As a cell grows, differentiates, and divides, it progresses through four distinct phases referred to as the cell cycle (Figure 2). The phases of the cell cycle are known as G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis). Growth and differentiation take place in the G1, S, and G2 phases, collectively known as interphase. Mitosis, the process of cell division, takes place during the M phase.[i]
Figure 2: Phases of the Cell Cycle
During the G1 phase, the cell grows and differentiates to carry out specialized functions. The cell is also engaged in RNA and protein synthesis during this phase. Following the G1 phase, the cell enters the S (synthesis) phase, in which it replicates its DNA in preparation for mitosis. During this phase, the amount of DNA in the cell doubles. Next the cell enters G2, a second growth phase in which the replicated DNA condenses into chromosomes. The cell is then ready to begin dividing.[i]
During M phase, the cell undergoes mitosis, during which the cell divides into two identical daughter cells, each of which receives a full set of genetic material. Following mitosis, actively proliferating cells directly enter the G1 phase to prepare for further replication. Nonproliferating cells enter a quiescent state referred to as G0. Cells in phases G1, S, G2, or M are said to be cycling, while those in G0 are dormant, or noncycling.[i]
[i] Raven P, Johnson G, Losos J, et al. Cell cycle and cell division. In: Biology. New York, NY; McGraw-Hill, 2011: Chapter 10.