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) The lac operon is constitutively active in these cells.
(B) They cannot synthesise functional beta-galactosidase.
(C) In the presence of glucose, E.coli cells do not utilize lactose.
(D) They cannot transport lactose from the medium into the cell.

Answer 1

Hint: For the transport of lactose in E.coli and many other enteric bacteria, the lactose operon is a crucial operon. Genes encoding for functionally related proteins are grouped along the DNA in a genetic regulatory system found in bacteria and their viruses. This characteristic enables coordinated control of protein production in response to cellular requirements.

Complete Step by Step Answer:
LacZ, LacY, and LacA are the three genes that make up the lac operon. Under the direction of a single promoter, these genes are translated into a single mRNA. Proteins that help the cell in using lactose are specified by genes present in the lac operon. Three genes that code for proteins involved in lactose metabolism can be found in the lac operon. The terms lac z, lac y, and lac an are used to describe them. Beta-galactosidase is produced by the lac z gene, a permease by the lac y gene, and the transacetylase enzyme by the lac a gene.
The beta-galactosidase (beta-gal) enzyme, which is produced by the z gene, is largely in charge of hydrolysing the lactose disaccharide into its monomeric components, galactose and glucose. Because they are unable to produce functional galactosidase, E.coli with mutant z genes of the lac operon cannot grow in media containing solely lactose as a source of energy.
So, option (B) is the correct answer.

Note: All the gene products of lac operon that is all the proteins synthesised by the genes present on it are involved in lactose metabolism. Lactose or allolactose to be more specific is the inducer of lac operon.