4 Apr


sterile 1. unable to reproduce. 2. free from living microorganisms; axenic. sterile male technique a technique used in con- trolling noxious insects. Large numbers of artificially reared males are given nonlethal but sterilizing doses of ionizing radiation and then released in nature. The natural populations are so overwhelmed by these males that females are almost always fertilized by them. As a result, the fertilized eggs produced are rendered inviable, and a new generation cannot be produced. sterilization 1. elimination of the ability to repro- duce. 2. the process of killing or removing all living microorganisms from a sample. steroid a lipid belonging to a family of saturated hydrocarbons containing 17 carbon atoms arranged in a system of four fused rings. The hormones of the gonads and adrenal cortex, the bile acids, vitamin D, digitalis, and certain carcinogens are steroids.

steroid receptor a cytoplasmic receptor protein that can bind to a specific steroid hormone. The re- ceptor-hormone complex then moves into the nu- cleus and binds to a specific DNA site to regulate gene activity. steroid sulfatase (STS) gene a pseudoautosomal gene in the mouse. See human pseudoautosomal re- gion. sterol a compound with the general chemical ring structure of a steroid, but with a long side chain and an alcohol group. Cholesterol (q.v.) is an example of a sterol. sticky ends complementary single-stranded pro- jections from opposite ends of a DNA duplex or from different duplex molecules that are terminally redundant. Sticky ends allow the splicing of hybrid molecules in recombinant DNA experiments. Many restriction endonucleases (q.v.) create sticky ends by making staggered cuts (q.v.) in a palindromic restric- tion site. Also called cohesive ends. See Appendix C, 1970, Smith and Wilcox.

stigma the receptive surface usually at the apex of the style of a flower on which compatible pollen grains germinate. stillbirth the birth of a dead fetus. stochastic process a process that can be visualized as consisting of a series of steps, at each of which the movement made is random in direction. stock 1. that part of a plant, usually consisting of the root system together with part of the stem, onto which is grafted a scion. 2. an artificial mating group, as, for example, a laboratory stock of mutant Dro- sophila. See strain. stoloniferous referring to a plant having a creep- ing horizontal stem that takes root at several points to produce new plants. See modular organism. stop codon a ribonucleotide triplet signaling the termination of the translation of a protein chain (UGA, UAG, UAA).

See Appendix C, 1965, Brenner et al. Compare with start codon. strain an intraspecific group of organisms possess- ing only one or a few distinctive traits, usually genet- ically homozygous (pure-breeding) for those traits, and maintained as an artificial breeding group by hu- mans for domestication (e.g., agriculture) or for ge- netic experimentation. There is no clear distinction between the terms strain and variety, but the latter is generally applied when the differences between such intraspecific groups is substantial. See cultivar, patho- var, stock. STR analysis STR stands for short tandem repeat, and the method relies on the variability in the STRs that are scattered along the human chromosomes to distinguish the DNAs of different individuals. The FBI uses a standard set of probes that bind to 13 specific STR regions to generate DNA profiles. The odds that any two humans (except identical twins) will have a match at all 13 loci is about one in a billion. See CODIS, DNA fingerprint technique, micro- satellites, repetitious DNA. strand displacement a replication mechanism, used by certain viruses, in which one DNA strand is displaced as a new strand is being synthesized. strand-specific hybridization probes specifically designed RNA transcripts used for blot or in situ hy- bridization experiments. A special plasmid vector is synthesized that contains a promoter for a phage RNA polymerase and an adjacent polylinker site (q.v.) which allows insertion of a DNA fragment in a specific direction. The vector is then cleaved with an appropriate restriction enzyme, and the gene

strand terminologies
fragment to be analyzed is ligated into the vector and propagated in E. coli. After purification, the plasmid DNA is used as a template for transcription by the specific phage RNA polymerase. By using appropriately labeled ribonucleoside triphosphates, radioactive transcripts of high specific activity are produced. These have two advantages over DNA probes obtained by nick translation (q.v.). (1) Since the RNA is strand specific, one strand of DNA can be analyzed at a time. (2) The sensitivity of hybrid- ization is increased, since the RNA will not self-an- neal. DNA probes, on the other hand, compete with their own complementary strands. strand terminologies names given to distinguish the two strands of a DNA molecule. Each strand of a DNA molecule has a 5′ end and a 3′ end. The 5′ end has a PO4 molecule connected to the number 5 carbon of the first sugar. The 3′ end has an OH group connected to the number 3 carbon of the last sugar.

The two strands of each DNA molecule are aligned in an antiparallel configuration, that is, they point in opposite directions. Terminologies for these strands depend on conventions adopted for messen- mRNA is considered to be a “sense” molecule, and therefore a synthetic RNA molecule with a comple- mentary nucleic acid sequence has been named an antisense RNA (q.v.). Here the prefix anti signifies opposite to or lying alongside. When the nucleotide sequence of an mRNA is printed in a scientific publi- cation, the 5′ end is always drawn above (as in the diagram below) or to the left. The direction of tran- scription is then down or from left to right. When mRNA is translated in a ribosome, the amino end of the new protein is the first and the carboxyl end the last to be formed.

The DNA strand that serves as the template for mRNA is called the template strand. The other strand will contain segments that are iden- tical in sequence to the codons in the mRNA, as- suming one substitutes Ts for Us. For this reason, the DNA strand complementary to the template strand is labeled “sense.” It is the sense strand that is drawn when a “gene sequence” is presented in the literature. Upstream refers to the 5′ direction and downstream to the 3′ direction on the sense strand. For example, the promotor sequence will be up- stream (to the left) of the first exon, and the polya- denylation site will be downstream (to the right) of the last exon. Other terms such as coding strand,

stratigraphic time divisions

anticoding strand, and antisense strand are found in the literature, but since they are used inconsistently, they should be avoided in the future. See deoxyribo- nucleic acid, leader sequence, plus (+) and minus (−) viral strands, polyadenylation, posttranscriptional pro- cessing, trailer sequence, transcription unit. stratigraphic time divisions geologic time divi- sions (q.v.). strawberry See Frageria. streak plating a technique of spreading microor- ganisms over the surface of a solidified medium for the purpose of isolating pure cultures. streptavidin a biotin-binding protein synthesized by Streptomyces avidinii. See biotinylated DNA. Streptocarpus the genus containing the Cape primroses. The inheritance of flower pigmentation has been thoroughly studied in various species in this genus. See anthocyanins. Streptococcus a genus of Gram-positive bacteria that occur as parasites and pathogens, particularly in the lungs and intestines of various animal species. Two species of immense medical importance are de- scribed below. Streptococcus pyogenes is responsible for more human diseases than other bacterial spe- cies. These include impetigo, rheumatic fever, scar- let fever, septicemia, “strep” throat, and toxic shock syndrome (q.v.).

The genome is a circular DNA molecule containing 1,852,442 bp. Ten percent of the 1,752 ORFs are located within resident pro- phages. S. pyogenes can produce at least 40 different virulence factors. Streptococcus pneumoniae is the cause of bacterial pneumonia, and the Pneumoccus Transforming Principle (PTP) (q.v.) was isolated from the organism. The genome consists of a single circular chromosome composed of 2,160,837 bp of DNA. There are 2,236 genes and biological roles have been assigned to 64% of the proteins they are predicted to encode. The genome of S. pneumococcus is rich in insertion sequences (q.v.), but most of these are nonfunctional because of insertions, dele- tions, and point mutations. The virulence of this pneumococcus is associated with its ability to syn- thesize a polysaccharide capsule. A 13 gene cluster has been identified that is likely to be involved in the biosynthesis and secretion of this structure.

See Appendix A, Bacteria, Endospora; Appendix C, 1928, Griffith; 1944, Avery, MacLeod, and McCarty; 1964, Fox and Allen; 2001, Ferretti et al., Tettelin et al. streptolydigins a group of antibiotics that, when bound to the beta subunit of bacterial RNA poly- merase, prevent transcriptional elongation. Streptomyces a genus of soil-inhabiting bacteria containing over 500 species. Some of these are nota- ble for their synthesis of many useful compounds, including the majority of the antibiotics used in hu- man and veterinary medicine, immunosuppressants, and herbicides.

Streptomyces-derived antibiotics in- clude streptomycin, streptonigrin, neomycin, chlor- amphenicol, and tetracyclines (all of which See). Streptomyces are also of interest for use in bioreme- diation (q.v.), since they are able to break down a diverse range of molecules, including aromatic com- pounds, organic acids, sugars, and alcohols. Of all the species, S. coelicolor is the most widely studied and has become the model organism for genetic analysis. It has a linear chromosome containing the largest number of predicted genes (7,825) for any prokaryote. Its genome contains an unprecedented number of regulatory genes. See Appendix A, Bacte- ria, Actinobacteria; Appendix E, Species Web Sites; Appendix F; antibiotic, streptavidin. streptomycin an antibiotic produced by Streptomy- ces griseus that binds to the 30S subunit of the bacte- rial ribosome and leads to faulty translation of the ad- vancing messenger tape. See ribosome, translation. streptomycin suppression seen in bacterial mu- tants with an altered ribosomal protein (S12).

This enables them to initiate polypeptide synthesis in the presence of streptomycin, and it also reduces the ex- tent of misreading induced by that antibiotic. Such cells are converted from streptomycin-sensitive to streptomycin-resistant. streptonigrin an antibiotic produced by Strepto- myces flocculus that causes extensive chromosomal breakage. stress fibers bundles of parallel-aligned, actin- containing microfilaments underlying the plasma membrane of cultured eukaryotic cells. Stress fibers permit cells to attach to the substratum and generate the stress or tension that causes them to assume a flattened shape. See fibronectin. stringency the condition with regard to tempera- ture, ionic strength, and the presence of certain or- ganic solvents such as formamide (q.v.), under which nucleic acid hybridizations are carried out. With conditions of high stringency, pairing will oc- cur only between nucleic acid fragments that have a high frequency of complementary base sequences. Conditions of weaker stringency must be used if the nucleic acids come from organisms that are geneti- cally diverse. Thus, if one were trying to isolate an alcohol dehydrogenase gene from a silkworm geno- mic library using a cloned gene from Drosophila mel-

struggle for existence



anogaster as a probe, less stringent conditions would be used than if the library came from D. virilis. stringent control See plasmid. stringent response the cessation of tRNA and ri- bosome synthesis by bacteria under poor growth conditions. stRNAs small temporal RNAs (q.v.). stroma the protein background matrix of a chloro- plast or mitochondrion. stromatolites living or fossil microbial mats domi- nated by cyanobacteria and fine sediment (usually calcium carbonate) trapped by these photosynthetic microbial communities.

The oldest stromatolites are more than 3 billion years old and are among the old- est known fossils. See Appendix C, 1980, Lowe. Strongylocentrotus purpuratus a common sea ur- chin used in studies of molecular developmental ge- netics. During oogenesis and egg maturation, large reservoirs of histone mRNAs are produced by fe- males. Histone genes (q.v.) were first isolated from this species. Its estimated genome size is 845,000 ki- lobases. See echinoderm. strontium90 a radioisotope of strontium with a half-life of 28 years generated during the explosion of nuclear weapons. 90Sr is one of the major sources of radiation due to fallout. structural change chromosomal aberration (q.v.). structural gene a DNA segment whose own struc- ture (nucleotide sequence) determines the structure (amino acid sequence) of a specific polypeptide. See gene, lac operon. structural heterozygote a cell or an individual multicellular organism characterized by a pair of ho- mologous chromosomes, one normal and the other containing an aberration, such as an inversion or a deficiency.

structural protein any protein that substantially contributes to shape and structure of cells and tis- sues: e.g., the actin and myosin components of mus- cle filaments, the proteins of the cytoskeleton, colla- gen, etc. struggle for existence the phrase used by Darwin to describe the competition between animals for en- vironmental resources such as food or a place to live,

hide, or breed. Darwin wrote in On the Origin of Species “I use the term struggle for existence in a large and metaphorical sense . . . including (which is more important) not only the life of the individual, but success in leaving progeny.” STS sequence tagged site (q.v.). Student t test a statistical method used to deter- mine the significance of the difference between two sample means. The method was developed by the British statistician W. S. Gosset, who used the pseu- donym “Student” in his publications. See page 74 for t distribution. style a slender column of tissue arising from the top of the ovary and through which the pollen tube grows. Stylonychia a genus of ciliates in which the ma- cronuclear anlage undergo endomitotic DNA repli- cation to form giant, banded, polytene chromosomes. Subsequently, the macronucleus undergoes a major reorganization of its DNA. The polytene chromo- somes are destroyed, and over 90% of the DNA is eliminated. The remaining DNA molecules are pres- ent as gene-sized pieces, and these undergo a series of replications as the macronucleus matures.

There- fore, the macronucleus comes to contain multiple copies of a subset of the genes found in the micronu- cleus. A similar sort of chromatin elimination occurs in ciliates of the related genus Oxytricha. In Stylo- nychia lemnae, UAA and UAG encode the amino acid glutamine rather than serving as termination co- dons. See Appendix A, Protoctista, Ciliophora; Ap- pendix 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 medium. subdioecy a sexual state of certain plants in which some unisexual individuals show imperfect sexual differentiation. sublethal gene See subvital mutation. submetacentric a chromosome that appears J- shaped at anaphase because the centromere is nearer one end than the other. subpopulations breeding groups within a larger population or species, between which migration is restricted to a significant degree. subspecies 1. a taxonomically recognized subdivi- sion of a species. 2. geographically and/or ecologi- cally defined subdivisions of a species with distinc- tive characteristics. See race. substitutional load the cost to a population in ge- netic deaths of replacing an allele by another in the course of evolutionary change.

See genetic load. substitution vector See lambda cloning vector. substrain a population of cells derived from a cell strain by isolating a single cell or groups of cells hav- ing properties or markers not shared by all cells of the strain. substrate 1. the specific compound acted upon by an enzyme. 2. substratum. substrate-dependent cells See anchorage-depen- dent cells. substrate race a local race of organisms selected by nature to agree in coloration with that of the sub- stratum. substratum the ground or other surface upon which organisms walk, crawl, or are attached. subtertian malaria See malaria. subtractive hybridization See representational dif- ference analysis (RDA). 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 glucose and fructose. sucrose gradient centrifugation See centrifuga- tion separation. Sudan black B a commonly used lysochrome. sue mutations See suppressor-enhancing muta- tions. sugar See carbohydrate, glucose. suicide genes genes whose products facilitate ap- optosis (q.v.).

Sulawesi a peculiarly shaped island east of Borneo. Sulawesi straddles the equator, with the Celebes Sea to the north and the Molucca Sea to the east. In the middle Miocene, about 15 million years ago, the Australian plate, bounded on the north by New Guinea, collided with the Eurasian plate. Sulawesi received land from both plates. This explains why this island has animals, some of whose ancestors came from Asia and others from Australia. See bio- geographic realms, plate tectonics, Wallace line.

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