The relation between the chromosome size gradient and the sequence of DNA replication in rye

Abstract

In eukaryotes there is a gradient in chromomere size along the chromosome which is particularly common in plants. The relation between this gradient and the sequence of DNA replication was studied in rye. The sequence of DNA replication along the chromosome follows the chromomere size gradient. Larger chromomeres not only replicate later but also require more time for replication than smaller ones. It is the chromomere size that determines the duration of replication and not the physical length of the chromosome region. The chromomere gradient is also a gradient of DNA amount. The more DNA present in the chromomeres of a given region, the longer time they take to replicate. The results extend the rule of late chromosome replication in heterochromatin, established for whole chromosomes or chromosome regions, demonstrating its validity for single chromomeres. The chromomere size gradient not only discloses a system of structural order in the chromosomes of eukaryotes but also represents a system of replicating order in these chromosomes. The replication pattern is well fixed and is not easily disturbed by rearrangements as shown by the pattern of replication of the large iso-chromosomes of rye. In the process of evolution, rearrangements which would disrupt too much the ordered pattern of replication would not be favoured by selection at the molecular level. This phenomenon would tend to preserve efficiently given original gene sequences which would remain relatively unaltered throughout evolution.