epistatic gene See epistasis. epithelium a tissue that forms the surface of an or- gan or organism. For example, the outer skin is an epithelial tissue; the cells lining the gut and respira- tory cavities are also epithelial cells. epitope the antigenic determinant on an antigen to which the paratope on an antibody binds. epizoite a nonparasitic sedentary protoctist or ani- mal living attached to another animal. epoch in geological time, a major subdivision of a geological period. See geologic time divisions. eponym a word, phrase, or abbreviation derived from the name of a person or place.
Eponyms often honor the discoverers of laws or other phenomena and the inventors of methods or techniques. Exam- ples are Hardy-Weinberg law, Lyon hypothesis, Hal- dane rule, Batesian mimicry, Rabl orientation, Ouchterlony technique, and Miller spread. Genetic diseases are often named after the physicians who first described them (e.g., Down syndrome and Tay- Sachs disease), or, more rarely, after the patients in which the conditions were first recognized Christmas disease and hemoglobin Lepore). Names of places and institutions are sometimes used as medical eponyms (e.g., Bombay blood group and he- moglobin Barts). The names of scientists who gave the first descriptions of various organs, organelles, and submicroscopic life forms are also honored with eponyms (e.g., Malpighian tubule, Golgi apparatus, Cajal body, and Epstein-Barr virus). In most epo- nyms the possessive form (’s) is avoided. There are about 120 eponyms distributed among the definition headings of this dictionary. epoxide a family of chromosome-breaking, alkyl- ating agents. Di(2,-3 epoxy)propyl ether is an ex- ample.
Epstein-Barr virus (EBV) a DNA virus of the her- pes group discovered in 1964 by M. A. Epstein and Y. M. Barr in cultures of Burkitt lymphoma cells. EBV is the cause of infectious mononucleosis, and it has an integration site on human chromosome 14. See Burkitt lymphoma, virus. equational division a division of each chromo- some into equal longitudinal halves that are incorpo- rated into two daughter nuclei. It is the type of divi- sion seen in mitosis. equatorial plate See mitosis. Equidae the horse family and its extinct ancestors. Living species include the domesticated horse, Equus caballus; the donkey, E. asinus; and three spe- cies of zebras. The earliest horse genus in the fossil record is Hyracotherium (q.v.). See Appendix A, Mammalia, Perissodactyla; horse; horse-donkey hy- brids.
equilibrium centrifugation See centrifugation sep- aration. equilibrium dialysis See dialysis. equilibrium population a population in which the allelic frequencies of its gene pool do not change through successive generations. An equilibrium can be established by counteracting evolutionary forces (e.g., a balance between selection and mutation pressures) or by the absence of evolutionary forces. See Hardy-Weinberg law. equine referring to members of the horse family, especially the domestic horse Equus caballus. Equus the genus that contains two domesticated species: E. caballus, the horse (2N = 64), and E. asi- nus, the donkey (2N = 62). E. przewalskii, the Asi- atic wild horse or taki (2N = 66), can produce fertile hybrids with domesticated horses. The species name comes from Nikolai Przewalski, the Russian natural- ist who described takis in 1883. Takis are descen- dants of the horses like those shown in the ancient cave paintings of Lascaux, France. About 50 genes have been mapped for E. caballus. See Appendix E; horse (for a listing of breeds), horse-donkey hybrids. ER endoplasmic reticulum (q.v.). era in geological time, a major division of a geo- logical eon. See geologic time divisions.
ergastoplasm rough-surfaced endoplasmic retic- ulum. ergosome polysome (q.v.). error-prone repair a process first demonstrated in E. coli exposed to UV light that allows DNA replica- tion to occur across thymine dimers (q.v.), but at the cost of fidelity of replication. The resulting strand often has incorrect nucleotides inserted opposite the UV photoproducts in the template. See Appendix C, 1967, Witkin; SOS response.
errors See statistical errors. erythroblastosis the discharge of nucleated red blood corpuscles from the blood-forming centers into the peripheral blood. erythroblastosis fetalis a hemolytic disease of in- fants due to Rh incompatibilities between the fetus and its mother. See Appendix C, 1939, Levine and Stetson; Rh factor, RhoGAM. erythrocyte the hemoglobin-containing cell found in the blood of vertebrates. erythrolabe See color blindness. erythromycin an antibiotic (illustrated above) pro- duced by Streptomyces erythieus. It inhibits protein synthesis by binding to the 50S subunits of 70S ribo- somes. See cyclohexamide, ribosome, ribosomes of organelles. erythropoiesis the production of erythrocytes (q.v.). erythropoietin a glycoprotein cytokine (q.v.) pro- duced by the kidney; it regulates the production of red blood cells. See Janus kinase 2. escaper See breakthrough.
Escherichia coli the “colon bacillus,” the organism about which the most molecular genetics is known. The genus is named after Theodore Escherich, the German bacteriologist who first described E. coli in 1885. The distances between genes presented on its circular linkage map are measured in minutes, based on interrupted mating experiments (q.v.). E. coli is of preeminent importance in recombinant DNA re- search, since it serves as a host for a wide variety of viral, plasmid, and cosmid cloning vectors. Given glucose and inorganic salts, E. coli can synthesize all the compounds it requires for life. Its chromosome is a circular DNA molecule that contains 4,639,221 base pairs. Some 4,288 ORFs have been identified. The functions of 40% of these are unknown. The average distance between genes is 118 bp. The aver- age size of a protein encoded by an ORF is 317 amino acids. The arrangement of genes that control flagellar synthesis is nearly identical to that of Salmo- nella typhimurium. The largest functional group of genes consists of 281 that encode transport and binding proteins. Of these, 54 are ABC transporters (q.v.).
The E. coli chromosome is a host for many prophages and cryptic prophages (q.v.). See Appen- dix A, Bacteria, Proteobacteria; Appendix C, 1946, Lederberg and Tatum; 1953, Hayes; 1956, 1958, Ja- cob and Wollman; 1961, Jacob and Monod, Niren- berg and Matthaei; 1963, Cairns, Jacob and Brenner; 1969, Beckwith et al.; 1972, Jackson et al.; 1973, Cohen et al.; 1997, Blattner et al., Lawrence and Oschman; Appendix E, Individual Databases; inser- tion sequence, lambda (λ) bacteriophage, operon, Sal- monella, sympatric speciation, virulence plasmids. Escherichia coli databases See Appendix E. essential amino acids amino acids required in the diet of a species because these molecules cannot be synthesized from other food materials; in contrast to nonessential amino acids that can be synthesized by normal members of the species. See amino acid. EST expressed sequence tag. See sequence tagged site (STS). established cell line a cell line (q.v.) that demon- strates the potential to be subcultured indefinitely in vitro. HeLa cells (q.v.) represent an established cell line. estivate aestivate (q.v.). estradiol a steroidal estrogen. estrogen an ovarian hormone that prepares the mammalian uterus for implantation of an embryo; also responsible for development of secondary sex- ual characteristics in females.
estrous cycle a seasonal cycle of reproductive ac- tivity dependent upon endocrine factors. If the or- ganism has one estrous period per year, it is called monestrous, if more than one, polyestrous. estrus 1. the period of reproductive activity. 2. the estrous cycle. ethidium bromide a compound used to separate covalent DNA circles from linear duplexes by den- sity gradient centrifugation. Because more ethidium bromide is bound to a linear molecule than to a co- valent circle, the linear molecules have a higher den- sity at saturating concentrations of the chemical and can be separated by differential centrifugation. It is also used to locate DNA fragments in electropho- retic gels because of its fluorescence under ultravio- let light.
Ethiopian designating or pertaining to one of the six biogeographic realms (q.v.) of the world; it in- cludes Africa, Iraq south of the Tropic of Cancer, Madagascar, and the adjacent islands. ethological behavioral. ethological isolation the failure of related species or semispecies to produce hybrid offspring because of differences in their mating behaviors. See alles- thetic trait, courtship ritual, mate choice, species rec- ognition.
ethology the scientific study of animal behavior, particularly under natural conditions. ethylene a gas (H2C = CH2) generated by plant cells, which functions as a growth regulator. It inhib- its the elongation of stems, while promoting their growth in diameter. It also accelerates the ripening of fruit and, in conjuction with auxin (q.v.), can stimulate the growth of lateral roots. In Arabidopsis thaliana, mutations of the ETRI gene confer resis- tance to ethylene. The protein encoded by ETRI contains ethylene-binding sites and so plays a role in transduction of the ethylene signal. ethylene dinitrilotetra-acetic acid See EDTA. ethyl methane sulfonate (EMS) one of the most commonly used mutagenic alkylating agents. The most common reaction of EMS is with guanine (q.v.), where it causes an ethyl group to be added to the number 7 nitrogen. Alkylation of guanine allows it to pair with thymine. Then, during replication, the complementary strand receives thymine rather than cytosine. Thus, EMS causes base substitutions of the transition type.
etiolation a plant syndrome caused by suboptimal light, consisting of small, yellow leaves and abnor- mally long internodes. etiology the study of causes, especially of disease. E1 trisomy syndrome See Edwards syndrome. Euarchontoglires a clade which groups together five mammalian orders because of similarities in the sequences of the nucleotides in a sample of their genes. It contains the Rodentia, Lagomorpha, Scan- dentia, Dermoptera, and Primates. See Appendix A. Eubacteria a subkingdom of the Prokaryotae (see Appendix A) composed of bacteria that, unlike the archaebacteria, contain neuraminic acid their cell walls. They also differ from archaebacteria in the composition of their tRNAs, rRNAs, and their RNA polymerases. See TATA box-binding protein. eucaryote See eukaryote. Eucaryotes See Eukaryotes. euchromatic containing euchromatin. euchromatin the chromatin that shows the stain- ing behavior characteristic of the majority of the chromosomal complement. It is uncoiled during in.