The temperature dependence of the 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  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 is shown to be due to the thermally generated bubbles  . For the case of intrinsic rigidity, the elastic constant is found to be proportional to the bubble number fluctuation.
The temperature dependence of the 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 the thermally generated bubbles has been obtained exactly . For the case of intrinsic rigidity, the elastic constant is found to be proportional to the bubble number fluctuation.
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A
has been obtained exactly