Chromosome Replication

Precise chromosomal DNA replication during S phase of the cell cycle is a crucial factor in the proper maintenance of the genome from generation to generation. The current "once-per-cell-cycle" model of eukaryotic chromosome duplication describes a highly coordinated process by which temporally regulated replicon clusters are sequentially activated and subsequently united to form two semi-conserved copies of the genome. Replicon clusters, or replication domains, are comprised of individual replication units that are synchronously activated at predetermined points during S phase. Bi-directional replication within each replicon is initiated at periodic AT-rich origins along each chromosome. Origins are not characterized by any specific nucleotide sequence, but rather the spatial arrangement of origin replication complexes (ORCs). Given the duration of the S phase and replication fork rate, adjacent origins must be appropriately spaced to ensure the complete replication of each replicon. Chromatin arrangement by the nuclear matrix may be the underpinning factor responsible for ORC positioning. The six subunit ORC binds to origins of replication in an ATP-dependent manner during late telophase and early G1. In yeast, each replication domain simply contains a single ORC binding site. However, more complex origins are characterized by an initiation zone where DNA synthesis may begin at numerous locations. A single round of DNA synthesis at each activated origin is achieved by "licensing" of the ORC. Replication initiation is limited to ORCs bound by the proper licensing proteins. These licensing proteins are removed upon commencement of replication. Inactivation of licensing machinery prior to S phase prevents re-licensing of origins and multiple rounds of replication within a domain. Progression through, and termination of, chromosome replication is controlled by S phase specific cyclins and CDKs which direct the timely phosphorylation of ORCs. Origin firing prompts the unwinding of DNA, displacement of histones and activation of replication forks. Different regions of chromosomal DNA replicate at characteristic periods during the S phase. Actively expressed euchromatin generally replicates during early S phase while highly-repetitive heterochromatin replicates later. During the "timing decision point" (TDP) in early G1, a replication timing plan is created. This plan confers the order of origin firing during