An _____ is a group of related genes under the control of one promoter site, and they function to conserve resources.

• A. Promoter unit
• B. Operon
• C. Gene cluster
• D. Transcription unit

Answer: (B)

In bacteria, a group of related genes that share a single promoter and are transcribed together into one mRNA is called an operon. This setup allows coordinated expression of all the enzymes needed for a pathway, so the cell can turn on or off an entire process at once and conserve energy and resources. An operon typically includes a promoter where RNA polymerase binds, an operator site that can bind a repressor to block transcription, and several structural genes that encode the pathway's enzymes. Because they’re co-transcribed, the cell can produce all the necessary proteins in one go when the pathway is needed, and halt production when it isn’t. The lac operon is a classic example, with multiple enzymes encoded in one transcriptional unit and regulated in response to lactose availability. Terms like promoter unit aren’t standard for this concept, a gene cluster simply notes physical proximity without implying shared regulation, and a transcription unit refers to the piece of DNA that is transcribed—which can be a single gene and doesn’t necessarily reflect a single promoter controlling multiple genes.