Topological structure of closed circular DNA

In this paper we set up a topological model which can be used to describe closed circular (cc) DNAs on the basis of Seifert construction in knot theory. We visually prove that the backbones of T-2n-ccDNAs (n = 1,2,...) in the crystal which are topological chiral possess the point symmetry group C-n, and that the backbones of T-2n-ccDNAs in solution which are topological achiral possess the point symmetry groups S-n/2, S-n and S-2n. Topological achirality leads us to believe that right-handed DNA can be flipped over into a left-handed helix. Further, it also provides a sound basis for the investigation of this surprising manifestation of DNA supercoiling, replication, transcription and recombination. (C) 1998 Elsevier Science B.V.