Qualifying for the license to replicate

A single replication fork would take more than a year to replicate the genome of Xenopus. By dividing the task among many thousands of replicons, each replicated by forks emanating from individual origins, replication is instead completed in as little as 30 min. While this efficient strategy has been adopted by all eukaryotes, it introduces a complication. To maintain the integrity of the genome, multiple replicons must now be coordinated so that all sequences are replicated exactly once per cell cycle. Because different replicons are often replicated at different times during S phase, replicated regions must be distinguishable from unreplicated regions to avoid problems of rereplication. We suggest that this distinction is based upon a fundamental feature of replication initiation. Two things happen at origins of replication: proteins are recruited to origins to assemble multiprotein replication machines, and these assemblies are triggered to initiate replication forks. Replication components accompany the departing forks, leaving behind a spent origin. Consequently, reinitiation should require assembly of new components at the origin. If this assembly is restricted to one part of the cell cycle and the initiation of forks to another, then origin firing would occur only once per cell cycle. The transition between replication-competent and replication-incompetent phases of the cell cycle has been explored in a series of early and influential cell fusion experiments. Upon fusion with an S phase cell, nuclei from G1 cells, but not from G2 cells, replicate their DNA. Thus, even when present in cytoplasm capable of supporting S phase, the G2 nucleus is incompetent to replicate. Since G2 nuclei are converted into G1 nuclei by the passage through mitosis, mitosis must provide replication competence to the G2 nucleus. In the last several years, in vitro experiments using Xenopus egg extracts as well as genetic experiments using fission yeast have given rise to two rather different models for the basis of this mitotic transition. We outline each of these areas of research below and evaluate features of each model in an attempt to bring us closer to a unified understanding of these events.