What can cause curved DNA bands in a gel run?

Correct answer: Uneven heating or gel running issue

The direct answer is A: uneven heating or another gel-running problem can cause curved bands. During electrophoresis, excessive voltage can generate heat. If the gel, buffer depth, electrical field, or temperature is uneven, DNA may migrate at different speeds across the lane, producing bowed or smiling bands. Old or incorrectly prepared buffer and a poorly positioned gel can also contribute. Option A is correct because these are physical running conditions that distort migration. Option B is wrong because perfect digestion does not by itself create curved bands; digestion concerns cutting, whereas curvature concerns separation conditions. Option C is wrong because absent DNA would usually give no visible band, not a curved one. Option D is wrong because enzyme naming has no physical effect on migration. Use the recommended voltage, fresh appropriate buffer, level the gel, and avoid overheating. Memory cue: curved bands usually suggest a field, heat, or buffer problem.