Which statement best defines an operon in bacteria?

• A. A single gene with multiple promoters.
• B. A cluster of genes transcribed as a single mRNA under one promoter, allowing coordinated regulation of related functions.
• C. Genes transcribed separately from each other.
• D. A cluster of genes transcribed as different mRNAs under multiple promoters.

Answer: (B)

An operon is a gene regulation unit in bacteria where several genes that work in the same pathway are controlled together by a single promoter and nearby regulatory elements. They are transcribed as one long polycistronic mRNA, so the genes are expressed simultaneously and coordinately. This setup allows bacteria to efficiently turn on or off a group of related functions as a unit, such as enzymes needed for a particular nutrient's metabolism, with regulation often mediated by a repressor or activator that binds near the promoter.

This description fits best because it highlights transcription of multiple genes as a single mRNA under one promoter and the coordinated control of related functions. The other options describe scenarios that don’t match operons: a single gene with multiple promoters, genes transcribed separately, or a cluster of genes producing different mRNAs from multiple promoters, none of which involve the shared promoter and single polycistronic transcript that characterize operons.