CONFORMATIONS OF LINEAR DNA

We examine the conformations of a model for underwound and overwound D NA. The molecule is represented as a cylindrically symmetric elastic s tring subjected to a stretching force and to constraints corresponding tb a specification of the link number. We derive a fundamental relati on between the Euler angles that describe the curve and the topologica l linking number. Analytical expressions for the spatial configuration s of the molecule in the infinite-length limit are obtained. A unique configuration minimizes the energy for a given set of physical conditi ons. An elastic model incorporating thermal fluctuations provides exce llent agreement with experimental results on the plectonemic transitio n.