Evidence for a second order supercoiling of plectonemic DNA

Abstract

DNA has been known to have a double helical structure since the pioneering work of Crick and Watson. Since then much work has been done on the conformation of various macromolecules. Advances in electron microscopy, small angle x-ray scattering (SAXS), small angle neutron scattering (SANS), synchrotron sources and the like made it possible to investigate larger correlation distances. 
Supercoiling is the coiling of the axis of the Crick-Watson helix. It occurs when there is an excess or insufficiency in the twist of the relaxed form of the helix. There are two known conformations for supercoiled DNA. One is toroidal, where the resulting superhelical axis is a circle. The other is interwound or plectonemic, where the superhelical axis is a line. One form can convert to the other since they are topologically equivalent. It is not entirely known which form is favored given certain environmental conditions.
Here we present evidence for another order of supercoiling for plectonemic DNA.