  The temperature dependence of DNA flexibility is studied in the presence of
stretching and unzipping forces. Two classes of models are considered. In one
case the origin of elasticity is entropic due to the polymeric correlations,
and in the other the double-stranded DNA is taken to have an intrinsic rigidity
for bending. In both cases single strands are completely flexible. The change
in the elastic constant for the flexible case due to thermally generated
bubbles is obtained exactly. For the case of intrinsic rigidity, the elastic
constant is found to be proportional to the square root of the bubble number
fluctuation.
