Flexibility of DNA.

A model for the flexibility of DNA is proposed that is based on discrete variations in the direction of propagation in going from one subunit to the next. Expansion of the local free energy in terms of the local bending gives a Gaussian distribution function. The assumption of the independence of local bends on neighbors lead to very simple formulae for the persistence length and the characterist.ic ratio. Emphasis, however, is placed on the application of the formulae for molecules of finite size where the persistence length and C, are not defined. The formulae are worked out for two models, which should serve as limits for the real physical situation.