PCR (Polymerase Chain Reaction)

Correct answer: A. Because recovery from the gel is not complete and some DNA can be lost

Gel extraction requires several handling steps: identifying and cutting the desired band, dissolving or melting the agarose, binding DNA to a membrane or resin, washing away impurities, and eluting the DNA. At each stage, a fraction of the DNA may remain in the gel, fail to bind, be lost during washing, or remain in the column instead of entering the eluate. The final eluate may therefore contain less total DNA and a lower measured concentration than the starting sample. Option A is correct. DNA is not amplified during extraction, restriction enzymes do not become DNA, and agarose does not become protein; these make B, C, and D incorrect. Measuring the recovered sample again is important before ligation or another downstream reaction.

Correct answer: A. To release the desired DNA fragment from the agarose matrix for recovery

After electrophoresis, the desired DNA fragment is physically trapped within the three-dimensional agarose matrix. To recover it, the excised gel slice is treated with an appropriate chaotropic or dissolving solution, often with controlled warming, so that the agarose structure breaks down or becomes fluid. The released DNA can then bind to a silica membrane or another purification material under suitable conditions, followed by washing and elution. Therefore option A is correct. Melting does not change DNA into protein and does not generally activate every enzyme. It also is not performed merely to remove the ladder; the target band itself must be released. Excessive heat or unsuitable conditions should be avoided because they can reduce DNA recovery or quality.