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semidominance
selector gene that binds to specific genomic sites, activating one or more of the genes necessary for the new pattern of development. See Antennapedia, apetala-2, bicoid, bithorax, caudal, compartmentalization, decapentaplegic, developmental control genes, dorsal, downstream genes, engrailed, eyeless, fushi tarazu, goosecoid, gurken, hedgehog, Hox genes, hunchback, metamerism, nanos, oskar, paired, Polycomb, sevenless, SRY. self to undergo self-pollination (q.v.) or self-fertilization (q.v.). self-assembly the spontaneous aggregation of multimeric biological structures involving formation of weak chemical bonds between surfaces with complementary shapes. For example, most of the components of the phage T4 capsid (head, tail, base plate, and tail fibers) are self-assembled. self-compatible fertilized. said of a plant that can be self-

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able to recognize only one promoter. Thus the transmission of DNA by horizontal mobile elements (q.v.) tends to select the stepwise formation of gene clusters that function as operons. See Appendix C, 1996, Lawrence and Roth; sympatric speciation. self-pollination the transfer of pollen to the stigmas of the same plant. self-splicing rRNA See ribozyme.

self-sterility the inability of some hermaphrodites to form viable offspring by self-fertilization. self-sterility genes genes that prevent the deleterious effects of inbreeding in monoecious plants by controlling the rate of growth of the pollen tube down the style. Self-incompatibility is controlled by a highly polymorphic S locus. Growth of a pollen tube in the style is arrested when the S allele carried by the pollen matches one of the two S alleles carried by the pistil. S genes have been cloned and shown to encode glycoproteins with RNase activity. In self-pollinations, the RNase is internalized by receptors on the pollen tube surface. Once inside the pollen tube, the enzyme degrades RNAs essential to its further growth. SEM scanning electron microscope. See electron microscope. semelparity reproduction that occurs only once in the life of an individual (e.g., annual plants, Pacific salmon). Compare with iteroparity. semen a biochemically complex nutrient fluid containing spermatozoa which is transferred to the female during copulation. semiconservative replication the method of replication of DNA in which the molecule divides longitudinally, each half being conserved and acting as a template for the formation of a new strand. See Appendix C, 1953, Watson and Crick; 1957, Taylor et al.; 1958, Meselson and Stahl; 1963, Cairns; 1964, Luck and Reich. Compare with conservative replication. semidiscontinuous replication a mode of DNA replication in which one new strand is synthesized continuously, while the other is synthesized discontinuously as Okazaki fragments. See replication of DNA. semidominance the production of an intermediate phenotype in individuals heterozygous for the gene concerned; also known as partial dominance. See incomplete dominance.

self-fertilization the fusion of male and female gametes from the same individual. self-incompatibility self-sterility (q.v.).

selfish DNA (also called junk DNA or parasitic DNA) 1. functionless segments of DNA that are replicated along with the rest of the chromosomal regions that serve vital functions. Examples would be pseudogenes (q.v.) and tandemly repeated and dispersed repetitive DNA segments that appear to serve no function, yet accumulate by unequal crossing over (q.v.). 2. the term is also used to refer to a parasitic DNA that has the ability to engineer its host genetically so that the host cell is better able to survive in nature. Examples would be R (resistance) plasmids and Ti plasmids (both of which see). See Appendix C, 1980, Doolittle and Sapienza, Orgel and Crick; 1997, Yoder, Walsh, and Bestor; C value paradox, DNA methylation, repetitious DNA. selfish operon a model to explain the origin in bacteria of clusters of genes that have similar functions and are conditionally expressed. The idea is that genes whose products are used only during conditions that happen infrequently are continually mutating to inactive alleles. When conditions that require the missing gene products reoccur, the bacteria die off unless horizontal transfer of active alleles can take place. But only small segments of DNA are transferred by episomes. Therefore, when two or more genes are needed simultaneously, selection will favor genes that are near one another. Also, cotranscribed genes are more likely to function adaptively in a new host, since the host needs to be