Question

which term describes a protein that prevents an operon from being turned on in a prokaryotic cell?
a. repressor
b. promoter
c. operator
d. enhancer

Explanation:

• A repressor protein binds to the operator region of an operon in prokaryotic cells. When it binds, it physically blocks RNA polymerase from transcribing the genes in the operon, thus preventing the operon from being turned on.
• A promoter is a DNA sequence where RNA polymerase binds to initiate transcription. It does not prevent an operon from being turned on.
• An operator is a DNA sequence (not a protein) that the repressor binds to.
• An enhancer is a DNA sequence (not relevant in prokaryotic operon regulation in the context of this question as it is more associated with eukaryotic gene regulation to increase the rate of transcription).

Answer:

A. Repressor