Covalently Closed, Circular DNA

Let consider topologicl properties of covalently closed, circular DNA, which is referred as cccDNA. Because there are no interruptions in either polynucleotide chain, the two strands of cccDNA cannot separated from each other without breaking of covalent bond. If we wished to separate two circular strands without permanently breaking bonds in the sugar phosphate backbones, we would have to pass one strand through ther strand repeatedly. The number of times one strand would have to be passed through the other strand in order for the two strands to be entirely separated from each other is called the linking number. The linking number, which is always an integer, is an invariant topological property of cccDNA, no matter how much the DNA molecule is distorted.

Topological States of Covalently Closed Circular (ccc) DNA

Topological States of Covalently Closed Circular (ccc)  DNA. The figure shows conversion of the relaxed (a) to  the negatively supercoiled (b) form of DNA. The strain in the  supercoiled form may be taken up by supertwisting (b) or  by local disruption of base pairing (c). [Adapted from a diagram provided by  Dr. M. Gellert.] (Source: Modified from Kornberg, A.  and  Baker, T. A. 1992. DNA Replication. Figure 1-21, page 32)