The number of times that the two strands of a closed-circular, double-helical molecule cross each other. The twisting number (T) of a relaxed closed-circular DNA is the total number of base pairs in the molecule divided by the number of base pairs per turn of the helix. For relaxed DNA in the normal B form, L is the number of base pairs in the molecule divided by 10. The writhing number (W) is the number of times the axis of a DNA molecule crosses itself by supercoiling. The linking number (L) is determined by the formula: L = W + T. For a relaxed molecule, W = 0, and L = T. The linking number of a closed DNA molecule cannot be changed except by breaking and rejoining of strands. The utility of the linking number is that it is related to the actual enzymatic breakage and rejoining events by which changes are made in the topology of DNA. Any changes in the linking number must be by whole integers. Molecules of DNA that are identical except for their linking numbers are called topological isomers.
Subjects: Genetics and Genomics — Chemistry.