What should be compared before band excision in restriction digestion?

Correct answer: band size with ladder and expected map / Band size with ladder and expected map

The direct answer is A: compare the band size with the ladder and the expected restriction map. A DNA ladder provides size references, while the expected map predicts which fragments should appear after digestion. First note the expected fragment size, then compare the sample band’s position with the ladder, and finally check whether the pattern agrees with the map. This helps identify the intended band before cutting it from the gel. Option A is correct because it uses objective size and mapping information. Option B is wrong because tube smell is not a reliable way to identify DNA bands. Option C is wrong because enzyme color does not tell the size or identity of a fragment. Option D is wrong and unsafe: buffer must never be tasted, and its taste gives no useful information. A correct band choice prevents purification of the wrong DNA. Memory cue: before cutting, check “ladder, expected size, map.”