G1 and S-phase checkpoints, chromosome instability, and cancer.

Mitogen-dependent progression through the first gap phase (G1) of the mammalian cell-division cycle is precisely regulated so that normal cell division is coordinated with cell growth, while the initiation of DNA synthesis (S phase) is precisely ordered to prevent inappropriate amplification of the DNA that may cause genome instability. To ensure that these fundamental requirements of cell division are met, cells have developed a surveillance mechanism based on an intricate network of protein kinase signaling pathways that lead to several different types of checkpoints. Since these checkpoints are central to the maintenance of the genomic integrity and basic viability of the cells, defects in these pathways may result in either tumorigenesis or apoptosis, depending on the severity and nature of the defects. This review summarizes the genetic and molecular mechanisms of checkpoint activation in the G1/S and S phases of the mammalian cell cycle that monitor DNA damage and replication. The relevance of these mechanisms to the origin of cancer is also discussed.