Simple procedure for distinguishing CCC, OC, and L forms of plasmid DNA by agarose gel electrophoresis

A simple and rapid method discriminating between covalently closed circular (CCC), open circular (OC), and linear (L) forms of plasmid DNA is presented. Two consecutive steps of agarose gel electrophoresis with a single DNA sample are used; use is made between the steps of ultraviolet light irradiation which introduces single-strand nicks in ethidium bromide-stained DNA, converting CCC into OC forms. Unambiguous assignments of OC and CCC forms are possible in samples containing several plasmids.