Past exam of the biochemistry course of the University of Cambridge / 2024 / Paper 5 / Section 1 / k / Solution

The normal patterns are A3, B2 and C4. In each, two principal DNA fluorescence in situ hybridization signals lie at corresponding positions on the two replicated sister chromatids. A single chromosome in mitosis has already replicated its DNA, so a unique locus normally has one copy on each sister, rather than two signals because two homologous chromosomes are being shown.

The other examples show a single unresolved or missing partner signal, extra fragmented signals, or an abnormal distribution. The images are small and relative intensities differ, especially in row C, so the appropriate criterion is the paired principal locus signals at matching positions, not perfect equality of brightness. Counts from many chromosomes, adequate optical resolution and hybridization controls are necessary before classifying borderline examples.