Question

E.coli cells with a mutated Z gene of the lac operon cannot grow in a medium containing only lactose as the source of energy because

• A. Lactose cannot be transported from the medium into the cell.
• B. The lac operon cannot function at all without this gene.
• C. They cannot synthesise β-galactosidase.
• D. The acetyl group from acetyl CoA cannot be transferred to lactose.

Answer 1

The correct option is C They cannot synthesise β-galactosidase.

The structural gene lacZ codes for the enzyme β-galactosidase, which breaks down lactose into glucose and galactose. Hence, this cannot happen if the gene is mutated. The gene lacA codes for β-galactoside transacetylase, which transfers an acetyl group from acetyl CoA to lactose. The gene lacY codes for the enzyme lactose permease, which helps the lactose molecules enter the bacterial cells from the surrounding medium.