Abstract
A long-standing problem in chromosome biology concerns the dynamic nature of centromeres. These chromosomal sites assemble the protein machines called kinetochores that connect chromosomes to the spindle microtubules for segregation to daughter cells during mitosis and meiosis. In multicelluar eukaryotes, centromeres are typically composed of highly homogeneous tandem repeats that evolve rapidly despite their highly conserved function [1]. For tandem repeats to evolve, a mutation must spread by some recombinational process, but a persistent dogma is that centromeres do not undergo homologous chromosome recombination (the shuffling of genetic segments between chromosomal pairs). New evidence [2] challenges this dogma and addresses the problem of rapidly evolving centromeres.
Highlights
A long-standing problem in chromosome biology concerns the dynamic nature of centromeres
Centromeres in multicellular eukaryotes are typically embedded in heterochromatin, the permanently condensed chromatin found around centromeres, in contrast to the euchromatic chromosome arms, which decondense between mitoses
Heterochromatin has been implicated in facilitating cohesion of sister chromatids around the centromere
Summary
Centromeres do not act alone in orchestrating chromosome segregation. In order for sister kinetochores to properly disjoin (separate) and segregate chromosomes to daughter cells in mitosis, their sister chromatids must be linked so that the pulling forces from the two halves of the spindle generate tension to correctly orient the kinetochores, stabilize kinetochore attachments, and signal that kinetochores are ready to disjoin. In Drosophila [5], humans [6], and budding yeast (Saccharomyces cerevisiae) [17], non-disjunction events at the second meiotic division are enriched in centromere-proximal crossovers This suggests that crossovers that are too close to the centromere disrupt pericentric sister chromatid cohesion, leading to premature separation of sister chromatids, which segregate randomly. The same or similar repeats comprise the flanking pericentric heterochromatin, underscoring the epigenetic specification of centromeres by CenH3 nucleosomes. Both centromeres and pericentric heterochromatin are rich in repetitive elements, repeats per se do not appear to be necessary for crossover suppression. This suppression is eliminated by a point mutation in the centromere that renders it unable to assemble a functional kinetochore [25], strongly suggesting that the kinetochore mediates suppression
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