Centric fission--simple and complex mechanisms.

Centric fission describes a rather poorly molecularly defined process of the transverse division of a functional centromere to result in two new centric chromosomes. While centric fission occurs rarely in humans, this process has been attributed an important role in eukaryotic karyotype evolution. Recent studies have given insight into the complex molecular mechanisms that lead to apparent centric fission events, including evidence in support of a mechanism driven by centric preduplication. These studies suggest that the traditional definition of centric fission, based on gross cytogenetic and molecular cytogenetic observations, needs to be broadened. It is necessary to distinguish between simple centric fissions, that involve the direct transverse breakage of a functional centromere, and other more complex fission events at or around a centromere that may be preceded by chromosomal rearrangements.