MoBio Unequal Crossover Chapter 8

Due to significant sequence homology, the repetitive sequence region of a chromatid may not line up exactly with its corresponding region in a homologous chromatid or identical sister chromatid. As a result, different number of repeat units may be generated during meiosis. This is thought to be the major mechanism underlying VNTR (variable number of tandem repeats ).

Image

Figure 8-D-6. Unequal crossover and sister chromatid exchange.
(a) Two pairs of sister chromatids line up during meiosis. A repetitive region of one chromatid (the third one) does not line up exactly with its corresponding region in other chromatids.
(b)Strand breaks on nonsister chromatids (along line A) will result in unequal crossover, producing different number of repeat units in these chromatids.
(c) Strand breaks on sister chromatids (along line B) also produce different repeats. In this case, it is called sister chromatid exchange. The detailed mechanism of DNA crossover in (b) and (c) may be explained by the Holliday model.