Smooth endoplasmic reticulum (SER)

2 Apr

Smooth endoplasmic reticulum (SER)

tion of chromosomes or chromatics is affected dur- ing cell division. slime molds See Acrasiomycota, Myxomycota. slippage strand mispairing See microsatellites. slow component in a reassociation reaction, the last component to reassociate, usually consisting of nonrepetitive (unique) DNA sequences. slow stop mutants temperature-sensitive dna mu- tants of E. coli that complete the current round of DNA replication when placed at the restrictive tem- perature, but cannot initiate another round of repli- cation.

Slp sex-limited protein; a serum protein that is normally found only in male mice and encoded by a gene in the major histocompatibility complex (H-2). small angle x-ray diffraction the technique used in the analysis of widely spaced repetitions, such as the groups of atoms that form monomeric subunits of a polymer.

See large angle x-ray diffraction, x-ray crystallography. small cell lung carcinoma See p53. small cytoplasmic RNAs the cytoplasmic coun- terparts of small nuclear RNAs (q.v.); found in small ribonucleoprotein particles (q.v.) in their native state. Small eye (Sey) a gene in mice and rats that con- trols eye development. Animals heterozygous for a mutant allele have underdeveloped eyes.

Homozy- gotes lack eyes altogether. Sey, Pax-6, and ey are ho- mologous genes. See Aniridia, eyeless, paired. small interfering RNAs (siRNAs) noncoding RNAs, approximately 22 nucleotides long, which si- lence gene expression by binding with perfect ho- mology to complementary sequences in target mes- senger RNAs in the cell and mediating their destruction. siRNAs have also been shown to silence genes at the transcriptional level. siRNAs are derived from endogenous or exogenous, double-stranded RNA (q.v.) precursors that fold into hairpin struc- tures and are processed by the enzyme Dicer (q.v.), such that both strands of the dsRNA give rise to dif- ferent siRNAs. siRNAs have been used as an experi- mental tool to inhibit gene expression (see RNA in- terference (RNAi) (q.v.)). Compare with small temporal RNAs (stRNAs). small nuclear RNAs a family of small RNA mole- cules that bind specifically with a small number of proteins to form small nuclear ribonucleoprotein particles.

These snRNPs (pronounced “snurps”) play a role in the posttranscriptional modification of RNA molecules. See Appendix C, 1979, Lerner and Steitz; posttranscriptional processing, snurposomes, spliceosome, transcriptosomes, Usn RNAs. small nucleolar RNAs molecules (abbreviated snoRNAs) that are characterized by “boxes” that contain four to six nucleotides. These conserved se- quence elements are distant in the primary se- quence, but they are brought into proximity in the folded RNAs as a result of base pairing of comple- mentary sequences that flank the boxes.

The loop that results is called a stem-box structure, and it func- tions to target the molecule to the nucleolus (q.v.). There are at least 150 different snoRNAs, and these function in the cleavage of pre rRNA molecules and the subsequent modifications of the products. For example, there is a large group of snoRNAs that play a role in the methylation of ribose-2-hydroxyl groups at conserved positions in rRNAs.

See Miller trees, nucleolin, preribosomal RNA, ribose, ribosome, 16S RNA, transcriptosomes. smallpox vaccine the antigenic preparation used to elicit an active immunity to smallpox. Commer- cial vaccines contain freeze-dried Vaccinia virus. See immunity, pox viruses, vaccine, Variola virus. small temporal RNAs (stRNAs) in Caenorhabditis elegans and presumably in other organisms, noncod- ing RNAs, approximately 22 nucleotides long, which regulate the timing of developmental events by binding to partially complementary sequences in the 3′ untranslated regions of protein-coding mes- senger RNAs and inhibiting their translation. Micro- RNAs (q.v.) encoded by the lin-4 and let-7 genes of C. elegans are examples of stRNAs.

These genes pro- duce approximately 70 nucleotides-long, double- stranded, stem loop precursor RNAs, which are pro- cessed by the enzyme Dicer (q.v.), such that only one strand of the dsRNA gives rise to the mature stRNA. Compare with small interfering RNAs (siRNAs). Smittia a genus of chironomid possessing giant po- lytene chromosomes (q.v.) and hence subjected to cytological study. See Chironomus. smooth endoplasmic reticulum (SER) endoplas- mic reticulum that lacks ribosomes; sometimes called the agranular reticulum. SER is common in cells that are actively synthesizing compounds other than proteins, that is, carbohydrates, lipids, or ste- roids. This intricate network of tubes and sacs is seen in cells such as oil gland cells in the epidermis, cells from glands that synthesize steroid hormones, and cells that line the small intestine. See endoplasmic reticulum.


smut 1. a fungus disease of cereals characterized by black masses of spores. 2. any basidiomycete fun- gus of the order Ustilagnales that causes smut dis- ease. snapdragon See Antirrhinum majus. sneak synthesis See background constitutive syn- thesis. snoRNAs small nucleolar RNAs (q.v.). SNPs See single-nucleotide polymorphisms. snRNA a small nuclear RNA (q.v.). snRNPs small nuclear ribonucleoproteins. See small nuclear RNAs. S1 nuclease an endonuclease from Aspergillus or- yzae that selectively degrades single-stranded DNA to yield 5′ phosphoryl mono- or oligonucleotides. snurposomes organelles found in amphibian ger- minal vesicles, equivalent to the speckles or clusters of interchromatin granules of other cell types.

Snur- posomes are composed of densely packed particles, each 20-30 nm in diameter.

These contain splicing snRNPs and other factors involved in splicing pre- mRNAs. Snurposomes are 1-4 µm in diameter, and they are observed in the matrix of Cajal bodies (q.v.) or attached to their surfaces. Snurposomes also float free in the nucleoplasm. They were given their name because they stain strongly with antibodies that lo- calize splicing snRNPs (snurps). See posttranscrip- tional processing, transcriptions. snurps See small nuclear RNAs. social Darwinism a theory originated by the Brit- ish philosopher Herbert Spencer, proposing that most of the “progress” in human societies has been brought about by competition (economic, military) and the “survival of the fittest.” Spencer believed that human progress required a struggle and compe- tition, not only between individuals but also be- tween social classes, nations, states, and races, and he ranked human races and cultures according to their assumed levels of evolutionary attainment. social evolution a continued increase in the com- plexity of human society resulting from the selec- tion, transmission, and utilization of the useful infor- mation gained in each generation. sociobiology the study of animal behavior from a genetic perspective. SOD See superoxide dismutase. sodium an element universally found in small amounts in tissues.

Atomic number 11; atomic weight 22.9898; valence 1+; most abundant isotope 23Na; radioisotopes: 24Na, half-life 15 hours, radia- tions emitted—beta particles and gamma rays; 22Na, half-life 2.6 years, radiations emitted—positrons and gamma rays. sodium dodecyl sulfate CH3(CH2)11SO3Na (so- dium lauryl sulfate), an anionic detergent used in the SDS-PAGE method of protein fractionation. See electrophoresis. Sogin’s first symbiont the hypothesized ancestor of eukaryotes. As outlined in the diagram (page 415), this organism arose by fusion of two prokaryotes with complementing metabolic capabilities. The first had a fragmented RNA-based genome that encoded the RNAs functioning in translation and the elaboration of cytoskeletal proteins.

Once equipped with a cyto- skeleton, the prokaryote could engulf other microor- ganisms. The second organism was a primitive arch- eon with a relatively unfragmented DNA genome that encoded metabolically active proteins. The first organism engulfed the second, which eventually functioned as the nucleus in the chimera. See Ap- pendix C, 1991, Sogin; serial symbiosis theory. Solanum tuberosum See potato. solenoid structure a supercoiled DNA produced during the condensation of chromosomes in the nu- clei of eukaryotes. This chromosome shortening is achieved by folding the linear array of nucleosomes into a helical fiber with six nucleosomes per turn. The molecules of histone 1 aggregate into a helical polymer along the center of the solenoid and stabi- lize it. A top view of the solenoid is shown. See Ap- pendix C, 1976, Finch and Klug; chromatin fibers, histones.

solution hybridization liquid hybridization (q.v.). soma the somatic cells of a multicellular organism in contrast to the germ cells. somatic cell any cell of the eukaryotic body other than those destined to become sex cells. In diploid organisms, most somatic cells contain the 2N num-

somatic recombination

Sogin’s first symbiont

ber of chromosomes; in tetraploid organisms, so- matic cells contain the 4N number, etc. somatic cell genetic engineering correction of genetic defects in somatic cells by genetic engineer- ing: e.g., insertion of genes for insulin production into defective pancreatic cells. Such correction would not be hereditary. somatic cell genetics the genetic study of asexu- ally reproducing body cells, utilizing cell fusion tech- niques, somatic assortment, and somatic crossing over. See Appendix C, 1964, Littlefield; 1965, Harris and Watkins; 1967, Weiss and Green; 1969, Boon and Ruddle; 1985, Smithies et al. somatic cell hybrid a hybrid cell resulting from cell fusion (q.v.). somatic cell nuclear transfer therapeutic cloning (q.v.). somatic crossing over the exchange of DNA be- tween non-sister chromatids in a somatic cell. Also called mitotic crossing over. See mitotic recombina- tion, site-specific recombination. somatic doubling the doubling of the diploid chro- mosome set. Such doubling may be induced experi- mentally by applying the alkaloid colchicine in a lano- lin paste to somatic tissues that are undergoing mitosis. somatic mutation a mutation occurring in any cell that is not destined to become a germ cell. If the mutant cell continues to divide, the individual will come to contain a patch of tissue of genotype differ- ent from the cells of the rest of the body. Compare with gametic mutation. somatic pairing the conjoining of the homologous chromosomes in somatic cells, a phenomenon seen in dipterans. The fact that the polytene chromo- somes of Drosophila undergo somatic pairing makes possible the identification of chromosomal rearrange- ments, the mapping of deficiencies, and, as a result, the cytological localization of genes. See Diptera, transvection. somatic recombination genetic recombination that does not involve germ cells but rather somatic cells, usually of a specific type and at a particular developmental stage. For example, somatic recombi-

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