  We introduce a three-state model for a single DNA chain under tension that
distinguishes between B-DNA, S-DNA and M (molten or denatured) segments and at
the same time correctly accounts for the entropy of molten loops, characterized
by the exponent c in the asymptotic expression S ~ - c ln n for the entropy of
a loop of length n. Force extension curves are derived exactly employing a
generalized Poland-Scheraga approach and compared to experimental data.
Simultaneous fitting to force-extension data at room temperature and to the
denaturation phase transition at zero force is possible and allows to establish
a global phase diagram in the force-temperature plane. Under a stretching
force, the effects of the stacking energy, entering as a domain-wall energy
between paired and unpaired bases, and the loop entropy are separated.
Therefore we can estimate the loop exponent c independently from the precise
value of the stacking energy. The fitted value for c is small, suggesting that
nicks dominate the experimental force extension traces of natural DNA.
