  When pulled along its axis, double-stranded DNA elongates abruptly at a force
of about 65 pN. Two physical pictures have been developed to describe this
overstretched state. The first proposes that strong forces induce a phase
transition to a molten state consisting of unhybridized single strands. The
second picture instead introduces an elongated hybridized phase, called S-DNA,
structurally and thermodynamically distinct from standard B-DNA. Little
thermodynamic evidence exists to discriminate directly between these competing
pictures. Here we show that within a microscopic model of DNA we can
distinguish between the dynamics associated with each. In experiment,
considerable hysteresis in a cycle of stretching and shortening develops as
temperature is increased. Since there are few possible causes of hysteresis in
a system whose extent is appreciable in only one dimension, such behavior
offers a discriminating test of the two pictures of overstretching. Most
experiments are performed upon nicked DNA, permitting the detachment
(`unpeeling') of strands. We show that the long-wavelength progression of the
unpeeled front generates hysteresis, the character of which agrees with
experiment only if we assume the existence of S-DNA. We also show that internal
melting (distinct from unpeeling) can generate hysteresis, the degree of which
is strongly dependent upon the nonextensive loop entropy of single-stranded
DNA.

Statistical Mechanics;Biomolecules