(b) Mention what would happen if lactose is withdrawn from the culture medium.

Explanation

### Step-by-Step Solution:

1. **Understanding the Lac Operon**: The lac operon is a cluster of genes responsible for the metabolism of lactose in E. coli. It consists of structural genes (Z, Y, A), a promoter, and an operator.

2. **Role of Lactose**: Lactose acts as an inducer for the lac operon. In the presence of lactose, it binds to the repressor protein, causing a conformational change that prevents the repressor from binding to the operator.

3. **Condition When Lactose is Present**: When lactose is present in the culture medium, it binds to the repressor protein. This allows RNA polymerase to bind to the promoter and initiate transcription of the structural genes (Z, Y, A), leading to the production of enzymes necessary for lactose metabolism.

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