Question

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. in the presence of glucose, E. coli cells do not utilize lactose
• B. they cannot transport lactose from the medium into the cell
• C. the lac operon is constitutively active in these cells
• D. they cannot synthesize functional $\beta$ - galactosidase

Answer 1

Answer: (D)

they cannot synthesize functional $\beta$ - galactosidase

Answer 2

Lac operon is a cluster of genes encoding three proteins that bacteria use to obtain energy from the sugar lactose. There are three structural genes in lac operon. Promoter for lac $i$ (i) Lac $Z \,(3063\, bp)$: This gene codes for the enzyme $\beta$ galactosidase which breaks lactose into glucose and galactose to be utilized in the cell. Therefore, $E$.coli cells with a mutated $Z$ genes of the lac operon can not grow in medium containing only lactose as the source of energy because they cannot synthesize functional $\beta$-galactosidase. (ii) Lac $Y (800 \,bp)$ : It codes for $\beta$ galactose permease enzyme which is a membrane bound protein and helps in transport of metabolites. (iii) Lac $\, (800 bp)$ : It codes for $\beta$ galactose transacetylase an enzyme that transfers an acetyl group from acetyl Co-A to $\beta$-galactosidase.