The lac operon is best described as an inducible operon for lactose metabolism regulated by which components?

• A. Activators CAP and glucose
• B. Repressor LacI and inducer allolactose
• C. TATA box and RNA polymerase
• D. Enhancers

Answer: (B)

Induction in this operon is driven by a repressor binding to the DNA and an inducer that releases that brake. The LacI repressor sits on the operator and blocks transcription when lactose isn’t present. When lactose is around, some of it is converted to allolactose, which binds to LacI and changes its shape so it can no longer hold the operator. With the repressor inactivated, RNA polymerase can transcribe the lac genes to metabolize lactose. A secondary layer exists: CAP bound to cAMP can enhance transcription when glucose is scarce, but the essential inducible control is the repressor LacI and its inducer allolactose. The other options describe elements not used in bacterial operons—TATA box and RNA polymerase are eukaryotic transcription tools, and enhancers are eukaryotic regulatory sequences.