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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 1. rDNA copy number dynamics through cyclical copy number loss and recovery.

Figure 1.

The size of rDNA loci comprised of tandemly duplicated repeats of the rRNA cistron (green arrows) is dynamic due to loss and recovery of individual rDNA repeats. rDNA copies are most notably lost by intra-chromatid recombination between distant rDNA copies, leading to excision of the intervening copies in the form of an extra-chromosomal rDNA circle (ERC). rDNA loci are capable of expanding their copy number to recover lost copies. The multiple proposed mechanisms for rDNA copy number expansion are discussed throughout this review. Overall rDNA copy number is proposed to be maintained through balanced copy number loss and expansion activity.