Cell Cycle

Mammalian cells undergo two gap, or growth, phases (G₁ and G₂). G₁ occurs prior to the S phase, and G₂ 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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