DNA replication and its control

In order for cells to divide, they must replicate their chromosomes precisely once. This replication is performed by a number of proteins that act together to form a replication fork. The major control over DNA replication occurs at replication origins, the sites where active replication forks are initiated. In the yeast S. cerevisiae, origins have been identified as ARSs which possess a defined DNA sequence. These ARSs are recognized by specific proteins including ORC and ABF1. The elements which define origins in higher eukaryotes have not yet been identified. The temporal sequence in which origins fire is dependent on both the chromosomal location of the origin and the transcriptional activity of the dividing cell, with euchromatin and actively-expressed genes tending to replicate early in S-phase. DNA replication is an all-or-nothing event and occurs only after commitment to cell division. Cell fusion studies imply that S-phase inducers must be supplied to a nucleus as a signal to start replication. In addition, cells which have replicated their DNA but not yet undergone mitosis, will not perform further replication, even in the presence of these S-phase inducers. This is currently best explained by the action of a licensing factor, an origin-binding protein which cannot cross the nuclear envelope and so requires a transient nuclear permeabilization in order to make the DNA replication competent. Cyclin-dependent kinases provide good candidates for the S-phase inducers, as cell-cycle regulated activities which can potentially act upon these origin complexes and actually induce initiation to take place. Precise identification of the proteins involved is still to be made.