Sequence of centromere separation: a possible role for repetitive DNA

The centromeres of a mitotic cell at the meta-anaphase junction separate in a non-random, genetically controlled sequence before anaphase migration ensues. In several, but not all, of the organisms studied so far it appears that the timing of separation of a centromere into two visible units depends upon the quantity of pericentric heterochromatin. A critical analysis of this parameter of cell cycle suggests that the sequence of centromere separation is influenced by repetitive DNA present in the pericentric region. In those cases with qualitatively uniform repetitive DNA in centromeric regions, the chromosomes carrying lesser quantities separate earlier than those with greater amounts. However, the overall pattern of separation may be determined by interactions between both quantitative and qualitative parameters of the repetitive DNA. It has been suggested that repetitive DNA has no transcriptional properties. It is probably only a structural component which acts as a site for the accumulation of protein molecules synthesized by some locus not present in the centromeric region. These proteins accumulate to saturate the centromeric repetitive DNA resulting in a (trilaminar) structure called the kinetochore. The longer the stretch to be saturated, the larger would be the kinetochore-like structure. Once saturated, the centromere splits into two subunits. Premature separation results in a lack of saturation and formation of an 'immature' kinetochore. This may lead to chromosome malsegregation. Thus, indirectly, one property of repetitive DNA in the centromeric region appears to be the maintenance of diploidy.