A Time and a Place - is the Choice of Origin of Replication within a Temporal Replicating Zone Stochastic or Determined?

For over 50 years, bacterial DNA has been known to start its replication from a predetermined, precise location. Metazoan genomes are divided into broad temporal replicating zones (early, middle, and late). Each replication domain (regions spanning in the range of 1 megabase) is characterized by its temporal replicating zone. However, the nature of order within a replication domain remains an open question. We want to explore two possible models of the temporal order within domains of replication. The replication could start at random at one of the origins in the domain. Alternatively, there could be a fixed position within each domain, from which the replication starts every time. The first model would seem to be much less regulated, which therefore would beg the question- what advantage is there to replicate the domains stochastically? If bacterial replication is initiated from the same site each time, what prevents eukaryotic replication domains (which are of similar sizes to bacterial genomes) from similar levels of regulation? The second model would require significant regulation which is yet to be discovered.

We have developed a FACS-based method to isolate a population of eukaryotic cells in the first minutes of S-phase. DNA sequencing, followed by bioinformatics analysis can then offer us a glimpse into the replication patterns of the very early origins of replication. We hope to use this to further our understanding of the chronology and regulation of replication within a replication domain.