Optical Tweezers Study of Topoisomerase Inhibition

Optical tweezers force-stretching of highly nicked dsDNA, as indicated by the large hysteresis area (black and red curves). Topoisomerase activity is evidenced by a higher level plateau and a complete vanishing of the overstretching hysteresis (green curve), indicating total repair of the DNA nicks. The arrow indicates a drop in the stretching curve resulting from topoisomerase cleavage during the cycle.