  We propose an Ashkin-Teller like model for elastic response of DNA molecule
to external force and torque. The base-stacking interaction is described in a
simple and uniform way. We obtain the phase diagram of dsDNA, and in
particular, the transition from B form to the S state induced by stretching and
twisting. The elastic response of the ssDNA is presented also in a unified
formalism. The close relation of dsDNA molecule structure with elastic response
is shown clearly. The calculated folding angle of the dsDNA molecule is
$59.2^o$.
