What can be done if one enzyme in double digest is not heat-inactivated?

Correct answer: Use cleanup or enzyme removal step

The direct answer is A: use a cleanup or enzyme-removal step. In a double digest, two restriction enzymes are used on the same DNA sample. If one enzyme is not heat-inactivated after the reaction, it may remain active and cut DNA during a later step, especially if the next buffer or temperature is suitable. The reasoning is: digestion ends, the unwanted active enzyme is removed or separated, and the cleaned DNA is then used for ligation or another procedure. Cleanup may use a column, magnetic beads, or another validated enzyme-removal method; the exact protocol depends on the enzymes and experiment. Option A is correct because it prevents unwanted continued cutting. Option B is absurd and unsafe; a reaction must never be eaten. Option C is wrong because restriction digestion does not make DNA become protein. Option D is wrong because removing the gel is not automatically the solution and is unrelated if the issue is an active enzyme in the reaction. Exam cue: when enzyme inactivation fails, remove the enzyme rather than guessing.