What is an operator in prokaryotic operons?

• A. RNA sequence that codes for a structural gene.
• B. DNA segment bound by a repressor to block RNA polymerase access or progression.
• C. A protein that binds to promoter to help RNA polymerase.
• D. The promoter region where transcription starts.

Answer: (B)

In prokaryotic operons, the operator is a regulatory DNA sequence that a repressor can bind to. When the repressor sits on the operator, RNA polymerase cannot access the promoter or cannot proceed, so the downstream genes aren’t transcribed. This lets the cell turn off the entire operon as needed. A classic example is the lac operon, where the lac repressor binds the operator to block transcription; when lactose is present, it binds the repressor and causes it to release, allowing transcription to occur.

This isn’t the RNA sequence that codes for a structural gene, nor a protein that helps RNA polymerase bind to the promoter, nor the promoter region itself. The defining feature of the operator is that regulatory DNA site bound by a repressor to block transcription.