  A unified model is constructed to study the recently observed DNA entropic
elasticity, cooperative extensibility, and supercoiling property. With the
introduction of a new structural parameter (the folding angle $\phi$), bending
deformations of sugar-phosphate backbones, steric effects of nucleotide
basepairs, and short-range basestacking interactions are considered. The
comprehensive agreement of theoretical results with experimental observations
on both torsionally relaxed and negatively supercoiled DNAs strongly indicates
that, basestacking interactions, although short-ranged in nature, dominate the
elasticity of DNA and hence are of vital biological significance.
