If both sample and positive control do not cut what will you check?

Correct answer: Enzyme buffer and incubation condition

Direct answer: Option A, check the enzyme buffer and incubation conditions. Restriction enzymes cut DNA only when the correct buffer, temperature, time, and enzyme activity are available. If both the sample and the positive control remain uncut, the problem is probably common to the reaction setup, not only to the sample DNA. Step by step: first confirm that the enzyme was added and stored correctly; then check its storage temperature, expiry, reaction buffer, buffer concentration, incubation temperature, and incubation time. Option A is correct because these factors affect both tubes. Option B, gel taste, is irrelevant and must never be tested. Option C, DNA colour, does not tell whether the enzyme cut DNA. Option D, tube shape, normally does not determine digestion. A positive control should cut; if it does not, troubleshoot the shared setup before blaming the sample. Memory cue: both fail means check what both share.