RNA polymerase doesn't pick which genes turn on
- The idea
- Predict
- See it
- Why
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The idea
“RNA polymerase or the ribosome decides which genes a cell expresses.”
RNA polymerase and ribosomes do the building, so it's easy to credit them with the choice. They work like a printer. Something else picks what gets printed.
The short version
The mistake: RNA polymerase or the ribosome decides which genes a cell expresses.
What's actually true: Regulatory proteins that bind specific DNA sequences decide which genes are on. The polymerase and ribosomes do the work.
- The same RNA polymerase transcribes thousands of genes, and the same ribosomes translate every mRNA. Machinery shared by every gene can't pick one.
- Transcription factors do the picking. They bind specific DNA sequences near a gene. Activators help the polymerase start there, and repressors block it.
- Different cell types make different transcription factors. That's how cells with the same DNA express different genes.
On the exam: When only some genes change, look for a protein that binds their DNA. Shared machinery would change them all.