Fig. 2.

a) In a dual beam optical tweezers instrument, two laser beams are focused to a small spot, creating an optical trap that attracts polystyrene beads. Single DNA molecules are attached at one end to a bead in the trap, while the other end is attached to another bead held by a glass micropipette. As the DNA molecule is stretched by moving the micropipette, the resulting force on the bead in the trap is measured. b) Typical force extension curves for double stranded DNA are shown as dotted lines. As the stretching force is increased, dsDNA reveals an entropic elastic response, followed by the overstretching region. The data in purple shows typical data for a full cycle of extension and relaxation, including some hysteresis upon reannealing. The data in blue and cyan show the response of the resulting single strands to yet higher forces, as the strands finally separate near 150 pN (thus there are no relaxation curves). The solid lines are DNA models for ssDNA and dsDNA, as described previously ⁶⁵.