Abstract
Normal cellular proliferation is a tightly regulated process. The two most important events of the eukaryotic cell cycle are DNA synthesis (replication of chromosomes) and mitosis (segregation of chromosomes into dividing cells),which occur during the S-phase and the M-phase of the cell cycle, respectively.1 The remainder of the cell cycle consists of two gap phases (G1 and G2) which occur prior to S-phase (G1) and prior to M-phase (G2) (see Figure 1). As growth quiescent cells enter and progress through the cell cycle, a number of “checkpoints” are encountered that are positioned at transition points between the different cell cycle states.2 These checkpoints are positioned at the initiation and completion of DNA replication (S-phase) and at the initiation and completion of cell division (M-phase). The most extensively studied of these checkpoints occurs late in G1, at the G1 to S transition, when the cell commits itself to another round of DNA replication.2 Bypass of this important cell cycle checkpoint may allow cells to replicate damaged DNA thereby resulting in the accumulation of genetic mutations that may eventually contribute to a neoplastic phenotype.