Fluctuations and Stiffness of Double-Stranded Polymers: Railway-Track Model

Many biopolymers are double-stranded with a tight coupling between the filaments. We introduce a simple model to study the consequences. It consists of two worm-like chains coupled by harmonic springs between points of identical contour length and displays a novel behaviour: a stiffness larger on large than on small lengthscales. Neglecting local deformations the global persistence length is found to be infinite. Otherwise, worm-like chain behaviour with a higher, renormalized bending stiffness is recovered on large lengthscales. We estimate the coupling contribution to the bending energy of double helices and argue that it could contribute to the stiffness of biopolymers.