Biochemistry 4th Edition by Christopher Mathews,Kensal van Holde, Dean Appling, Spencer Anthony Cahill

Exercise 13

A closed circular supercoiled DNA is relaxed by treatment with topoisomerase. No matter how much enzyme is used, or how long the experiment is run. the experimenter always finds a gel electrophoresis pattern indicating some DNA with one, two, and three helical turns in addition to the relaxed (nicked) circle see figure). Suggest an explanation for this observation.