In bacteria, what is an operon?

• A. A single gene regulated by multiple promoters in eukaryotes
• B. A protein complex that assists transcription in eukaryotes
• C. A sequence that terminates transcription in bacteria
• D. A cluster of genes regulated together by a single promoter; common in prokaryotes

Answer: (D)

In bacteria, an operon is a functioning unit that groups several related genes under the control of a single promoter, so they are transcribed together as one mRNA. This setup lets the cell turn on or off a whole metabolic pathway at once, coordinating the production of multiple enzymes needed for that pathway. The operon typically includes a promoter where RNA polymerase binds, an operator site where a repressor or activator can bind to regulate transcription, and multiple structural genes that are transcribed together (polycistronic mRNA). A classic example is the lac operon in E. coli, which is turned on when lactose is available and glucose is scarce, enabling the coordinated expression of all the needed enzymes to metabolize lactose. This arrangement is common in prokaryotes and contrasts with many eukaryotic genes, which often have their own promoters and are regulated more independently.