Which molecule binds to the operator to prevent transcription when lactose is absent?

• A. Lactose
• B. Repressor
• C. Promoter
• D. RNA polymerase

Answer: (B)

In the lac operon, transcription is turned off when lactose is absent by a repressor protein that binds to the operator. This binding physically blocks RNA polymerase from accessing the promoter, so the enzymes needed to metabolize lactose aren’t produced. When lactose is present, it’s converted to allolactose, which binds the repressor and changes its shape, causing it to release from the operator. With the operator no longer blocked, RNA polymerase can bind the promoter and transcribe the lac genes. Lactose acts as the inducer, not the binder to the operator; the promoter is just the DNA site where transcription begins; RNA polymerase is the enzyme that carries out transcription.