Centromere separation. Early replication of repetitive DNA associated with inactive centromeres.

Four types of stable dicentric and one octacentric chromosomes from mouse brain tumor cells and L-929 cells were analyzed for the timing of replication of repetitive deoxyribonucleic acid (DNA) located in the centric and pericentric regions associated with active versus inactive centromeres. The repetitive DNA present in the heterochromatin blocks of inactive centromeres replicates much earlier than similar DNA associated with the active centromeres. The former appears to replicate during early to mid S when several euchromatic segments are still replicating. There seems to be little or no overlap in the timing of replication of the repetitive DNA present in the vicinity of prematurely separating centromeres (which are accessory and nonfunctional) and those that separate at meta-anaphase junction (which are the functional centromeres). In the absence of any information about the mechanism(s) controlling initiation and completion of DNA synthesis in the two types of heterochromatic blocks, the differential timing of replication of the DNA with similar base composition remains an enigma.