The operator doesn't make the repressor
- The idea
- Predict
- See it
- Why
- Check
The idea
“The operator is the gene that makes the repressor.”
Operator and repressor sound like a matched pair. Since DNA codes for proteins, it's easy to guess the repressor comes from the operator.
The short version
The mistake: The operator is the gene that makes the repressor.
What's actually true: Operators and enhancers are stretches of DNA that proteins bind to. Separate genes code for those proteins.
- A gene is DNA that is transcribed, usually to make a protein. A regulatory site is DNA that a protein binds. It codes for no protein.
- The lac operator is one such site. The repressor comes from lacI, a separate regulatory gene, and binds the operator to block transcription.
- In eukaryotes, activator proteins from their own genes bind sites called enhancers. Different cell types make different activators.
On the exam: Ask if the DNA codes for a product or gets bound by one. Operators and enhancers get bound.