Using Langevin dynamics simulations, we study the hysteresis in unzipping of longer double stranded DNA chains whose ends are subjected to a time dependent periodic force with frequency $\omega$ and amplitude $G$ keeping other end fixed. We find that area loop, $A_{loop}$, scales as $1/\omega$ at higher frequencies, whereas it $(G-G_c)^{\alpha}\omega^{\beta}$ exponents $\alpha=1$ $\beta=1.25$ l...