Cell cycle checkpoint
Cell cycle checkpoints are control points that ensure the cell progresses through its cycle only when everything is working correctly. They check DNA, replication, and chromosome separation, and they pause the cycle to allow fixes if something goes wrong. The three main checkpoints are at the G1 phase (the Start/restriction point), the G2/M transition, and the metaphase-to-anaphase transition (the spindle checkpoint).
The cell cycle has four main stages. G1 is a growth and metabolism phase. S is when DNA is copied. G2 is a second growth phase where the cell prepares for division. M (mitosis) is when the copied chromosomes separate into two new nuclei, followed by cell division. In simple terms, the cycle makes sure DNA is copied accurately and the two daughter cells end up with the right amount of DNA.
Progress through the cycle is controlled by cyclins and cyclin-dependent kinases (CDKs). Different cyclin-CDK pairs become active at different stages to trigger the next steps. This system acts like a clock but also responds to the cell’s real-time conditions.
G1 checkpoint: This is where the cell decides whether to enter a new round of division. If DNA is damaged or conditions are not right, the cell delays or stops. A family of proteins called p53 can trigger production of p21, which blocks the Cyclin E-CDK2 complex and helps keep the cell from moving into S phase. The Rb protein and E2F transcription factors also play a key role: when Rb is intact, E2F is held back; when Rb is inactivated, E2F turns on genes needed for DNA replication. Signals from DNA damage sensors (ATM and ATR) help mount these responses.
G2/M checkpoint: After DNA is copied, the cell checks again for damage or incomplete replication. ATM/ATR activate kinases (Chk1/Chk2) that help keep the G2/M transition from starting too soon. They work with p53 and p21 to slow or stop the cycle if problems are found. When the cell is ready, enzymes called Cdc25 remove inhibitory phosphates from the Cdk1–Cyclin B complex, allowing mitosis to begin.
Spindle checkpoint: Once mitosis starts, chromosomes must line up correctly and attach to the spindle in a way that will pull sister chromatids apart evenly. If attachments are faulty, this checkpoint keeps mitosis on hold. When all chromosomes are properly attached, the cell activates the anaphase-promoting complex (APC/C), which leads to the destruction of cyclin B and the release of separase to separate the sister chromatids, letting the cell complete division.
Why checkpoints matter: They help prevent mutations and cancer. When checkpoint components fail (for example, ATM, Chk1, BRCA1/2, or p53 pathways), cells can accumulate DNA damage and become cancerous. Understanding these checkpoints helps explain how cells keep their genetic information safe and why failures can be dangerous.
In short, the cell cycle is a carefully timed sequence controlled by cyclins and CDKs. Checkpoints pause the cycle to fix problems, ensuring DNA is accurately copied and divided between the two daughter cells.
This page was last edited on 3 February 2026, at 19:24 (CET).