Figure 1.

(A) The substrate (top) and product (bottom) knots are each shown in two different configurations. The configurations on the right are standard minimal crossing representations of the substrate knot (top right) and product knot (bottom right). The representations on the left are shown in such a way such that the one crossing on which topoisomerase acts has been separated from the other crossings. A circle has been drawn around this crossing. The topoisomerase action is modeled by changing the overcrossing segment to an undercrossing segment (and vice versa), via intersegmental passage. (B) The top left knot after the DNA conformation not bound by topoisomerase has been simplified. White mesh sphere represents the topoisomerase.