Cell division is divided into four phases: G1, S, G2, and M (Fig. 1.6). The entire process is punctuated by two spectacular events, the replication of DNA during S phase and chromosome segregation during mitosis or M phase. Of the four cell cycle phases, three can be assigned to replicating cells and only the G1 phase, and a related quiesent phase, G0, are nonreplicative in nature.