Theory of DNA Superhelicity

When a duplex DNA molecule is covalently closed into a circle, conservation of the antiparallel chemical (5′–3′) orientations of the two strands dictates that the resulting ring molecule must have the topological structure of two closed, interlinked circles. This fixes the value of the molecular linking number Lk. This Lk is an integer topological invariant which measures the number of times either strand would have to be passed through the other before they could be physically disentangled (provided the molecule is not knotted). Its value remains fixed, independent of geometric deformation of the molecule, as long as both strands are covalently closed. Lk can only be varied by the introduction of transient strand breaks. A class of enzymes called DNA topoisomerases has been found whose function is to regulate Lk in this manner, suggesting a physiological function for this activity.1,2