  Metropolis Monte Carlo simulation is used to investigate the elasticity of
torsionally stressed double-stranded DNA, in which twist and supercoiling are
incorporated as a natural result of base-stacking interaction and backbone
bending constrained by hydrogen bonds formed between DNA complementary
nucleotide bases. Three evident regimes are found in extension versus torsion
and/or force versus extension plots: a low-force regime in which over- and
underwound molecules behave similarly under stretching; an intermediate-force
regime in which chirality appears for negatively and positively supercoiled DNA
and extension of underwound molecule is insensitive to the supercoiling degree
of the polymer; and a large-force regime in which plectonemic DNA is fully
converted to extended DNA and supercoiled DNA behaves quite like a torsionless
molecule. The striking coincidence between theoretic calculations and recent
experimental measurement of torsionally stretched DNA [Strick et al., Science
{\bf 271}, 1835 (1996), Biophys. J. {\bf 74}, 2016 (1998)] strongly suggests
that the interplay between base-stacking interaction and permanent
hydrogen-bond constraint takes an important role in understanding the novel
properties of elasticity of supercoiled DNA polymer.
