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. Author manuscript; available in PMC: 2020 Oct 1.
Published in final edited form as: Trends Genet. 2019 Aug 5;35(10):734–742. doi: 10.1016/j.tig.2019.07.006

Figure 3. rDNA copy number expansion by unequal sister chromatid exchange in mitotic Drosophila germ cells.

Figure 3.

(A) Unequal sister chromatid exchange (USCE) can create reciprocal rDNA copy number gain and loss between sister chromatids. When a double-stranded DNA break (DSB) occurs in an rDNA region post replication, the DSB can be repaired by homologous recombination with the sister chromatid. Homologous recombination between misaligned rDNA copies resolved by chromatid exchange increases rDNA copies on one sister while reducing the copies on the other sister. (B) USCE over successive cell divisions can rapidly expand rDNA copies (range from orange to blue). Repeated inheritance of the sister chromatid that gains copies during USCE will lead to rapid expansion of rDNA copies, beyond what could be gained by a single USCE event.