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 ).
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.
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