Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 2 / Section C / 14 / l / Solution

A strong specificity control is competition with a large excess of unlabelled cognate promoter DNA. It should reduce the shifted labelled complexes, whereas an unrelated or binding-site-mutant competitor should compete much less effectively. This tests sequence-dependent association rather than non-specific probe retention. A probe-only lane identifies the free probe, and an irrelevant isotype-matched antibody controls whether any added antibody produces aggregation or altered mobility. These controls address different uncertainties and should not be treated as interchangeable.