Practice question

In covalently closed circular DNA, each restriction cut increases fragment count by one when digestion goes to completion. A single site yields one linear band, two sites yield two bands, three sites yield three bands, provided no partial digestion occurs. Observing three separate fragments of 1.5, 2.0, and 0.5 kb that sum exactly to 4 kb indicates three cleavage events occurred around the circle. Therefore three recognition sites are distributed around plasmid. Mapping distances between them using fragment sizes allows reconstruction of circular restriction map important for vector engineering and insert localization.