06413_10 - J JacobMonod hypothesis The theory postulated by...

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J Jacob–Monod hypothesis The theory postulated by the French biologists François Jacob (1920– ) and Jacques Monod (1910–76) in 1961 to explain the control of * gene expression in bacteria ( see operon ). Jacob and Monod investi- gated the expression of the gene that codes for the enzyme β -galactosidase, which breaks down lactose; the operon that regulates lactose metabolism is called the * lac operon. jade A hard semiprecious stone consist- ing either of jadeite or nephrite. Jadeite , the most valued of the two, is a sodium aluminium pyroxene, NaAlSi 2 O 6 . It is prized for its intense translucent green colour but white, green and white, brown, and orange varieties also occur. The only important source of jadeite is in the Mogaung region of upper Burma. Nephrite is one of the amphibole group of rock-forming minerals. It occurs in a vari- ety of colours, including green, yellow, white, and black. Important sources in- clude Siberia, Turkistan, New Zealand, Alaska, China, and W USA. jadeite See jade . Jahn–Teller effect If a likely structure of a nonlinear molecule or ion would have degenerate orbitals (i.e. two molecu- lar orbitals with the same energy levels) the actual structure of the molecule or ion is distorted so as to split the energy levels (‘raise’ the degeneracy). The effect is observed in inorganic complexes. For example, the ion [Cu(H 2 O) 6 ] 2+ is octahe- dral and the six ligands might be ex- pected to occupy equidistant positions at the corners of a regular octahedron. In fact, the octahedron is distorted, with four ligands in a square and two opposite ligands further away. If the ‘original’ structure has a centre of symmetry, the distorted structure must also have a cen- tre of symmetry. The effect was predicted theoretically by H. A. Jahn and Edward Teller in 1937. jasper An impure variety of * chal- cedony. It is associated with iron ores and as a result contains iron oxide impurities that give the mineral its characteristic red or reddish-brown colour. Jasper is used as a gemstone. jaw The part of the vertebrate skeleton that provides a support for the mouth and holds the teeth. It consists of two bones, the upper jaw (maxilla) and the lower jaw (mandible). Members of the Ag- natha lack jaws. Jeans instability Instability in a cloud of gas in space due to Û uctuations in the density of the gas, causing the matter in the cloud to clump together and lead to gravitational collapse. The conditions under which this occurs were worked out by Sir James Hopwood Jeans (1877–1946) in terms of Newtonian gravity. The analo- gous analysis of this problem using gen- eral relativity theory is the basis of the theory of * structure formation. jejunum
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This note was uploaded on 05/29/2009 for the course CIVIL ENG 12345 taught by Professor Xhfxfh during the Spring '09 term at Amirkabir University of Technology.

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06413_10 - J JacobMonod hypothesis The theory postulated by...

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