DNA re-replication
[1] Rereplication is believed to lead to genomic instability and has been implicated in the pathologies of a variety of human cancers.
[2] DNA rereplication must be strictly regulated to ensure that genomic information is faithfully transmitted through successive generations.
In yeast, the origins contain autonomously replicating sequences (ARS), distributed throughout the chromosome about 30 kb from each other.
Along with other conserved B elements, they form the section where the origin recognition complexes (ORCs) assemble to begin replication.
Cdt1 binding and the ATPase activity of ORC and Cdc6 facilitate the loading of the minichromosome maintenance (MCM) proteins 2-7 onto the chromatin.
High CDK levels, which are maintained until the end of mitosis, inhibit or destroy pre-RC components and prevent the origin from relicensing.
A new MCM complex cannot be loaded onto the origin until the pre-RC subunits are reactivated with the decline of CDK activity at the end of mitosis.
[10] The identification and characterization of the ORC, Cdc6, Cdt1, and the MCM complex proteins as the licensing factor gives credence to this model and suggests a means by which the oscillatory nature of CDKs in the cell cycle can regulate rereplication.
[5] The phosphorylation of these components is initiated at the onset of S phase and is maintained throughout the rest of the cell cycle as CDK activity remains high.
Note: Since origins fire at different times throughout S phase, it is crucial that the inhibitory mechanisms that prevent new MCM2-7 recruitment do not destabilize existing pre-RCs.
Although CDK regulation of pre-RC assembly appears to be highly evolutionarily conserved, some differences across organisms are noted.
In multicellular eukaryotes pre-RC assembly is regulated by the anaphase-promoting complex (APC) in addition to CDKs.
In G1, APC activity is adequate to suppress the accumulation of geminin, thereby indirectly promoting pre-RC assembly.
[5] Rereplication and mitotic failure are generally not programmed events, but rather result spontaneously from defects in the cell cycle machinery.
[1] Rereplication appears to give rise to dsDNA breaks which triggers a DNA damage response and arrests cells in G2.
[13] Rereplication can be experimentally induced by simultaneously disrupting several of the mechanisms that prevent origin re-licensing.
[14] Note: Recent evidence suggests that although overlapping, the multiple replication regulation mechanisms should not be considered as functionally redundant; although a single mechanism may repress rereplication at greater than 99% efficiency, it may not be sufficient to maintain genome stability over many generations.
that the multiplicative effect of many overlapping mechanisms is what sufficiently prevents rereplication and ensures the faithful transmission of a cell's genome.
[16] This checkpoint response activates due to overexpression of cyclin E, which has been shown to be important in regulating the licensing system.
Ataxia telangiectasia mutated (ATM) activates after a larger amount of DSBs is detected at later stages of DNA re-replication.
