Mammalian cells undergo two gap, or growth, phases (G
1 and G
2). G
1 occurs prior to the S phase, and G
2 occurs before the M phase.
Control of the Cycle
During the G1 and G2 phases, cells grow and make sure that conditions are proper for DNA replication and cell division. During the G1 phase, cells monitor their environment and determine if conditions, including the availability of nutrients, growth factors and hormones, justify DNA replication. The decision to initiate replication is made at a specific "checkpoint" in G1 called the "restriction point."
The processes of DNA replication and mitosis, and intervening events during the cell cycle, occur in a highly ordered and specific manner. A complex network of proteins ensures that these events occur at the proper times. Intracellular and extracellular signals block cell-cycle progression at checkpoints if certain events have not yet been completed. After the restriction point, the cell is committed to replicating its genome and dividing, completing one round of the cell cycle. If, prior to the restriction point, cells sense inadequate growth conditions or receive inhibitory signals from other cells, they enter G0 (G-zero) phase, also called quiescence. In the G0 phase, they are maintained for prolonged periods in a nondividing state.
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