Correlated local bending of DNA and its effect on DNA flexibility

The flexibility of long DNA chains can be well described by the worm-like chain model (WLC) as a semi-flexible polymer with all local details coarse grained into one parameter, the persistence length l$_{\mathrm{p}}$ (approximately 150 base pairs). Recent experimental studies of DNA in the sub persistence length regime have shown a dramatic departure from WLC and suggested a length dependent DNA flexibility. Here we report an improved model of DNA flexibility with explicit considerations of a new length scale l$_{\mathrm{D}}$ (approximately 10 base pairs), over which DNA local bend angles are correlated (arxiv.org/abs/1309.7515). In this correlated worm-like chain (C-WLC) model, a finite length correction term is analytically derived and the persistence length is found to be contour length dependent. While our model reduces to the traditional worm-like chain model when treating long DNA at length scales much larger than l$_{\mathrm{p}}$, it predicts that DNA becomes much more flexible at shorter sizes, in good agreement with recent cyclization measurements of short DNA fragments around 100 base pairs.