  A simple model of DNA based on two interacting polymers has been used to
study the unzipping of a double stranded DNA subjected to a periodic force. We
propose a dynamical transition, where without changing the physiological
condition, it is possible to bring DNA from the zipped/unzipped state to a new
dynamic (hysteretic) state by varying the frequency of the applied force. Our
studies reveal that the area of the hystersis loop grows with the same
exponents as of the isotropic spin systems. These exponents are amenable to
verification in the force spectroscopic experiments.
