The cell cycle is tightly regulated to ensure accurate DNA replication and chromosome segregation. Failures in regulation can lead to uncontrolled cell proliferation — cancer.
The Three Major Checkpoints
G1 Checkpoint
Restriction Point
G2 Checkpoint
DNA Damage Check
M Checkpoint
Spindle Assembly
Cell cycle checkpoints are surveillance mechanisms that halt progression until critical conditions are met. They act as molecular brakes, preventing a cell from advancing with errors that could be catastrophic.
The most important decision point in the cell cycle. The cell evaluates its size, nutrient availability, growth factor signals, and DNA integrity. If conditions are favourable and no DNA damage is detected, the cell commits irreversibly to entering S phase and replicating its DNA. If conditions are unfavourable, the cell may enter a quiescent state called G0.
After DNA replication is complete, this checkpoint verifies that the entire genome has been accurately copied and that no DNA damage remains. The cell also confirms it has grown sufficiently and synthesised the proteins needed for mitosis. If replication errors or damage are detected, the cell cycle is arrested to allow repair before entering mitosis.
Active during metaphase, this checkpoint ensures that every chromosome is properly attached to spindle fibres from both poles (bi-oriented). Unattached kinetochores generate a “wait” signal by producing the mitotic checkpoint complex (MCC), which inhibits the anaphase-promoting complex (APC/C). Only when all chromosomes are correctly attached does the checkpoint release, allowing sister chromatid separation and the onset of anaphase.
Quick Check
What does the spindle assembly checkpoint (SAC) verify before allowing the cell to proceed?
The engine driving the cell cycle is a family of protein kinases called cyclin-dependent kinases (CDKs). CDK protein levels remain relatively constant throughout the cell cycle, but CDKs are only enzymatically active when bound to their regulatory partner proteins — the cyclins.
Cyclin levels rise and fall in a predictable pattern as the cell progresses through the cycle. Each cyclin-CDK complex phosphorylates specific target proteins to trigger events appropriate to that phase.
| Cyclin-CDK Complex | Phase | Key Function |
|---|---|---|
| Cyclin D — CDK4/6 | G1 | Responds to growth signals; initiates Rb phosphorylation |
| Cyclin E — CDK2 | G1/S transition | Commits cell to S phase; completes Rb phosphorylation |
| Cyclin A — CDK2 | S phase | Drives DNA replication; prevents re-replication |
| Cyclin B — CDK1 | M phase | Triggers mitotic entry; promotes nuclear envelope breakdown |
When a cyclin is degraded (typically by ubiquitin-mediated proteolysis), its CDK partner becomes inactive. This oscillating activation and inactivation of CDKs creates the unidirectional, irreversible progression through the cell cycle.
Fill in the Blank
CDKs are only active when bound to their regulatory partner proteins called________.
Two classes of genes play opposing roles in cell cycle control. Tumor suppressor genes encode proteins that restrain cell proliferation, while proto-oncogenes encode proteins that promote it. A healthy cell maintains a precise balance between these forces.
The p53 protein is a transcription factor activated by DNA damage, oncogene activation, and other stress signals. Once activated, p53 can halt the cell cycle at the G1 checkpoint by inducing p21 (a CDK inhibitor), activate DNA repair pathways, or trigger apoptosis (programmed cell death) if the damage is too severe to repair. Mutations in the TP53 gene are found in more than 50% of all human cancers, making it the most commonly mutated gene in cancer.
Rb acts as a molecular gatekeeper of the G1/S transition. In its active (hypophosphorylated) state, Rb binds and sequesters E2F transcription factors, preventing the expression of genes required for DNA replication. When growth signals trigger cyclin D-CDK4/6 and subsequently cyclin E-CDK2 activity, Rb is progressively phosphorylated. Hyperphosphorylated Rb releases E2F, allowing transcription of S phase genes and commitment to cell division.
Proto-oncogenes are normal cellular genes that encode proteins promoting cell growth, division, and survival — such as growth factors (e.g., PDGF), receptor tyrosine kinases (e.g., HER2), signal transduction proteins (e.g., Ras), and transcription factors (e.g., Myc). When these genes acquire gain-of-function mutations — through point mutations, gene amplification, or chromosomal translocation — they become oncogenes that drive uncontrolled cell proliferation independent of normal growth signals.
“The p53 protein has been called the ‘guardian of the genome’ because of its central role in preventing the propagation of cells that carry damaged DNA. It is mutated in more than half of all human cancers, making it the most commonly mutated gene in cancer.”
— Robert A. Weinberg, The Biology of Cancer, 2nd Edition (2013)
Quick Check
Which protein is known as the 'guardian of the genome'?
Cancer is fundamentally a disease of uncontrolled cell division. It arises when the carefully balanced regulatory mechanisms of the cell cycle break down due to accumulated mutations in genes that control cell growth, division, and death.
According to the multiple-hit hypothesis (Knudson hypothesis), a single mutation is usually insufficient to cause cancer. Instead, cancer typically develops through the progressive accumulation of several mutations — often in both oncogenes and tumor suppressor genes — over years or decades. Each successive mutation confers an additional growth advantage, gradually transforming a normal cell into a malignant one.
Hanahan and Weinberg (2011) identified key capabilities that cancer cells acquire as they become malignant:
Sustained Proliferative Signalling
Cancer cells produce their own growth signals or amplify growth factor receptors, maintaining constant division.
Evading Growth Suppressors
Inactivation of tumor suppressors like p53 and Rb removes the brakes on cell division.
Resisting Cell Death
Cancer cells disable apoptotic pathways, surviving conditions that would kill normal cells.
Enabling Replicative Immortality
Activation of telomerase prevents telomere shortening, allowing unlimited division.
Inducing Angiogenesis
Tumors stimulate new blood vessel growth to supply nutrients and oxygen.
Activating Invasion & Metastasis
Cancer cells acquire the ability to migrate and colonize distant tissues.
Fill in the Blank
Normal genes that promote cell growth are called proto-oncogenes. When they acquire gain-of-function mutations, they become________that drive uncontrolled cell proliferation.