  In this Letter, we investigate the link between thermal denaturation and
mechanical unzipping for two models of DNA, namely the Dauxois-Peyrard-Bishop
model and a variant thereof we proposed recently. We show that the critical
line that separates zipped from unzipped DNA sequences in mechanical unzipping
experiments is a power-law in the temperature-force plane. We also prove that
for the investigated models the corresponding critical exponent is proportional
to the critical exponent alpha, which characterizes the behaviour of the
specific heat in the neighbourhood of the critical temperature for thermal
denaturation.
