This paper aims at a comprehensive understanding on the novel elasticproperty of double-stranded DNA (dsDNA) discovered very recently throughsingle-molecule manipulation techniques. A general elastic model fordouble-stranded biopolymers is proposed and a new structural parameter calledthe folding angle $\phi$ is introduced to characterize their deformations. Themechanical property of long dsDNA molecules is then studied based on thismodel, where the base-stacking interactions between DNA adjacent nucleotidebasepairs, the steric effects of basepairs, and the electrostatic interactionsalong DNA backbones are taken into account. Quantitative results are obtainedby using path integral method, and excellent agreement between theory and theobservations reported by five major experimental groups are attained. Thestrong intensity of the base-stacking interactions ensures the structuralstability of DNA, while the short-ranged nature of such interactions makesexternally-stimulated large structural fluctuations possible. The entropicelasticity, highly extensibility, and supercoiling property of DNA are allclosely related to this account. The present work also suggests the possibilitythat negative torque can induce structural transitions in highly extended DNAfrom right-handed B-form to left-handed configurations similar with Z-formconfiguration. Some formulae concerned with the application of path integralmethod to polymeric systems are listed in the Appendix.
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