If glucose is absent but lactose is also absent, what happens to transcription of the lac operon?
ATranscription switches fully on, because CAP alone is enough once glucose is absent
BTranscription remains off, because the repressor still binds the operator when lactose is absent
CTranscription runs at half strength, since only one of the two conditions is met
DTranscription becomes permanently locked on regardless of lactose levels
Answer & Solution
Correct answer: B. Transcription remains off, because the repressor still binds the operator when lactose is absent
1. If glucose is absent, CAP can bind the operator sequence to help activate transcription.
2. If lactose is absent, however, the repressor binds the operator to prevent transcription.
3. If either requirement is unmet, meaning glucose is present or lactose is absent, transcription remains off.
4. In this scenario lactose is absent even though glucose is also absent, so the repressor still blocks transcription.
5. CAP binding by itself is not sufficient, since the option claiming full activation from CAP alone ignores the repressor's block.
6. Transcription is described as either on or off depending on both conditions together, not running at a partial or half strength.
7. Transcription is not permanently locked on regardless of lactose, since the repressor's binding is explicitly dependent on lactose being absent.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 16 "Gene Expression", section The lac Operon: An Inducer Operon_
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