When lactose is present, RNA polymerase binds to the promoter to transcribe the enzymes needed to digest lactose.

• A. RNA polymerase; promoter
• B. Repressor; operator
• C. tRNA; ribosome
• D. Helicase; promoter

Answer: (A)

Transcription in bacteria starts when RNA polymerase binds to a promoter, the DNA sequence that signals where transcription should begin. In the lac operon, lactose presence leads to inactivation of the repressor that sits on the operator; with the repressor out of the way, RNA polymerase can access the promoter and start transcribing the genes needed to digest lactose. The promoter is the key site for initiating transcription, so RNA polymerase binding there is what turns on the production of the lactose-digesting enzymes.

The repressor–operator pair describes repression, which would block transcription rather than enable it when lactose is present. tRNA and ribosome are involved in translation, not transcription, and helicase functions relate to DNA unwinding in replication, not initiation of transcription.