Before revision:  We propose a dynamic transition, which has potential application in understanding non-equilibrium processes such as transcription and replication of DNA, unfolding of proteins and RNA etc.
After revision: 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 dynamic transition, which has potential application in understanding non-equilibrium processes such as transcription and replication of DNA, unfolding of proteins and RNA etc.