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E. coli cells with a mutated z gene of the lac operon cannot grow in medium containing only lactose as the source of energy because -
A
They cannot synthesize functional beta galactosidase
B
They cannot transport lactose from the medium into the cell
C
The lac operon is constitutively active in these cells
D
In the presence of glucose, E. coli cells do not utilize lactose
Detailed Solution
The lac operon has three structural genes: z, y and a.
The z gene codes for $\beta$-galactosidase, which hydrolyses lactose into glucose and galactose.
The y gene codes for permease, which increases the permeability of the cell to lactose, and the a gene codes for transacetylase.
If the z gene is mutated, the cells cannot synthesise a functional $\beta$-galactosidase.
Even if lactose enters the cell (permease is still made by the y gene), it cannot be broken down into glucose and galactose and cannot be used as a source of energy.
So the cells cannot grow on lactose alone because they cannot synthesise functional beta galactosidase.
The z gene codes for $\beta$-galactosidase, which hydrolyses lactose into glucose and galactose.
The y gene codes for permease, which increases the permeability of the cell to lactose, and the a gene codes for transacetylase.
If the z gene is mutated, the cells cannot synthesise a functional $\beta$-galactosidase.
Even if lactose enters the cell (permease is still made by the y gene), it cannot be broken down into glucose and galactose and cannot be used as a source of energy.
So the cells cannot grow on lactose alone because they cannot synthesise functional beta galactosidase.