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Supercoiling Whites formula explains the phenomenon of supercoiling in DNA rings. Linear molecules of DNA assume a configuration known as the b-configuration, with about ten base pairs for every turn of the double helix. This configuration has minimum energy. In the ring form too, the DNA double helix tries to attain the state of minimum energy. Energy of the molecule changes if there is a change in pitch (that is, the number of bases per full turn) or bending of the double helix ring. The DNA ring approximates the b-configuration of the linear molecule while trying to attain the state of minimum energy. In a closed circular double helix, there is usually a deficit in the link. Whites formula dictates that this has to be compensated by a change in the twist or writhe of the DNA ribbon. Now, even a small change in the pitch of the DNA results in a large increase in energy. So, the twist remains almost constant. Hence, the change in link has to be compensated by a change in writhe, i.e.,
Thus, the writhe becomes non-zero and so, the DNA molecule supercoils and that too in the shape of an interwound helix. Here the increase in energy due to bending as well as change in pitch is minimum and so a state of minimum energy is achieved.
In the same way, supercoiling can be caused even by an increase in the linking number (though this does not occur in nature). Supercoils formed by deficit in link are called negative supercoils and the ones formed by an increase in link are called positive supercoils.
This alteration in link is achieved by enzymes, broadly catagorised as Topoisomerases. Topoisomerases are enzymes that change the topological structure, i.e., the link of a DNA molecule. They do so, by temporarily breaking one of the strands and passing the other strand through it.
Topoisomerases are categorised into two types: Type-1, which alter the link by 1, by cutting one strand and passing the other strand through the break. Type-2 Topoisomerases, on the other hand, alter the link by two, by breaking both the strands of the double helix at the same time and passing a segment of the double helix through the break. Importance of Supercoiling One obvious use of Supercoiling is that it helps to pack large circular rings of DNA in a small space by making these rings highly compact. It also helps in the unwinding process required for replication and transcription of DNA.
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