A cell's response can outlast its signal
- The idea
- Predict
- See it
- Why
- Check
The idea
“Once the signal molecule is gone, the response inside the cell stops right away.”
You'd think removing the signal ends the response. But the signal only flips switches inside the cell, and they stay flipped until something flips them back.
The short version
The mistake: Once the signal molecule is gone, the response inside the cell stops right away.
What's actually true: What a signal switched on stays on until the cell turns it off or breaks it down. Some responses keep themselves going.
- Removing the signal stops new activation at the receptor. It doesn't undo what's already on inside the cell.
- Phosphatases take phosphates off proteins, phosphodiesterase breaks down cAMP, and G proteins switch off by hydrolyzing GTP. Their speed sets how long a response lasts.
- The signal may also have turned on a gene. The new proteins keep working until they are broken down.
- Some responses keep themselves going. A transcription factor can keep its own gene on. A cell past the G1 checkpoint enters S phase without more growth factor.
On the exam: When the signal leaves, ask what switches each step off and whether anything keeps itself on.