Practice question

Circular DNA remains topologically closed, so a single unique restriction cut converts it into one linear molecule of unchanged length. A second unique cut at a different position divides that linear molecule into two distinct fragments. Counting bands on agarose gel therefore directly reveals number of cut sites when enzymes are unique. For circular plasmids, number of fragments equals number of sites cut. Two enzymes each cutting once generate two fragments whose sizes add up to total plasmid size, confirming double digestion and enabling circular map construction for vector analysis.