Cell Cycle Checkpoints & Cancer
- Identify the G1, G2, and M checkpoints and what each verifies
- Explain how cyclins and Cdks drive the cell cycle forward
- Relate loss of checkpoint control to the development of cancer
Checkpoints: quality control gates
The cell cycle is not allowed to run unchecked. At several points the cell pauses to verify that everything is in order before proceeding — these are checkpoints. If a check fails, the cycle halts until the problem is fixed (or the cell self-destructs). There are three key gates. The G1 checkpoint (the "restriction point") asks whether the cell is big enough, has enough nutrients, has undamaged DNA, and has received a go-ahead signal — this is the main decision to commit to dividing. The G2 checkpoint verifies that DNA replication finished completely and correctly. The M (spindle) checkpoint, during metaphase, confirms that every chromosome is properly attached to the spindle before chromatids are pulled apart.
Cyclins and Cdks: the engine
What actually pushes the cycle from one phase to the next is a pair of proteins. Cyclin-dependent kinases (Cdks) are enzymes that trigger cell-cycle events by phosphorylating target proteins — but a Cdk is inactive on its own. It only works when bound to a partner protein called a cyclin, whose concentration rises and falls across the cycle. When cyclin levels build high enough, they activate their Cdks to form cyclin-Cdk complexes that drive the cell past a checkpoint; afterward the cyclin is degraded and the complex switches off. This rise-and-fall of cyclins is the clock that times the cycle.
When control fails: cancer
Cancer is fundamentally a disease of the cell cycle: cells that ignore checkpoint controls and divide without stopping. The trouble usually starts with mutations in the genes that regulate division. Proto-oncogenes normally code for proteins that promote the cycle; a mutation can turn one into an oncogene stuck permanently "on," like a gas pedal jammed to the floor. Tumor suppressor genes (such as p53) normally halt the cycle to allow DNA repair or trigger cell death; if they are inactivated, the brakes fail. p53 is called the "guardian of the genome" because it stops damaged cells at the G1 checkpoint. With accelerators stuck on and brakes gone, cells divide uncontrollably, pile up into a tumor, and — if they spread through the body (metastasis) — become life-threatening.
A cell suffers a mutation that inactivates its p53 gene. Predict the consequence for that cell’s division, and explain why p53 loss is found in so many cancers.
- 1.p53 is a tumor suppressor that acts at the G1 checkpoint: when it detects DNA damage, it halts the cycle so the damage can be repaired, or triggers programmed cell death if it cannot.
- 2.With p53 inactivated, the G1 checkpoint is no longer enforced for DNA damage — a cell with damaged DNA is allowed to proceed through the cycle.
- 3.That cell replicates its damaged DNA and passes the mutations to its daughters, and further mutations accumulate unchecked, driving uncontrolled division.
- 4.Because p53 guards against exactly this, losing it removes a central brake — which is why p53 mutations appear in a very large fraction of human cancers.
What is the role of cyclins in the cell cycle?
Do not swap the two cancer genes. A proto-oncogene is a normal "go" gene; mutation makes it a hyperactive oncogene (accelerator stuck on). A tumor suppressor is a "stop" gene; its loss removes a brake. Oncogenes are gained-function; tumor suppressors are lost-function.
A tumor suppressor gene like p53 normally stops cells with damaged DNA from dividing. How does the loss of p53 function contribute to cancer?
The AP framing for cancer is "accelerators and brakes." Tie any loss of control back to a specific failure: an oncogene stuck on (gain of function) or a tumor suppressor switched off (loss of function), and name the checkpoint (often G1, guarded by p53) that fails as a result.
Answer the 2 checkpoints as you read.
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