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The cell cycle control system has three crucial checkpoints located in the G1, G2, and M phases. At the G1 checkpoint, regulator proteins verify cell size, DNA integrity, and nutrient availability before DNA synthesis begins. The G2 checkpoint checks for DNA errors and appropriate cell size before mitosis. During M-phase, the system verifies chromosome attachment and alignment to the spindle before cell division proceeds.

Cyclin-dependent kinases, or CDKs, form complexes with cyclins, which are phase-specific proteins expressed at fixed points in the cell cycle. These positive regulators cell cycle cyclins and CDKs complexes directly affect proteins involved in cell growth and DNA synthesis. Regulatory phosphorylation and dephosphorylation fine-tune cyclin-CDK function, ensuring well-defined progression through each cell cycle phase.

At the G1 checkpoint, regulator proteins assess whether the cell has reached critical size, verify DNA is free from errors, and confirm sufficient nutrients and growth factors are present to begin DNA synthesis. If the cell lacks necessary signals, it enters a resting state called G0 phase until all conditions are satisfied, preventing premature progression into the S-phase.

The cell cycle control system uses surveillance mechanisms at checkpoints to monitor essential events like DNA replication and chromosome segregation. If regulator proteins detect irreparable DNA damage, they activate cell cycle inhibitors that arrest the cycle until the damage is repaired. If damage cannot be fixed, cell death occurs, preventing the propagation of mutations that could lead to cancer.

The G0 phase is a resting state that cells enter when they do not receive necessary signals at the G1 checkpoint. Cells remain in G0 until conditions such as adequate cell size, DNA integrity, nutrients, and growth factors are met. This quiescent phase allows cells to pause proliferation and conserve resources when conditions are unfavorable for division.

During M-phase, regulator proteins verify that chromosomes are properly attached to the spindle and accurately aligned before cell division proceeds. This checkpoint ensures that each daughter cell receives the correct number of chromosomes. If chromosomes are misaligned or improperly attached, the control system halts progression until the defect is corrected or cell death is triggered.

Cyclin-CDK complexes drive cell cycle progression through protein phosphorylation cascades that transition cells between phases. Checkpoint control, by contrast, is a surveillance mechanism that monitors completion of essential events and delays progression if problems are detected. While cyclin-CDKs actively promote advancement, checkpoints act as brakes to ensure accuracy before allowing the cycle to continue.