Activators turn genes up, repressors turn them down
- The idea
- Predict
- See it
- Why
- Check
The idea
“Losing an activator should raise a gene's output, and losing a repressor should lower it.”
On a diagram, an activator and a repressor look alike. It's easy to assume that removing either one makes output drop.
The short version
The mistake: Losing an activator should raise a gene's output, and losing a repressor should lower it.
What's actually true: An activator raises transcription and a repressor lowers it. Lose an activator and output falls. Lose a repressor and it rises.
- Transcription factors bind DNA sequences that control a gene. Activators make RNA polymerase start more often. Repressors make it start less often.
- If mRNA falls without the protein or its site, it was an activator. If mRNA rises, it was a repressor.
- Many genes have sites for both, so output depends on the balance. One lost site affects only its own gene. A lost protein affects every gene it regulates.
On the exam: Ask what the protein did while it was bound. Remove a helper and output falls. Remove a brake and it rises.