ultramicrotome an instrument that cuts ultrathin sections (50-100 nanometers thick) of plastic em- bedded tissues using knives made from polished dia- monds or from triangles of broken plate glass. See Appendix C, 1950, Latta and Hartmann; 1953, Por- ter-Blum and Sjostrand; knife breaker. ultrasonic pertaining to ultrasound. ultrasound sound waves of frequency higher than the human audible limit of about 20,000 vibrations per second. ultrastructure fine structure, especially within a cell, that can be seen only with the high magnifica- tion obtainable with an electron microscope. ultraviolet absorption curve the curve showing the relation between the relative amount of ultravio- let radiation absorbed by a solution of molecules and the wave length of the incident light. ultraviolet microscope an optical system utilizing ultraviolet radiation. Since glass filters out UV, quartz transmitting or glass reflecting lenses must be used in the UV microscope. Such a microscope has double the resolving power of the light microscope.
Furthermore, if monochromatic UV of a wavelength absorbed by nucleic acids (260 nanometers) is used, nucleic acid-rich structures may be photographed in unstained cells. In combination with a spectropho- tometer, the UV microscope provides a method for the quantitative estimation of nucleic acids in cells. ultraviolet radiation that part of the invisible electromagnetic spectrum just beyond the violet with wavelengths between 1,000 and 4,000 A˚ng- stroms. Wavelengths around 260 nanometers are ab- sorbed by DNA.
See Appendix C, 1939, Knapp et al.; melanin, thymine dimer, UV reactivation. uncharged tRNA a tRNA molecule to which no amino acid is attached. underdominance the unusual situation where a heterozygote shows an attribute, such as viability or fertility, that is lower than either homozygote. For example, the New Zealand Black (NZB) strain of mouse spontaneously develops a disease that resem- bles lupus erythematosis in humans Zealand White (NZW) mice are normal in this re- gard.
The hybrid offspring from crossing these in- bred strains of mice (NZB × NZW) develop a more severe disease than that of the NZB strain. undersea vent communities chemosynthetic or- ganisms that live at great depths and in the absence of sunlight along tectonically active rifts where lava erupts from ocean floor. The first community of this type was discovered around the vents of sulfide hot springs at a depth of 8,000 feet about 380 miles north of the Galapagos Islands.
See Appendix C, 1977, Corliss and Ballard; hyperthermophile. underwinding coiling of a DNA molecule in a left-handed direction, i.e., opposite to that of the double helix; negative supercoiling. undulipodium any cellular projection surrounding a cylindrical shaft containing a bundle of eleven mi- crotubules, nine of which form a circle around the central pair, while the cortical microtubules are dou- blets. Cilia and flagellae (q.v.) are examples. Masti- gote protoctists possess undulipodia; amastigote pro- toctists do not. See axoneme, Rhizopoda. unequal crossing over a recombinational event that involves recombining sites that are misalinged, and nonreciprocal recombinant chromosomes are formed as a result.
The phenomenon was discovered in Drosophila at the Bar (q.v.) locus. Here improper pairing of a duplicated chromosomal segment was followed by unequal crossing over. The result was one crossover chromatid with one copy of the seg- ment and another with three copies. Unequal cross- ing over is responsible for generating duplications and deletions at many sites in human chromosomes where clusters of duplicated DNA segments occur.
See Appendix C, 1925, Sturtevant; cone pigment genes (CPGs), hemoglobin Lepore, silk, thalassemias. ungulate a hoofed mammal. Perissodactyla plus Artiodactyla. See Appendix A. uniformitarianism a geological theory that present is the key to the past.” In other words, the phenomena of volcanism, crustal movements, ero- sion, glaciation, etc., that can be seen today have been operating through billions of years of earth his- tory, and they are the primary forces that have made the earth what it is today. Contrast with catastro- phism. uniform resource locator (URL) the symbolic rep- resentation of an address used on the World Wide Web.
See Appendix E for examples of URLs. unineme hypothesis the concept that a newly formed chromatid contains only one DNA duplex extending from one end to the other. Contrast with polyneme hypothesis. See Appendix C, 1973, Kavenoff and Zimm. uniovular twins monozygotic twins (q.v.). uniparental disomy See disomy.