  The unzipping transition under the influence of external force of a dsDNA
molecule has been studied using the Peyrard-Bishop Hamiltonian. The critical
force $F_c(T)$ is found to depend on the potential parameters $k$, represents
the stiffness of single strand of DNA and the potential depth $D$. We used
constant extension ensemble to calculate the average force needed to stretch a
base pair $y$ distance apart. A very large peak around $y = 1 {\rm \AA}$ is
found. The value of $F(y)$ needed to stretch a base pair located far away from
the ends of a dsDNA molecule is found twice the value of the force needed to
stretch a base pair located at one of the ends to the same distance. The effect
of mismatching in the base pairs on the peak height and position is
investigated. The formation and behaviour of a loop of Y shape when one of the
ends base pair is stretched and a bubble of ssDNA with the shape of "an eye"
when a base pair far from ends is stretched are investigated.
