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The Journal of Chemical Physics , 18 , Chemistry - A European Journal , 18 35 , Journal of Physical Organic Chemistry , 25 8 , Angewandte Chemie , 19 , A cord, thread, or anything flexible, formed by winding strands or separate things round each other.
A kind of closely twisted, strong sewing silk, used by tailors, saddlers, and the like. A kind of cotton yarn, of several varieties. A roll of twisted dough, baked. A little twisted roll of tobacco. One of the threads of a warp, -- usually more tightly twisted than the filling.
A material for gun barrels, consisting of iron and steel twisted and welded together; as, Damascus twist. The spiral course of the rifling of a gun barrel or a cannon. A beverage made of brandy and gin. A twig. Example Sentences: 1 Aberrant forms elongated and twisted in the vacuole and double virions in the plasma membrane were observed as early as 65 h after infection.
Writhe Definition: v. For each additional helical twist being accommodated, the lobes will show one more rotation about their axis. The noun form "supercoil" is rarely used in the context of DNA topology. Instead, global contortions of a circular DNA, such as the rotation of the figure-eight lobes above, are referred to as writhe. The above example illustrates that twist and writhe are interconvertible.
The twist is the number of helical turns in the DNA and the writhe is the number of times the double helix crosses over on itself these are the supercoils.
The relationship of twist, writhe and supercoiling is expressed as the equation:. Extra helical twists are positive and lead to positive supercoiling, while subtractive twisting causes negative supercoiling. Many topoisomerase enzymes sense supercoiling and either generate or dissipate it as they change DNA topology.
DNA of most organisms is negatively supercoiled. In part because chromosomes may be very large, segments in the middle may act as if their ends are anchored. As a result, they may be unable to distribute excess twist to the rest of the chromosome or to absorb twist to recover from underwinding--the segments may become supercoiled , in other words. In response to supercoiling, they will assume an amount of writhe, just as if their ends were joined.
Supercoiled DNA forms two structures; a plectoneme or a toroid, or a combination of both. A negatively supercoiled DNA molecule will produce either a one-start left handed helix, the toroid, or a two-start right handed helix with terminal loops, the plectoneme.
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