Ribosomal DNA (rDNA) sequences are essential parts of many species’ genomes. The highly repetitive nature of these DNA sequences makes them susceptible to shrinkage over time—and if they shrink too much, cells die. If rDNA shrinks too much in germ cells, the cells that become eggs and sperm, then individuals may become infertile and their lineages may go extinct.
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Source: Phys.org