  The effect of electrostatic interactions on the stretching of DNA is
investigated using a simple worm like chain model. In the limit of small force
there are large conformational fluctuations which are treated using a
self-consistent variational approach. For small values of the external force f,
we find theoreticlly and by a simple blob picture that the extension scales as
fr_D where r_D is the Debye screening length. In the limit of large force the
electrostatic effects can be accounted for within the semiflexible chain model
of DNA by assuming that only small excursions from rod-like conformations are
possible. In this regime the extension approaches the contour length as
f^{-1/2} where f is the magnitude of the external force. The theory is used to
analyze experiments that have measured the extension of double-stranded DNA
subject to tension at various salt concentrations. The theory reproduces nearly
quantitatively the elastic response of DNA at small and large values of f and
for all concentration of the monovalent counterions. The limitations of the
theory are also pointed out.
