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The chromosomes of several viruses and bacteria are circular molecules of duplex DNA. The electron micrographs show a peculiar twisted appearance that is termed supercoiling or super-helicity. The mathematical expression for this phenomenon is:

$\mathrm{L}=\mathrm{T}+\mathrm{W}$

The relationship between linking number, twist and writhing number can be given as:

$\text{L = T + W}$

Here, L represents the linking number, it is the number of times a particular DNA strand winds around another DNA strand. The linking number cannot be altered by distorting or twisting the specific molecule, as long as both the strands stay intact covalently.

T represents the twist, which denotes the number of complete revolutions that one polynucleotide makes around the duplex axis. T is positive for right handed duplex. For B-DNA, the twist is the count of base pairs divided by 10.5.

W indicates the writhing number, it indicates the count of turns that the duplex axis makes around the superhelix axis. It computes the DNA’s superhelicity.