λ − DNA 0 10 20 30 40 S-DNA B-DNA U-DNA FIG. 2: (Color) Dynamic simulations of a microscop ic model accommodating U, M, B and S forms of DNA [11]. Here we show microscopic configurations (v ertical axis) as a function of time (horizontal axis) from an illustrativ e simulation of a 300-basepair fragmen t of λ -DNA at 21 ◦ C and salt concentration 150 mM, extended at 1000 nm/s. The molecule possesses one free end, permitting unpeeling. White indicates B-DNA, red indicates unhybridized DNA (unpeeled or molten), and blue indicates S-DNA. The spatial scale indicates molecular length. The maximum force attained is 90 pN (central dotted line). At forces in excess of ab out 50 pN, S-form DNA is themo dynamically unstable with respect to unpeeled DNA. However, unp eeling takes time to proliferate, and S-DNA is seen to be transiently stable even near forces of about 90 pN. The step-by-step processivity of unp eeling confers upon overstretc hing an anomalous kinetics that, we argue, rationalizes several nontrivial experimental observations [16–18]. 9