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Force–extension curve for a 1.2 kbp hairpin. At low forces the hairpin is annealed and displays a constant extension. As the force is increased above ~16 pN, the hairpin is mechanically unzipped and its extension abruptly increases. The force needs to be reduced below 15 pN for the hairpin to fully reanneal; a certain hysteresis is observed (due to the nucleation of a dsDNA seed in the vicinity of the hairpin loop).
