height represents 40 kbp) at a range of thermodynamic states at times 0 and 10 seconds after setting the loading rate to zero. Two features are apparent. First, S-DNA is sufficiently long-lived to be seen after 10 seconds of waiting at low temperatures. Second, near and above room temperature, S-DNA is short-lived. We show a similar diagram in Fig. 4 for identical thermodynamic conditions but a smaller stretching rate of 100 nm/s. Because the target force f$_{0}$ is attained more slowly, and molecules spend more time at high forces than do their counterparts in Fig. 3, more unpeeling is observed. We show additional ‘dynamic phase diagrams’ at salt concentrations of 50 mM (Fig. 5) and 500 mM (Fig. 6), and individual molecular compositions as a function of sequence in Fig. 7. We find that S-DNA is short-lived and that unpeeling occurs readily under the conditions considered in Ref. [9]: we would not expect S-DNA to be observed at forces near 65 pN after a few seconds under those conditions. Repeating such experiments at high salt concentrations and at low temperatures would provide a test of the predictions presented here. We have argued previously [10, 11] that the transient emergence of S-DNA rationalizes several recent experimental observations. The basepair-by-basepair processivity of U-DNA means that its emergence and disappearance is slow, which renders hysteretic a stretching-shortening cycle [4, 6]. Moreover, competition between S-DNA’s basepairing-stacking energy and U-DNA’s entropy con- fers a strong temperature dependence upon the degree of hysteresis accompanying a stretching- shortening cycle, in agreement with the striking kinetic data of Ref. [17]. The transient stability of S-DNA also rationalizes [6, 10] a multi-stage force-extension behavior (for DNA at sufficiently low temperature, high salt concentration and possessing sufficiently low AT content), in which a pulling rate-independent plateau at 65 pN is succeeded at high forces by a second, rate-dependent transition, as seen experimentally [16, 18]. By contrast, we predict that DNA forced to dehy- bridize during overstretching (as would happen if S-DNA does not exist) gives rise to considerable hysteresis under all solution conditions (rather than only at high temperatures) and a strongly pulling rate-dependent overstretching force [10]. Finally, we note that the ‘force-induced melting’ picture proposes that overstretched DNA near 65 pN consists of internal molten bubbles (M-DNA) punctuated by small regions of B-DNA, and that unpeeling occurs at higher forces [1–3]. The ob- servation of Ref. [9] that DNA at low salt concentrations progressively unpeels near 65 pN from nicks and free ends appears to challenge this picture. Statistical mechanical considerations cannot suggest likely atomistic structures for S-DNA (possible manifestations of which have been reported in recent atomistic simulations [19, 20]). However, we argue that overstretching dynamics revealed by experiments, especially the recent Ref. [12], implies the existence of an elongated phase of DNA energetically preferred to U-DNA. We have noted previously the limitations of the coarse-grained picture that we advocate, as well as the fact that a different interpretation of M-DNA (does it retain some energetic advantage over U-DNA, for instance?) might yield agreement between our model calculations and experimental data in III. SUMMARY 4