  We study three-dimensional deformations of thin inextensible elastic rods
with non-vanishing spontaneous curvature and torsion. In addition to the usual
description in terms of curvature and torsion which considers only the
configuration of the centerline of the rod, we allow deformations that involve
the rotation of the rod's cross-section around its centerline. We derive new
expressions for the mechanical energy and for the force and moment balance
conditions for the equilibrium of a rod under the action of arbitrary external
loads. Several illustrative examples are studied and the connection between our
results and recent experiments on stretching of supercoiled DNA molecules is
discussed.
