture of microorganisms or in a tissue culture statistic the value of some quantitative characteristic in a. .. Transfer RNA molecules are among the smallest biologically active nucleic acids known For example, an alanine transfer RNA isolated from yeast (shown above) contains 77 nucleotides and is folded back upon itself and kept in a “clover leaf” configuration by the characteristic pairings of the bases G to C and A to U All transfer RNAs attach to their amino acids by the 3′ end, which contains a terminal adenylic . corollary of Darwin’s the Megaloptera and the Thysanoptera. See Appen- theory of natural selection; namely, that as a result dix A, Eumetazoa, Bilateria, Coelomata, Arthro- of the elimination by natural. dispersion of a favorable chromosomal re-the nasal passages and S. epidermis on the skin of normal humans. S. aureus has a genome size of 2 .80 arrangement that yields homozygotes that are adap- glucose Starch steric. undergoes a major which organisms walk, crawl, or are attached. reorganization of its DNA. The polytene chromo- subtertian malaria See malaria. somes are destroyed, and over 90% of the DNA is eliminated.