Question

$Escherichia$ $coli$ with mutated $z$ gene of $lac$ operon cannot grow in medium containing only lactose as the source of energy because

• A. In the presence of glucose, Escherichia coli cannot utilise lactose
• B. They cannot synthesise functional beta-galactosidase
• C. The bacterium cannot transport lactose from the medium into the cell
• D. $lac$ operon is constitutively active in these cells

Answer 1

The correct option is B They cannot synthesise functional beta-galactosidase

$lac$ operon
The $lac$ operon is an inducible operon system that remains usually in a repressed condition and gets activated only when the substance to be metabolised (lactose) is present.
Statement 1
An operon is a coordinated group of genes that function together to regulate gene expression. $lac$ operon is responsible for the transport and metabolism of lactose in bacteria like $E$. coli. It has three structural genes ($lac$ $z$, $lac$ $y$ and $lac$ $a$) and a regulator gene ($lac$ $i$ gene).
The lac operon is not constitutively active in these cells but gets switched off and on depending on the presence of inducers and repressors.
Statement 2
The $lac$ $z$ gene in the $lac$ operon is a structural gene that codes for the enzyme beta-galactosidase.
This enzyme is responsible for the hydrolysis of lactose to produce glucose and galactose.
Therefore, if the $lac$ $z$ gene is mutated, the bacteria present in the lactose-only medium cannot survive.
Statement 3
E. coli prefers to grow in glucose. The lac operon is switched off in presence of only glucose in the medium because of some catabolic activities. These catabolic activities are not governed by the $z$ gene.
Statement 4
Transport of lactose from the medium into the cell is controlled by the permease enzyme encoded by the $lac$ $y$ gene of $lac$ operon. Hence, inability to transport lactose from the medium into the bacterial cell is not the result of mutated $z$ gene.
Final answer
(B) They cannot synthesise functional beta-galactosidase