THE UNIVERSITY OF ROCHESTER

Many thanks for your recent letter which has done wonders to clarify my own confusion about handedness in superhelical DNA structures, The stiff rubber tubing experiment you described is really beautiful in that it demonstrates how a left-handed toroidal helix (the state of DNA in SV-40 when bound to histones) is converted to a right-handed interwound helix when histones have been removed. In the absence of histones, the latter structure has lower free energy than the former structure and therefore arises spontaneously. It is therefore the form readily prepared in the laboratory.