Plasmid linearization in restriction digest can prove what?

Correct answer: The enzyme cut at one site in plasmid

The direct answer is A: the enzyme may have cut the circular plasmid at one site. A circular plasmid cut exactly once becomes one linear DNA molecule. During gel electrophoresis, this can appear as a band near the plasmid’s expected total size, although migration can differ from uncut circular forms because DNA shape affects movement. Thus, a linearized band supports the idea of a single restriction site for that enzyme, provided the band pattern and controls agree. Option A is correct but should be interpreted with the expected size and controls. Option B is wrong because DNA does not become protein during restriction digestion. Option C is wrong because a band shape or linearization does not prove that the gel is always bad. Option D is wrong because a linearized product suggests enzyme activity, not enzyme absence. Remember that one cut in a circle opens it into a line; multiple cuts would produce multiple fragments.