belong to this subdivision. See Appendix A, Ani- malia, Eumetazoa, Bilateria, Protostomia, Coelo- mata, Articulata. echinoderm an animal with an external skeleton of calcareous plates and internal water-vascular sys- tem (starfish, sea urchins, sea lilies, sea cucumbers, etc.).
Echinoderm embryos were used to isolate the first histone genes (q.v.) and the first cyclins (q.v.). See Appendix A, Echinodermata. eclipsed antigens antigenic determinants of para- site origin that resemble antigenic determinants of their hosts to such a degree that they do not elicit the formation of antibodies by the host. The forma- tion of eclipsed antigens by parasites is termed molec- ular mimicry. eclipse period in virology, the time interval be- tween infection and the first intracellular reappear- ance of infective phage particles. See one-step growth experiment.
Compare with latent period. eclosion the emergence of the adult insect from its pupal case. ecodeme a deme (q.v.) associated with a specific habitat (a cypress swamp, for example). ecogeographical divergence the evolution from a single ancestral species of two or more different spe- cies, each in a different geographical area and each adapted to the local peculiarities of its habitat. ecogeographic rules any of several generalizations concerning geographic variation within a species that correlate adaptations with climate or other en- vironmental conditions: e.g., Allen’s rule, Bergman’s rule. E. coli Escherichia coli (q.v.). ecological genetics the analysis of the genetics of natural populations and of the adaptations of these populations to environmental variables.
ecological isolation a premating (prezygotic) iso- lating mechanism in which members of different species seldom, if ever, meet because each species prefers to live (is adapted to) different habitats. ecological niche the position occupied by a plant or animal in its community with reference both to its utilization of its environment and its required as- sociations with other organisms. ecology the study of the relationships between or- ganisms and their environment. ecophenotype a nongenetic phenotypic modifica- tion in response to environmental conditions. See phenotypic plasticity. EcoRI See restriction endonuclease.
ecosystem an assemblage of interacting popula- tions of species grouped into communities in a local environment. Ecosystems vary greatly in size (e.g., a small pool vs. a giant reef). See biome. ecotype race (within a species) genetically adapted to a certain environment. See Appendix C, 1948, Clausen et al. ectoderm a germ layer forming the external cover- ing of the embryo and neural tube (from which de- velop the brain, spinal cord, and nerves).
Ectodermal derivatives include all nervous tissues, the epidermis (including cutaneous glands, hair, nails, horns, the lens of the eye, etc.), the epithelia of all sense or- gans, the nasal cavity sinuses, the anal canal, and the mouth (including the oral glands, and tooth enamel), and the hypophysis. See Appendix C, 1845, Remak. ectopic out of place; referring to a biological struc- ture that is not in its proper location.
In develop- mental genetics the adjective is sometimes used to describe the expression of a regulatory gene in the wrong place. An example is the cluster of ommatidia on the antenna of the fly head in the Frontispiece illustration. ectopic pairing nonspecific pairing of intercalary and proximal heterochromatic segments of Drosoph- ila salivary chromosomes. ectoplasm the superficial cytoplasm of ciliates. ectotherm vertebrates such as fishes, amphibians, and reptiles that have little or no endogenous mech- anisms for controlling their body temperature.
Their temperature is determined by environmental condi- tions. Contrast with endotherm. editing See proofreading, RNA editing. EDTA an abbreviation for ethylene diaminetet- racetic acid. A molecule capable of reacting with metallic ions and forming a stable, inert, water-solu- ble complex. EDTA is used to remove metals that occur in minute amounts even in distilled water.
Edwards syndrome a well-defined set of congeni- tal defects in humans caused by the presence of an extra chromosome 18. Also called trisomy 18 syn-
drome or E1 trisomy syndrome. The mean survival is two months, and the incidence is 1/3,500 live births. The condition was first described in 1960 by J. H. Edwards and four colleagues. See human mitotic chromosomes. EEG electroencephalogram (q.v.).
Ef-1 , Ef-Tu See translation elongation factors. effective fertility See fertility. effective lethal phase the stage in development at which a given lethal gene generally causes the death of the organism carrying it. effective population size the average number of individuals in a population that contribute genes to succeeding generations.
If the population size shows a cyclical variation as a function of season of the year, predation, parasitism, and other factors, the effective population size is closer to the number of individuals observed during the period of maxi- mal contraction. effector 1. a molecule that affects (positively or negatively) the function of a regulatory protein. 2. an organ or cell that reacts to a nervous stimulus by doing chemical or mechanical work; for example, muscles, glands, electric organs, etc.
effector cell in immunology, a cell (usually a T lymphocyte) that carries out cell-mediated cytotox- icity. effector molecules small molecules that combine with repressor molecules and activate or inactivate them with respect to their ability to combine with an operator gene.
See inducible system, repressible system, regulator gene. efferent leading away from an organ, cell, or point of reference. In immunology, the efferent branch of the immune response includes the events occurring after the activation of the immune system (e.g., anti- bodies combining with antigens or cytokines stimu- lating specific cells). Compare with afferent. efflux pump any cellular mechanism that expels drugs (such as antibiotics) or other environmental toxins from the cell.
EGF epidermal growth factor (q.v.). egg a female gamete; an ovum. See Appendix C, 1651, Harvey; 1657, de Graff; 1827, von Baer. egg chamber the insect ovarian follicle. In Dro- sophila, it consists of a cluster of 16 interconnected cystocytes surrounded by a monolayer of follicle cells.
The oocyte is the most posterior cystocyte. The remaining 15 cystocytes function as nurse cells (q.v.). Ehlers-Danlos syndrome a family of hereditary diseases characterized by overelasticity and brittle- ness of the skin and by excessive extensibility of the joints.
The underlying defects involve blocks in the synthesis of collagen (q.v.). In type VI, for example, a hydroxylysine-deficient collagen is produced that is unable to form intermolecular cross-links. Both X-linked and autosomal genes are involved.
The syn- drome gets its name from Edward Ehlers, a Danish dermatologist, and Henri Danlos, a French physi- cian, who published descriptions of the condition in 1901 and 1908, respectively. einkorn wheat Triticum monococcum “one-grained” wheat, so called because it has a single seed per spikelet.
A wheat in cultivation since stone age times. See wheat. ejaculate 1. the process of semen release in higher vertebrates; also called ejaculation. 2. the semen re- leased in a given copulatory interaction or in a given artificially induced response. elaioplast an oil-rich plastid.
elastin a rubber-like, that is the main component of the elastic fibers found in tendons, ligaments, and the walls of bron- chi and arteries. electroblotting See blotting. electrode either terminal of an electrical appa- ratus. electroencephalogram the record of the rhythmi- cal changes in the electrical potential of the brain. electrofusion See Zimmermann cell fusion. electrolyte a substance that when dissolved in wa- ter conducts an electric current.
electron a negatively charged particle that is a con- stituent of every neutral atom. Its mass is 0.000549 atomic mass units. Compare with positron. electron carrier an enzyme, such as a flavoprotein or cytochrome, that can gain and lose electrons re- versibly. electron-dense said of a dense area seen on an electron micrograph, since the region has prevented electrons from passing through it. High electron density may mean that the area contains a high con- centration of macromolecules or that it has bound
the heavy metals (Os, Mn, Pb, U) used as fixatives and/or stains. electron microscope a magnifying system that uses beams of electrons focused in a vacuum by a series of magnetic lenses and that has a resolving power hundreds of times that of the best optical mi- croscopes. Two main types of electron microscopes are available. In the transmission electron microscope (TEM), the image is formed by electrons that pass through the specimen.
In the scanning electron micro- scope (SEM), image formation is based on electrons that are reflected back from the specimen. Thus, the TEM resembles a standard light microscope, which is generally used to look at tissue slices, and the SEM resembles a stereoscopic dissecting microscope, which is used to examine the surface properties of biologi- cal materials. See Appendix C,1932, Knoll and Ruska; 1963, Porter and Bonneville.
electron microscope techniques See freeze-etch- ing, freeze-fracture, Kleinschmidt spreading tech- nique, negative staining. electron pair bond convalent bond. electron transport chain a chain of molecules lo- calized in mitochondria and acting as hydrogen and electron acceptors.
The chain functions to funnel electrons from a given substrate to O2. The energy released is used to phosphorylate ADP. See cyto- chrome system, mitochondrial proton transport. electron volt a unit of energy equivalent to the amount of energy gained by an electron passing through a potential difference of one volt.
Larger multiple units of the electron volt (eV) are fre- quently used: keV = kilo (thousand) eV; MeV = mega (million) eV; and GeV = giga (billion) eV. electropherogram a supporting medium contain- ing a collection of molecules that have been sepa- rated by electrophoresis. The medium is generally in the form of a sheet, much longer and wider than it is thick, and it is often a gel such as agarose.
electrophoresis the movement of the charged molecules in solution in an electrical field. The solu- tion is generally held in a porous support medium such as filter paper, cellulose acetate (rayon), or a gel made of starch, agar, or polyacrylamide. Electro- phoresis is generally used to separate molecules from a mixture, based upon differences in net electrical charge and also by size or geometry of the mole- cules, dependent upon the characteristics of the gel matrix.
The SDS-PAGE technique is a method of separating proteins by exposing them to the anionic detergent sodium dodecyl sulfate (SDS) and poly- acrylamide gel electrophoresis (PAGE). When SDS binds to proteins, it breaks all noncovalent interac- tions so that the molecules assume a random coil configuration, provided no disulfide bonds exist (the latter can be broken by treatment with mercapto- ethanol).
The distance moved per unit time by a random coil follows a mathematical formula involv- ing the molecular weight of the molecule, from which the molecular weight can be calculated. See Appendix C, 1933, Tiselius; pulsed field gradient gel electrophoresis, zonal electrophoresis. electroporation The application of electric pulses to animal cells or plant protoplasts to increase the permeability of their membranes.
The technique is used to facilitate DNA uptake during transformation experiments. electrostatic bond the attraction between a posi- tively charged atom (cation) and a negatively charged atom (anion), as in a crystal of common table salt (NaCl); also known as an ionic bond. Electrostatic bonds are called salt linkages in the older literature. In proteins, the positively charged side groups of ly- sine and arginine form electrostatic bonds with the negatively charged side groups of aspartic and glu- tamic acids. This stabilizes the tertiary and quater- nary structures of these molecules. See amino acids, protein structure.
element a pure substance consisting of atoms of the same atomic number and that cannot be decom- posed by ordinary chemical means. See chemical ele- ments, periodic table. ELISA enzyme-linked immunosorbent assay (q.v.). Ellis-vanCreveld (EVC) syndrome a hereditary disease characterized by shortening of the forearms and lower legs, extra fingers, and congenital heart malformations. The condition was first described by Richard Ellis and Simon vanCreveld in 1940.
Fifty cases of EVC syndrome that occurred in the Amish (q.v.) community of Lancaster County have been traced back to a single couple who emigrated to Pennsylvania in 1744. The disease has been shown to be due to a mutation in an intron of the EVC gene that causes aberrant splicing. This gene has been mapped to 4p16, and it encodes a protein that contains 992 amino acids. The function of the EVC protein has not been determined. See posttranscrip- tional processing. Elodea canadensis a common pond weed that be- haves as a facultative apomict (q.v.). See Appendix C, 1923, Santos.