  The force induced unzipping transition of a double stranded DNA is considered
from a purely thermodynamic point of view. This analysis provides us with a set
of relations that can be used to test microscopic theories and experiments. The
thermodynamic approach is based on the hypothesis of impenetrability of the
force in the zipped state. The melting and the unzipping transitions are
considered in the same framework and compared with the existing statistical
model results. The analysis is then extended to a possible continuous unzipping
transition.
