PerspectiveCell Biology

Competing chromosomes explain junk DNA

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Science  03 Nov 2017:
Vol. 358, Issue 6363, pp. 594-595
DOI: 10.1126/science.aaq0200

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The vast majority of eukaryotes have two copies of each chromosome and reproduce sexually. Meiosis is a vital process that produces gametes (eggs and sperm) by reducing the number of chromosome copies to one; fertilization between egg and sperm restores the chromosome copy number to two. During female meiosis, one set of chromosomes is expelled into a tiny cell called a polar body, whereas the other is segregated into the egg. It is a fundamental tenet of genetics that there is a random, 50% chance for any particular chromosome to be segregated into the egg versus the polar body. However, cases in which one copy of a chromosome is inherited with greater than 50% frequency have been reported in many species (1), but the molecular mechanism of this preferential inheritance has remained obscure. Recent work has indicated that centromeres, the chromosomal regions that form attachments to microtubules that mediate chromosome segregation during meiosis, compete with each other for inheritance during female meiosis (2). Thus, the essential DNA sequences that mediate accurate chromosome segregation are actually “selfish” (or parasitic) genetic elements that have invaded our genome. On page 668 of this issue, Akera et al. (3) provide the most detailed molecular mechanism to date that explains how a parasitic DNA sequence has used the asymmetry of oocyte meiosis to ensure its own inheritance and therefore its spread through populations.