Which statement about operons is true?

• A. Operons cluster genes under a single promoter for coordinated expression
• B. Each gene has its own promoter
• C. They are found in eukaryotes only
• D. They are not regulated by repressors or activators

Answer: (A)

Genes in an operon are organized so that several functionally related genes are controlled by a single promoter and regulatory region, allowing them to be transcribed together as one mRNA for coordinated expression. This arrangement lets the cell respond quickly to internal and external signals by turning on or off an entire pathway with one regulatory input. The promoter is where RNA polymerase binds to start transcription, and nearby regulatory elements like an operator can be bound by repressors or activators to modulate that transcription. A classic example is the lac operon in E. coli, where the absence of lactose allows a repressor to block transcription, while lactose (through its inducer) relieves repression and enables production of multiple enzymes needed to metabolize lactose. This shared control of several genes is what makes operons efficient for coordinated expression. The other statements don’t fit because each gene having its own promoter would prevent coordination, operons are characteristic of prokaryotes (not exclusive to eukaryotes), and they are indeed regulated by repressors or activators rather than being unregulated.