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If a DNA segment under twist strain is closed into a circle by joining its two ends, and then allowed to move freely, it takes on different shape, such as a figure-eight. This shape is referred to as a supercoil. (The noun form "supercoil" is often used when describing DNA topology.) The DNA of most organisms is usually negatively supercoiled.
Contrary to intuition, a topological property, the linking number, arises from the geometric properties twist and writhe according to the following relationship: L k = T + W, where L k is the linking number, W is the writhe and T is the twist of the coil. The linking number refers to the number of times that one strand wraps around the other.
There is also some relevant info at DNA supercoil and Nucleic acid tertiary structure. Of note "Additionally, certain enzymes such as topoisomerases are able to change DNA topology to facilitate functions" -- e.g. there are enzymes that can add/remove twist, which changes the whole structure.
In DNA double helix, the two strands of DNA are held together by hydrogen bonds. The nucleotides on one strand base pairs with the nucleotide on the other strand. The secondary structure is responsible for the shape that the nucleic acid assumes. The bases in the DNA are classified as purines and pyrimidines. The purines are adenine and guanine ...
After introducing a nick in the system, the negative supercoil gradually unwinds (c) until it reaches its final, circular, plasmid state (d). [2] Nicked DNA can be the result of DNA damage or purposeful, regulated biomolecular reactions carried out in the cell. During processing, DNA can be nicked by physical shearing, over-drying, or enzymes.
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