Cell-Cycle Checkpoints Explained: G1/S, Intra-S, G2/M and Mitosis
Cell division is policed by a series of checkpoints that verify DNA integrity and replication status before a cell commits to the next phase. Cancer cells routinely weaken these controls, but in doing so they often become reliant on the checkpoints that are still intact. Understanding which checkpoint a gene or drug acts on helps put a molecular report in context.
Quick Answer
Cell division is policed by a series of checkpoints that verify DNA integrity and replication status before a cell commits to the next phase. Cancer cells routinely weaken these controls, but in doing so they often become reliant on the checkpoints that are still intact. Understanding which checkpoint a gene or drug acts on helps put a molecular report in context.
Why Checkpoints Exist
The cell cycle moves through four phases: G1 (growth and preparation), S (DNA synthesis), G2 (further growth and repair) and M (mitosis). Checkpoints are surveillance mechanisms that pause progression when conditions are wrong, giving the cell time to repair damage or, if the damage is irreparable, to exit the cycle or die.
Each checkpoint converges on the same core machinery: cyclin-dependent kinases (CDKs) paired with cyclin partners. Checkpoint signalling ultimately keeps the relevant CDK inactive until the problem is resolved. This is why so many cancer-associated alterations cluster around CDKs, cyclins and their regulators.
The G1/S Checkpoint and Restriction Point
In late G1 the cell decides whether to enter S phase. Growth-factor signalling drives expression of D-type cyclins, which activate CDK4 and CDK6 to begin phosphorylating the retinoblastoma protein (RB). Once RB is fully inactivated by cyclin E-CDK2, E2F transcription factors are released and S-phase genes switch on.
A parallel arm responds to stress: p53 is stabilised, induces the CDK inhibitor p21 (CDKN1A) and halts the cell in G1. Loss of p53, RB or CDKN2A removes this brake, which is one reason G1/S control is disrupted in the majority of human cancers.
The Intra-S and Replication-Stress Checkpoint
During S phase, stalled or collapsed replication forks generate stretches of single-stranded DNA that activate the ATR-CHK1 axis. This slows origin firing, stabilises forks and delays completion of replication until problems are resolved.
Many oncogenes increase replication stress by forcing premature or excessive origin firing. Tumours carrying such oncogenes can become unusually dependent on ATR and CHK1, which is the rationale behind ATR and CHK1 inhibitors now in clinical trials.
The G2/M DNA-Damage Checkpoint
Before mitosis, the cell checks that DNA replication is complete and that no double-strand breaks remain. ATM and ATR signal through CHK2 and CHK1 to inhibit the CDC25 phosphatases and activate WEE1 kinase, keeping the mitotic driver CDK1-cyclin B switched off.
Cells that have lost p53 depend more heavily on this G2/M arrest because they cannot sustain a G1 block. That dependency is being explored therapeutically with WEE1, CHK1 and ATR inhibitors, which aim to push damaged cells into a lethal mitosis.
The Spindle-Assembly Checkpoint
In mitosis, the spindle-assembly checkpoint (SAC) delays chromosome separation until every kinetochore is correctly attached to spindle microtubules. Unattached kinetochores generate a diffusible signal that inhibits the anaphase-promoting complex, preventing premature loss of sister-chromatid cohesion.
A weakened SAC contributes to aneuploidy and chromosomal instability, a common feature of aggressive tumours. Because cancer cells still need a minimally functional SAC to survive division, drugs that target SAC kinases such as MPS1 are being studied as a way to tip instability past a survivable threshold.
Key Takeaways
- ·Checkpoints act at G1/S, within S phase, at G2/M and during mitosis, all converging on CDK activity.
- ·Cancers disable some checkpoints and become dependent on those left intact.
- ·Checkpoint-targeting drugs (WEE1, ATR, CHK1, MPS1 inhibitors) exploit that dependency and remain investigational in most settings.
- ·A checkpoint-gene result should be read with tumour type, co-alterations and assay context, not in isolation.
Put these genes in pathway context
Frequently asked questions
What is the key idea in Cell-Cycle Checkpoints Explained: G1/S, Intra-S, G2/M and Mitosis?
Cell division is policed by a series of checkpoints that verify DNA integrity and replication status before a cell commits to the next phase. Cancer cells routinely weaken these controls, but in doing so they often become reliant on the checkpoints that are still intact. Understanding which checkpoint a gene or drug acts on helps put a molecular report in context.
What should be kept with the result or mechanism?
Cancers disable some checkpoints and become dependent on those left intact. Checkpoint-targeting drugs (WEE1, ATR, CHK1, MPS1 inhibitors) exploit that dependency and remain investigational in most settings. A checkpoint-gene result should be read with tumour type, co-alterations and assay context, not in isolation.
References
- 1Cell cycle control in cancer. Nature Reviews Molecular Cell Biology, 2021. PubMed
- 2The cell cycle, cancer development and therapy. Molecular Biology Reports, 2022. PubMed
- 3The DNA-damage response in human biology and disease. Nature, 2009. PubMed
- 4Hallmarks of Cancer: The Next Generation. Cell, 2011. PubMed
Continue Reading
The Cyclin D-CDK4/6-RB Axis: How Cells Enter the Division Cycle
3 min read
The G1/S Restriction Point: Where a Cell Commits to Dividing
3 min read
The G2/M DNA-Damage Checkpoint: ATR, CHK1 and WEE1
3 min read
The Spindle-Assembly Checkpoint and Chromosomal Instability
3 min read
TP53, RB1, CDKN2A and MDM2: Tumour-Suppressor Pathways Compared
6 min read
E2F Transcription Factors: Activators, Repressors and Cancer
3 min read
Choose your next research step
Move from this explanation into a gene profile, a pathway map, or the next evidence update.
TP53 has 100+ trials currently recruiting on ClinicalTrials.gov. The GeneAnalyses digest summarises the new and changed ones each day.