The big idea: Cyclins are proteins whose concentrations rise and fall during the cell cycle. A cell passes each checkpoint only when a particular cyclin reaches its threshold level.
The points to remember
- There are several different cyclins; each rises and falls at a different point in the cycle.
- Each checkpoint needs a specific cyclin above a threshold concentration.
- Below the threshold the cell waits; above it, the cell moves on to the next stage.
- After its checkpoint, each cyclin is quickly broken down.
- This makes sure the stages happen in the right order, one at a time.
Remember it as: No cyclin above the line, no way through the checkpoint.
Real example: discovering cyclins. Tim Hunt found cyclins in dividing sea urchin eggs, where their levels rose and fell with every division. He shared the 2001 Nobel Prize in Physiology or Medicine for discoveries about control of the cell cycle.
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Checkpoints: Checkpoints are points in the cycle where the cell is held until conditions are right.
| Checkpoint | Allows the cell to | Needs |
|---|---|---|
| End of G1 | Start replicating its DNA (enter S) | A cyclin above its threshold |
| End of G2 | Start mitosis | A different cyclin above its threshold |
| Metaphase | Separate the chromatids (anaphase) | Another cyclin above its threshold |
If the control fails: A cell that passes checkpoints when it should not can divide out of control. Mutations in the genes that control the cycle are the subject of 4.4.16.
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How it is asked: A graph of cyclin levels through the cycle: identify where a checkpoint is passed, or explain what would happen if a cyclin stayed low. Also short written answers on how cyclins control the cycle.
IB-style questionExplain[3 marks]
Explain how cyclins control the progress of a cell through the cell cycle.
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See the answer broken into clear points, with guidance on how each point earns marks.