Francois Jacob and Jacques Monod described such a cluster of genes and the DNA sequences that control its transcription as a(n) ______________.

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

Answer: (D)

This question focuses on how bacteria organize genes and their control regions into a single regulatory unit. An operon is a unit of related genes that are controlled together by DNA elements nearby. It includes the promoter, where RNA polymerase binds to start transcription, and the operator, a regulatory DNA sequence that binds a repressor to switch transcription on or off, along with the structural genes themselves that are transcribed as a single mRNA when the operon is active. This concept, described by Jacob and Monod, uses the lac operon as a classic example of how multiple genes can be co-regulated.

So, the cluster of genes plus the DNA sequences that control its transcription is best described as an operon. The promoter is part of the operon, but by itself it’s only the initiation site. The operator is a regulatory element within the operon, not the whole unit. A plasmid is a separate circular DNA molecule and not what the question is describing.