Practice question

A vector of 16500 bp subjected to digestion that yields a small 453 bp fragment displaying double fluorescence intensity suggests two copies of that fragment co-migrating at same position. In cases of tandem duplication, restriction sites flanking duplicated segment are repeated, causing same small fragment to be excised twice per plasmid molecule, doubling its molar amount and thus ethidium bromide staining or fluorescence intensity relative to single-copy fragments. Single-site fragment would show normal intensity approximating larger bands. Enhanced signal indicates duplicated site explaining extra dosage and is important for assessing plasmid stability and recombination events.