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. 2009 Dec 16;97(12):3158–3167. doi: 10.1016/j.bpj.2009.09.040

Figure 1.

Figure 1

(a) Schematic of a single-molecule DNA stretching experiment. The 5′ ends of a short, double-stranded DNA duplex are attached to a surface and an atomic force microscope cantilever via elastic poly(ethylene glycol) (PEG) polymers. Separation of the substrate and the cantilever at constant velocity leads to an increasing end-to-end distance and thus to an increasing force. (b) Superposition of 20 experimentally obtained force-extension traces obtained from the same 30-basepair 1 × 2 DNA duplex with a separation velocity of 1 μm/s. The duplex dissociates at ∼60–65 pN. (c) Schematic of the three-state model. Every basepair of the DNA duplex appears in one of three states: B-DNA, S-DNA, or single-stranded DNA. Every state s of an N-basepair-long DNA may thus be represented by a list of length N with entries 0 (B-DNA), 1 (S-DNA), and 2 (ssDNA) for every basepair.