DNA under high tension: Overstretching, undertwisting, and relaxation dynamics

Single-molecule experiments indicate that a double-stranded DNA increases in length if put under tension greater than 10 pN; beyond this point its conformation can no longer be described using an inextensible wormlike-chain model. The simplest extensible wormlike chain with twist rigidity is considered as a model for DNA under tension, and it is found that the fact that DNA is chiral demands that stretching be coupled to twisting at Linear order in elastic theory; stretching a DNA is thus a way to perturb its twist degress of freedom. Nonlinearities are essential to stabilize the experimentally observed "overstretched" state which is roughly 1.6 times the length of the unperturbed double helix, and undertwisted. If DNA linkage is held fixed, the transition to the overstretched state is strongly broadened. The overstretching model is also used to study DNA conformational change driven by the binding of RecA proteins to the double helix, and indicates that RecA cannot be dissociated from DNA by application of tension. Finally, relaxation dynamics of a stretched DNA are discussed. The existence of a dynamical writhe instability for a chain without fixed linkage number is demonstrated; it is also argued that the (undertwisted) interior of an overstretched DNA will supercoil as it relaxes.