What is an operon and its basic purpose in bacteria?

• A. A cluster of genes under one promoter regulated together, allowing coordinated expression.
• B. A sequence in RNA that binds ribosomes
• C. A single gene with multiple promoters
• D. A protein complex that regulates transcription

Answer: (A)

In bacteria, the idea being tested is that an operon is a unit of gene regulation where several genes that work together are controlled from a single promoter, so they’re transcribed as a group into one mRNA. This means the enzymes needed for a particular pathway can be produced together, making the response efficient. An operon often includes an operator site where a repressor protein can bind to block transcription, and an inducer or corepressor can influence that binding to turn expression on or off as needed. A classic example is the lac operon, where the presence or absence of lactose determines whether the genes for lactose metabolism are expressed.

This isn’t just a sequence in RNA that binds ribosomes, nor is it a single gene with multiple promoters, nor a separate protein complex regulating transcription. Those ideas describe different aspects of gene expression, not the coordinated group regulation that defines an operon.