question_answer

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

Correct Answer: D

Solution :

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. (i) Lac Z (3063 bp): This gene cods 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 cannot 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 A (800 bp): It codes for \[\beta \] galactose transacetylase an enzyme that transfers an acetyl group from acetyl Co-A to \[\beta \]- galactosidase.