Region of DNA that controls RNA polymerase's access to a set of genes with related functions.

• A. Promoter
• B. Operator
• C. Enhancer
• D. Operon

Answer: (B)

In bacterial gene regulation, a group of related genes is often organized as an operon, and the key control point is the operator. The operator is a DNA sequence that serves as a binding site for a repressor protein. When the repressor is bound, it blocks RNA polymerase from accessing the promoter and transcribing the entire set of genes. When a specific signal or molecule binds the repressor, it changes shape and releases from the operator, allowing RNA polymerase to bind the promoter and transcribe the operon. This switch-like control precisely regulates expression of functionally related genes as a unit. The promoter is simply where RNA polymerase binds to start transcription, not the regulatory switch itself; an enhancer is a eukaryotic element that boosts transcription at a distance, and the operon refers to the whole gene cluster, not the specific control site.