Figure 2. Efficient rewinding of DNA at very large forces is monitored in real time using optical tweezers.

(a) Schematic of the OT experimental setup and progress of the reaction. A DNA hairpin substrate was tethered between two beads—one trapped in the optical trap and the other fixed in a tip of a micropipette. In this passive configuration, the DNA rewinding or unwinding reaction was followed by monitoring changes in force. (b) Hairpin states can be precisely followed using OT, and typical progress curves in the absence of protein are shown. The force as a function of X_T for the H_GCapex hairpin at 25 °C shows a broad region resulting in a partially unzipped hairpin configuration. (c,d) Representative traces of rewinding reactions catalysed by RecG (c) and UvsW (d) proteins. Experimental traces showing force as a function of time starting with the initial partially denatured hairpin configuration. Rewinding bursts were detected as force rips.