Control of chromosome duplication

[1] This flexibility in genome size comes at a cost: there has to be a high-fidelity control system that coordinates multiple replication origins so that they are activated only once during each S phase.

If this were not the case, daughter cells might inherit an excessive amount of any DNA sequence, which could lead to many harmful effects.

[3] In eukaryotes, it is still unclear what exact combinations of DNA sequence, chromatin structure, and other factors define these sites.

One of these is the A element (ACS), an 11 bp consensus sequence rich in adenines and thymines that is essential for initiation.

[3][5] The ORC, a component of the initiation complex, binds the ACS in vivo throughout the cell cycle, and in vitro in an ATP dependent manner.

[3] At the sequence level, AT rich elements and CpG islands have been found at origins, but their importance or role is not yet clear.

Features identified at the chromatin level include nucleosome free regions, histone acetylation and DNAse sensitive sites.

Assembly of these pre-replicative complexes (pre-RCs) is regulated in a manner that coordinates DNA replication with the cell cycle.

[6] The ORC is a six subunit complex that binds DNA and provides a site on the chromosome where additional replication factors can assemble.

[6] Studies in Xenopus revealed the Mcm2-7 complex is a critical component of DNA replication machinery.

[3] Evidence for this is that inactivation of CDKs in cells arrested in G2/M or in S phase drives reassembly of pre-RCs.

CDK phosphorylates Cdc6 to mark it for degradation by the SCF in late G1 and early S phase.

It has been suggested that phosphorylation affects the ability of the ORC to bind other components of the pre-RC.

Having multiple mechanisms to prevent re-replication is beneficial because it the regulatory network continues to function even if one of the components fails.

[6] At the same time, S-Cdks suppress formation of new pre-RCs during S phase, G2 and early M, when S cyclin levels remains high.

It is not known whether CDKs and Cdc7 just regulate protein assembly at origins, or whether they directly activate components of the pre-initiation complex.

[6] Cdc7 levels remain relatively constant throughout the cell cycle, but its activity varies.

[6] In vitro biochemical studies have shown that Cdc7-Dbf4 phosphorylates individual components of the Mcm complex.