  A double stranded DNA molecule when pulled with a force acting on one end of
the molecule can become either partially or completely unzipped depending on
the magnitude of the force F. For a random DNA sequence, the number M of
unzipped base pairs goes as M~(F-Fc)^(-2) and diverges at the critical force Fc
with an exponent \chi=2. We find that when excluded volume effect is taken into
account for the unzipped part of the DNA, the exponent \chi=2 is not changed
but the critical force Fc is changed. The force versus temperature phase
diagram depends on only two parameters in the model, the persistence length and
the denaturation temperature. Furthermore a scaling form of the phase diagram
can be found. This scaling form is parameter independent and depends only on
the spatial dimension. It applies to all DNA molecules and should provide a
useful framework for comparison with experiments.
