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When a single DNA molecule is stretched beyond its normal B-form contour length, it will undergo one of two types of conformational transitions at high or low force, depending on its attachment and ability to rotate. The structure of the DNA during these transitions is difficult to measure with traditional structural methods because it only occurs on a single molecule. More than a decade after its discovery, van Mameren et al. (1) have, in this issue of PNAS, directly visualized the mechanics of DNA overstretching for the first time by combining single-molecule force spectroscopy and fluorescence imaging. They show that, regardless of whether or not the molecule can freely rotate, when DNA is stretched to high force it will melt, as the work done by the force to increase the length of the DNA converts double-stranded duplex DNA into ssDNA. A typical force-extension curve for torsionally relaxed DNA is shown in Fig. 1. As the DNA extension reaches its B-form contour length of 0.34 nm/bp, the force required to stretch the molecule increases rapidly as the double helix is straightened out and resists further increase in length. At ≈65 pN, very little additional force is required to increase the DNA length to ≈1.7 times its contour length. Upon initial inspection, the transition appears to be from dsDNA to ssDNA. However, when the transition was discovered, several puzzling facts appeared to preclude a melting model (2). The most important fact was that the two strands do not come apart when the transition reaches the contour length of ssDNA. Instead, the strands do not separate until forces >100 pN are exerted. Therefore, the transition was originally attributed to a conversion from B-form dsDNA to a new form of dsDNA termed “S-DNA,” in which the neighboring base pairs unstack but the base-pairing …

Peeling back the mystery of DNA overstretching