Why can a nicked plasmid create confusion in restriction digest?

Correct answer: It can show different mobility and look like an extra band

The direct answer is A: a nicked plasmid can migrate with a different mobility and may look like an extra band. A nick is a break in only one strand of the plasmid DNA. The other strand remains continuous, so the molecule is not completely fragmented, but its topology changes, often relaxing a supercoiled plasmid toward an open circular form. Step by step: one strand receives a nick; supercoiling is relaxed; the molecule moves through the gel differently; its band may be mistaken for another DNA product. A is correct. B is wrong because nicking does not convert DNA into protein. C is wrong because a nicked plasmid does not normally destroy the restriction enzyme. D is wrong because nicked DNA can still enter and migrate through the gel. An uncut plasmid control helps identify common conformations and prevents incorrect interpretation. Memory cue: “one-strand nick changes shape, and shape changes mobility.”