AP Biology · Unit 4 · Topic 4.6
A cell does not divide because it can. It divides because nothing stopped it. Try to push a damaged cell past each checkpoint in turn.
Learning objective 4.6.A Describe the events that occur at checkpoints in the cell cycle.
Set the cell's condition with the switches, then watch how far it gets. Each checkpoint asks about a few of these and lets the cell through only if every one of them holds. Try to find a way through with something switched off.
The G1 checkpoint comes before S because copying the DNA is expensive and there is no point starting if the cell should not be dividing. The G2 checkpoint comes after S because that is the first moment there is a copy to check. The spindle checkpoint comes before anaphase because once the chromatids have been pulled apart there is no putting them back.
It is the only condition on two lists. At G1 the question is whether the cell should copy a damaged sequence at all. At G2 the question is whether anything went wrong during the copying itself — damage that did not exist an hour earlier.
Switch DNA undamaged off and step through: the cell never reaches G2, because it was already stopped at G1. The second check matters for damage that arises after the first one.
The enzyme is there all the time. What changes is the protein it needs as a partner. Watch which line has the shape, and where the shaded window — the part of the cycle in which the transition can actually be driven — comes from.
Drag to a tick after the fall. The CDK line has not moved — it is exactly where it was — and the activity is zero. Switching a transition off by destroying one partner is quick and complete, and it cannot be undone by accident, which is part of why the cycle runs forwards and not backwards.
Raise the threshold and the shaded window narrows; lower it and the window widens. The cyclin curve is unchanged either way. The threshold is a property of the cell, not of the cyclin, and it is one of the things a cell can adjust to change how readily it divides.
Turn CDK off entirely. The cyclin keeps rising and falling exactly as before, and nothing at all happens — which is what the name cyclin-dependent kinase is telling you: neither half does anything alone.
A cell arrives at the G1 checkpoint with damaged DNA. Four things can happen next. Three of them keep the fault out of the tissue, and one does not — and the difference is not visible for a long time.
Notice what the model does not say. It does not say that cancer cells divide unusually fast — in this model the ignored lineage divides at exactly the same rate as the repaired one. It says they divide when they should have been stopped, and that they carry damage forward. Speed is not the defining problem; missing permission is.
It is easy to read the apoptosis row as the worst outcome, since the lineage ends at zero cells. For the organism it is one of the good ones: a single cell is a very small price for keeping a fault out of a tissue, and apoptosis is a controlled, tidy process rather than a cell bursting and damaging its neighbors.
Answer these in your own words before you open the answers.
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| Checkpoint | When | What it asks | What it checks |
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A question that gives you an unfamiliar fault — a drug that stops spindle fibers forming, a mutation that removes a growth-signal receptor — is asking you to find which row it belongs in.