Properties of DNA crossovers and their link to DNA topology.

<p>In (+)sc DNA, the plectoneme forms a left-handed superhelix with right-handed crossovers (clockwise rotation for closing the small angle). In (−)sc DNA, the plectoneme forms a right-handed superhelix with left-handed crossovers. (a) Schematic representation of (+)sc DNA and catenanes with right-handed crossovers formed by groove-backbone interaction between two B-DNA duplexes, as found in crystallographic structures. (b) Schematic representation of (−)sc DNA with left-handed crossovers formed by the juxtaposition of the major grooves of two B-DNA duplexes, as found in crystallographic structures.