Practice question

A circular plasmid with a single BamHI recognition site cut to completion linearizes into a single molecule equal to plasmid length, observed as one discrete 4 kb band on agarose gel. Two sites would produce two bands whose sizes sum to 4 kb, with varying intensities. Absence of sites would leave supercoiled, nicked, and linear forms unchanged. Incomplete digestion might show additional supercoiled and nicked forms besides linear band. Therefore appearance of single discrete 4 kb linear band after BamHI digestion strongly supports presence of one unique BamHI site valuable for linearizing vector.