  This paper aims at a comprehensive understanding on the novel elastic
property of double-stranded DNA (dsDNA) discovered very recently through
single-molecule manipulation techniques. A general elastic model for
double-stranded biopolymers is proposed and a new structural parameter called
the folding angle $\phi$ is introduced to characterize their deformations. The
mechanical property of long dsDNA molecules is then studied based on this
model, where the base-stacking interactions between DNA adjacent nucleotide
basepairs, the steric effects of basepairs, and the electrostatic interactions
along DNA backbones are taken into account. Quantitative results are obtained
by using path integral method, and excellent agreement between theory and the
observations reported by five major experimental groups are attained. The
strong intensity of the base-stacking interactions ensures the structural
stability of DNA, while the short-ranged nature of such interactions makes
externally-stimulated large structural fluctuations possible. The entropic
elasticity, highly extensibility, and supercoiling property of DNA are all
closely related to this account. The present work also suggests the possibility
that negative torque can induce structural transitions in highly extended DNA
from right-handed B-form to left-handed configurations similar with Z-form
configuration. Some formulae concerned with the application of path integral
method to polymeric systems are listed in the Appendix.
