  We model unzipping of DNA/RNA molecules subject to an external force by a
spin-oscillator system. The system comprises a macroscopic degree of freedom,
represented by a one-dimensional oscillator, and internal degrees of freedom,
represented by Glauber spins with nearest-neighbor interaction and a coupling
constant proportional to the oscillator position. At a critical value $F_c$ of
an applied external force $F$, the oscillator rest position (order parameter)
changes abruptly and the system undergoes a first-order phase transition. When
the external force is cycled at different rates, the extension given by the
oscillator position exhibits a hysteresis cycle at high loading rates whereas
it moves reversibly over the equilibrium force-extension curve at very low
loading rates. Under constant force, the logarithm of the residence time at the
stable and metastable oscillator rest position is proportional to $(F-F_c)$ as
in an Arrhenius law.
