Cell Cycle Checkpoint

The cell cycle is controlled at three checkpoints (the G₁, or restriction point, M, and the G₂ checkpoint) by both external and internal signals (Figure 1). These signals report whether crucial cellular processes that should have occurred by that point have been completed correctly. If all is in order, the cell may proceed to the next cell cycle phase; if not, the cell will exit the cell cycle and enter the G₀, or non-dividing, phase.

G₁ checkpoint. This is the first checkpoint, also known as the restriction point. This checkpoint happens in the late G1 phase, when the cell begins to enter the cell cycle and chromosome duplication. Cells pass the G1 checkpoint when they are stimulated by appropriate external growth factors.

G₂ checkpoint. This checkpoint checks for damage to DNA after it has been replicated.

M checkpoint. The metaphase (M) checkpoint checks that the mitotic spindles/ microtubules are properly attached to the kinetochore. If the cell passes this checkpoint, sister-chromatids will begin to separate, leading to the completion of mitosis and cytokinesis.

Figure 1. Cell cycle checkpoints.

There are three checkpoints present in the cell cycle which control its progression. The first checkpoint G1, or the restriction checkpoint, occurs during normal cell growth and is passed when certain growth factors are present. The G2 checkpoint assesses for DNA damage following replication. Finally, the M checkpoint checks that the mitotic spindles and microtubules are properly attached to the kinetochore.