When lactose is absent, transcription does not occur because the ________ binds to the ________ and prevents ________ from binding to the promoter.

When lactose is absent, transcription does not occur because the ________ binds to the ________ and prevents ________ from binding to the promoter.

• A. Transcription; promoter; RNA polymerase
• B. Translation; promoter; RNA polymerase
• C. Replication; activator; RNA polymerase
• D. Transcription; repressor; RNA polymerase

Answer: (D)

The situation is a classic example of negative regulation of transcription. In the lac operon, when lactose is absent, the Lac repressor is active and binds to the operator region on the DNA. This binding physically blocks RNA polymerase from accessing the promoter, so transcription cannot start. The best way to describe this is that transcription is prevented because the repressor binds to the operator and prevents RNA polymerase from binding to the promoter. This captures why transcription stops without lactose, and why the highlighted combination matches the mechanism.

The situation is a classic example of negative regulation of transcription. In the lac operon, when lactose is absent, the Lac repressor is active and binds to the operator region on the DNA. This binding physically blocks RNA polymerase from accessing the promoter, so transcription cannot start. The best way to describe this is that transcription is prevented because the repressor binds to the operator and prevents RNA polymerase from binding to the promoter. This captures why transcription stops without lactose, and why the highlighted combination matches the mechanism.