If linear DNA has two restriction sites how many fragments form after complete digestion?

Correct answer: Three

The direct answer is option C: three fragments. A linear DNA molecule has two ends. Each complete cut made at an internal restriction site divides one continuous piece into two pieces. Therefore, for linear DNA, the number of fragments is the number of cuts plus one: \(F=C+1\). Here there are two restriction sites and complete digestion means both sites are cut, so \(F=2+1=3\). Visualise the order as end—site 1—middle section—site 2—end; the cuts separate the molecule into a left end fragment, a middle fragment, and a right end fragment. Option A, one fragment, would mean no effective cut. Option B, two fragments, would result from one cut in linear DNA, not two. Option C is correct because two cuts produce three separate pieces. Option D, four fragments, would require three cuts in linear DNA, assuming all cuts are distinct and complete. This rule is specifically for linear DNA. For a circular DNA molecule, two distinct cuts generally produce two fragments because there are no free ends, so do not apply the linear formula blindly. Memory cue: linear DNA fragments = cuts plus one; circular DNA fragments = cuts, under the usual complete-digestion assumptions.