In the lac operon, the repressor inhibits transcription when
a. The repressor is bound to the inducer.
b. The repressor is not bound to the inducer.
c. The repressor is bound to glucose.
d. The repressor is not bound to the operator.

Verified step by step guidance

1. Understand the role of the lac operon: It is a set of genes involved in lactose metabolism in E. coli, regulated by the presence or absence of lactose.
2. Identify the components of the lac operon: The operon includes the promoter, operator, and structural genes (lacZ, lacY, and lacA). The repressor protein is encoded by the lacI gene.
3. Recognize the function of the repressor: The repressor binds to the operator region to block RNA polymerase from transcribing the structural genes when lactose is absent.
4. Determine the effect of the inducer: The inducer (allolactose, a derivative of lactose) binds to the repressor, causing a conformational change that prevents the repressor from binding to the operator.
5. Analyze the given options: The repressor inhibits transcription when it is bound to the operator, which occurs when the inducer is not present. Therefore, the correct condition is when the repressor is not bound to the inducer.

Key Concepts

Lac Operon

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It includes structural genes, a promoter, an operator, and a repressor. The operon is a classic example of gene regulation, where the presence or absence of lactose influences the transcription of these genes.

Repressor Protein

The repressor protein in the lac operon binds to the operator region to inhibit transcription. When lactose is absent, the repressor binds to the operator, preventing RNA polymerase from transcribing the genes. When lactose is present, it binds to the repressor, causing a conformational change that releases the operator, allowing transcription.

Inducer

An inducer is a molecule that initiates gene expression. In the lac operon, allolactose acts as an inducer by binding to the repressor protein, altering its shape, and preventing it from binding to the operator. This allows RNA polymerase to access the promoter and transcribe the genes necessary for lactose metabolism.