How should band identity be confirmed before extracting desired fragment from gel after restriction digest?

Correct answer: By comparing with ladder and expected map

After a restriction digest, DNA fragments appear as bands at positions related to their sizes. The desired fragment should have a size predicted by the restriction map or experimental design. A DNA ladder provides reference bands of known sizes, allowing the sample band to be estimated and compared with the expected fragment length.

The correct procedure is therefore to compare the band with the ladder and with the expected restriction map before cutting it from the gel. This reduces the chance of purifying an unwanted fragment, which could lead to an unsuccessful cloning or downstream experiment. Enzyme smell, tube colour, and buffer taste provide no reliable information about DNA fragment identity and are not appropriate laboratory methods. Thus option A is correct.