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What is it?
The structure of the DNA's helix allows for the entire molecule to be 'twisted' together like a rope. In a relaxed state, the helix completes one complete turn every 10.4 base pair connections. However, the DNA polymer chains can be either more or less 'twisted', thus 'tightening' or 'loosening' the twist in the helix structure.
Kinds of supercoiling:
There are two basic variations on the supercoil, the positive supercoil and the negative supercoil. The positive supercoil occurs when the DNA structure becomes more tightly wound. This results in the base pairs being closer to each other, as well there being more 'twists' in the helix per cm of DNA. The other variation, negative supercoiling, occurs when the strands are 'loosened', such that helix itself attains less base pair connections per individual turn.
The function of supercoiling is, what is typically referred to, as an abstract mathematical property. The 'Supercoiling' factor (S) is equal to the sum of the twist (T) and the 'writhe' (W) of the DNA's structure. The relationship is often expressed S=T+W. If 'T' in the equation is positive, then the twists enacted on the DNA structure during the supercoiling process will cause the DNA to compress. If 'T' is negative, then the DNA will expand.
The supercoiling of most DNA in biological organisms is, to some degree, negatively coiled. This allows for the enhanced 'local unwinding' of the DNA which is a useful function in genetic behaviours. Also, due to the enlarged size of many chromosomes, their middle segments may often act as if having 'anchored' ends resulting in an inability to redistribute 'twist' in the structure typically acquired from underwinding the structure.
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