Figure 2.

Typical room temperature force (F) as a function of extension per base pair (b) curve for a single dsDNA molecule in 10 mM Hepes, pH 7.5, 100 mM [Na⁺] (ionic strength). The data obtained while stretching (▴) and relaxing the λ-DNA (□) are very similar except for the region between b = 0.34 nm/bp and b = 0.42 nm/bp, which shows some hysteresis. The solid line on the left is the theoretical curve for an extensible wormlike chain (43) (dsDNA) with a persistence length of 50 nm, a contour length of 0.34 nm/bp, and an elastic stretch modulus of 1,000 pN. The solid line on the right is the curve for an extensible freely jointed chain (36) (ssDNA) with a persistence length of 0.75 nm, a contour length of 0.56 nm/bp, and an elastic stretch modulus of 800 pN (19). The thick solid line connecting the stretching data is the result of a fit to the Bragg-Zimm melting model by using the curves for dsDNA and ssDNA with σ = 10⁻³.