Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Checkpoints are internal controls that let a cell continue only when conditions are right.
- Cyclin-dependent kinases (CDKs) are enzymes that phosphorylate target proteins. Their amount stays about constant, but they are active only when bound to a cyclin.
- Cyclins are made steadily, then destroyed quickly, so their concentration rises and falls each cycle.
- An active cyclin-CDK complex phosphorylates targets that drive the next stage; destroying the cyclin switches the CDK off.
- Apoptosis (programmed cell death) is orderly self-destruction. It removes damaged or unneeded cells and shapes organs in development.
- Cancer: mutations make “drive” proteins overactive or “stop” proteins fail, so cells divide with damaged DNA and avoid apoptosis. Usually several mutations are needed.
- Not required: specific cyclin-CDK pairs or specific growth factors.
Key relationships
| Checkpoint | Checks | If not passed |
|---|---|---|
| G1 | cell size, nutrients, signals, DNA damage | pause and repair, G0, or apoptosis |
| G2 | DNA fully replicated and undamaged | pause until fixed, or apoptosis |
| M (spindle) | every chromosome attached to both poles | anaphase delayed |
| Change | Chain of effects |
|---|---|
| Cyclin not made | CDK stays inactive → targets not phosphorylated → cell stops at that checkpoint |
| Cyclin not broken down | CDK stays active → cell cannot leave that stage (for a mitotic cyclin, stuck in mitosis) |
| Damage-checkpoint protein lost | damaged cells keep dividing, little apoptosis → mutations build up → cancer risk |
| Too much apoptosis | healthy cells lost → tissue shrinks or fails |
Assumptions behind the numbers
- Concentrations in model graphs are in arbitrary units (au); compare them only with each other.
- Cell-culture results show what can happen; a whole body has extra controls (neighbouring cells, the immune system).
Mistakes to avoid
- Calling cyclins enzymes (CDKs are the kinases).
- Saying CDK amount rises and falls (CDK activity does).
- Saying a failed checkpoint always kills the cell.
- Describing apoptosis as harmful in every case.
- Saying one mutation causes cancer.
- Skipping links in a prediction: write cyclin → CDK activity → phosphorylation → progress.
Quick self-check
- Why does CDK activity peak when cyclin peaks? (CDK needs bound cyclin to be active; more cyclin means more active complexes.)
- UV raises apoptosis in a culture from 4% to 30%. Percentage increase? ((30 − 4) ÷ 4 × 100 = 650%)
- Which checkpoint stops a cell entering anaphase with an unattached chromosome? (The M, or spindle, checkpoint.)
Next: practice questions.