We analyze whether the “overstretched,” or “$S$” form of double-stranded DNA consists of essentially separated, or essentially interacting, polynucleotide strands. Comparison of force-extension data for $S$-DNA and single-stranded DNA shows $S$-DNA to be distinct from both double helix and single-stranded forms. We use a simple thermodynamical model for tension-melted double-stranded DNA, which indicates that the overstretching transition near 65 piconewtons cannot be explained in terms of conversion of double helix to noninteracting polynucleotide strands. However, the single-strand-like response observed in some experiments can be explained in terms of “unpeeling” of large regions of one strand, starting from nicks on the original double helix. We show that $S$-DNA becomes unstable to unpeeling at large forces, and that at low ionic strength, or for weakly base-paired sequences, unpeeling can preempt formation of $S$-DNA. We also analyze the kinetics of unpeeling including the effect of sequence-generated free energy inhomogeneity. We find that strongly base-paired regions generate large barriers that stabilize DNA against unpeeling. For long genomic sequences, these barriers to unpeeling cannot be kinetically crossed until force exceeds approximately 150 piconewtons.

• Received 8 October 2003

DOI:https://doi.org/10.1103/PhysRevE.70.011910