Early-replicating heterochromatin.

Euchromatin, which has an open structure and is frequently transcribed, tends to replicate in early S phase. Heterochromatin, which is more condensed and rarely transcribed, usually replicates in late S phase. Here, we report significant deviation from this correlation in the fission yeast, Schizosaccharomyces pombe. We found that heterochromatic centromeres and silent mating-type cassettes replicate in early S phase. Only heterochromatic telomeres replicate in late S phase. Research in other laboratories has shown that occasionally other organisms also replicate some of their heterochromatin in early S phase. Thus, late replication is not an obligatory feature of heterochromatin.