What can cause an open circular plasmid band?

Correct answer: A nick in one DNA strand of the plasmid

A nick in only one strand of plasmid DNA relaxes its supercoiling, producing the open circular (relaxed) form. Therefore, a single-strand nick is the main cause of an open circular plasmid band. In contrast, cutting both strands at the same position produces a linear plasmid. Exam tip: remember the forms—nicked = open circular, one cut in both strands = linear, intact = supercoiled.