Which statement best describes an operon?

• A. A cluster of genes under a single promoter regulated together by a repressor or activator; examples lac and trp.
• B. A single gene with multiple promoters.
• C. A DNA sequence that codes tRNA.
• D. A protein complex that initiates translation.

Answer: (A)

Operons organize several genes that participate in the same process into a unit that is transcribed together from a single promoter, with regulation by a regulator protein that can block or enhance transcription by binding near an operator. This setup enables coordinated control of all the genes involved in a pathway. In the lac operon, for example, the genes lacZ, lacY, and lacA are transcribed together when lactose is present, because the inducer inactivates the repressor. In the trp operon, production of tryptophan itself acts as a corepressor to help the repressor bind the operator and stop transcription, conserving resources when tryptophan is plentiful. That combination of a gene cluster, a single promoter, and regulation by a repressor or activator is what defines an operon. The other options describe different genetic elements—individual genes with multiple promoters, tRNA genes, or protein complexes involved in translation initiation—none of which capture the coordinated, polycistronic transcription characteristic of operons.