  As sections of a strand duplexed DNA denature when exposed to high
temperature, the excess linking number is taken up by the undenatured portions
of the molecule. The mechanical energy that arises because of the overwinding
of the undenatured sections can, in principle, alter the nature of the thermal
denaturation process. Assuming that the strains associated with this
overwinding are not relieved, we find that a simple model of strain-altered
melting leads to a suppression of the melting transition when the unaltered
transition is continuous. When the melting transition is first order in the
absence of strain associated with overwinding, the modification is to a third
order phase transition.
