  Overstretching of B-DNA is currently understood as force-induced melting.
Depending on the geometry of the stretching experiment, the force threshold for
the overstretching transition is around 65 or 110 pN. Although the mechanisms
behind force-induced melting have been correctly described by Rouzina and
Bloomfield \cite{RouzinaBloomfield2001a}, neither force threshold has been
exactly calculated by theory. In this work, a detailed analysis of the
force-extension curve is presented, based on a description of single-stranded
DNA in terms of the discrete Kratky-Porod model, consistent with (i) the
contour length expected from the crystallographically determined monomer
distance, and (ii) a high value of the elastic stretch modulus arising from
covalent bonding. The value estimated for the ss-DNA persistence length,
$\lambda = 1.0 $ nm, is at the low end of currently known estimates and
reflects the intrinsic stiffness of the partially, or fully stretched state,
where electrostatic repulsion effects are expected to be minimal. A detailed
analysis of single- and double-stranded DNA free energies provides estimates of
the overstretching force thresholds. In the unconstrained geometry, the
predicted threshold is 64 pN. In the constrained geometry, after allowing for
the entropic penalty of the plectonemic topology of the molten state, the
predicted threshold is 111 pN.

Biological Physics;Soft Condensed Matter;Biomolecules