  We have investigated the denaturation bubble life time for a homogeneous as
well as for a heterogeneous DNA within a Poland-Scheraga model. It is shown
that at criticality the bubble life time for a homogeneous DNA is finite
provided that the loop entropic exponent c>2 and has a scaling dependence on
DNA length for c<2. Heterogeneity in the thermodynamical limit makes the bubble
life time infinite for any entropic exponent.
