When can magnetic bead cleanup be useful in restriction digestion?

Correct answer: In recovering DNA fragments by size or purification

The direct answer is A: magnetic bead cleanup can recover DNA fragments by purification or size selection. Magnetic beads bind DNA under particular salt and polyethylene-glycol conditions. A magnet then holds the beads while unwanted liquid is removed, followed by washing and elution of purified DNA. With an appropriate bead-to-sample ratio, larger DNA fragments may be retained while smaller fragments and impurities remain in the liquid, although the exact selection range depends on the kit and protocol. Option A is correct because beads are widely used after restriction digestion to remove salts, enzymes, and other contaminants or to select a desired fragment size. Option B is wrong because beads do not heat or activate an enzyme. Option C is wrong because making an agarose gel requires buffer, agarose, heating, and casting equipment, not magnetic beads as the main method. Option D is wrong because cleanup does not directly perform protein expression; it only prepares DNA. Remember: beads bind, a magnet separates, washing cleans, and elution recovers DNA.