Visualizing topoisomers. In this experiment, DNA molecules (plasmids) have an identical number of base pairs but differ in the degree of supercoiling. In lane 1, highly supercoiled DNA migrates as a single band. Lanes 2 and 3 show the effect of treating supercoiled DNA with a type I topoisomerase; the DNA in lane 3 was treated for a longer time than the DNA in lane 2. Each individual band in the bracketed region of lane 3 contains DNA plasmids with the same linking number; Lk changes by 1 from one band to the next.