Preventing re-replication of chromosomal DNA

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Abstract
To ensure precise duplication of chromosomal DNA, replication origins are 'licensed' for replication during late mitosis and early G1 by loading complexes of the Mcm2–7 proteins. The licensing system is turned off during S phase and G2, thereby ensuring that no replication origins can fire more than once in a single cell cycle. Recent structural studies have revealed that the Mcm2–7 proteins form a hexameric ring that can potentially encircle double-stranded DNA. The ability of Mcm2–7 to be clamped around DNA potentially explains why the licensed state can be stably maintained over long time periods. The Mcm2–7 structure resembles that of SV40 T antigen, which is a replicative helicase, consistent with the idea that Mcm2–7 functions as a replicative helicase. Recent evidence indicates that the licensing system is regulated in different ways in different organisms. In yeasts, high CDK activity from late G1 until the end of mitosis inhibits different components of the licensing system in a range of different ways. In metazoans, however, the Cdt1 component of the licensing system seems to be the main component that is down-regulated late in the cell cycle. On progression into S phase, metazoan Cdt1 is degraded, and a Cdt1 inhibitor called geminin becomes active. If metazoans over-express Cdt1 in S phase, or lack geminin, re-replication of DNA occurs. In cells that are unable to down-regulate the licensing system in S phase and G2, re-replication of DNA takes place. Metazoan cells respond to this by activating a number of different checkpoint pathways, which cause cell-cycle arrest or apoptosis. When cells exit from the cell cycle, Mcm2–7 and other components of the licensing system are degraded. This potentially provides a barrier to prevent the inappropriate proliferation of such cells. To re-enter the cell-cycle, quiescent cells must first re-license their DNA. Recent work has shown that mouse embryos lacking cyclins E1 and E2 are unable to re-license their DNA on exit from quiescence. The cyclin E-null embryos are also unable to produce some cell types that are normally polyploid. These results indicate that cyclin E has a central role in activating the licensing system.