Bending and Twisting Elasticity: a Revised Marko-Siggia Model on DNA   Chirality
  A revised Marko-Siggia elastic model for DNA double helix [Macromolecules 27, 981 (1994)] is proposed, which includes the WLC bending energy and a new chiral twisting energy term. It is predicted that the mean helical repeat length (HRL) for short DNA rings increases with the decreasing of chain length; while for very long chains, their mean HRL is the same, independent of both the chain length and whether the ends are closed, it is longer than the value for rectilinear DNAs. Our results are in good agreement with experiments. 
Zhou Haijun and Ou-Yang Zhong-can