Which checkpoint checks what
- The idea
- Predict
- See it
- Why
- Check
The idea
“Any DNA problem stops the cell at G1/S, spindle trouble stops it before mitosis, and any cyclin runs any step.”
Three checkpoints on one circle diagram blur together. Each can only check what has already happened when the cell gets there.
The short version
The mistake: Any DNA problem stops the cell at G1/S, spindle trouble stops it before mitosis, and any cyclin runs any step.
What's actually true: Each gate checks what exists when the cell arrives: G1/S before copying, G2/M after it, and the spindle checkpoint attachment.
- G1/S checks cell size, nutrients, growth signals and DNA damage. Passing it commits the cell to replication.
- G2/M checks that copying is done and the DNA is undamaged. In metaphase, the spindle checkpoint checks that every chromosome is attached.
- Cyclins switch on cyclin-dependent kinases, or CDKs. G1 cyclin pushes the cell into S. Mitotic cyclin starts mitosis, and its breakdown lets the cell finish.
On the exam: Ask when the gate sits. G1/S: safe to copy? G2/M: is copying done? Spindle: is every chromosome attached?