Question

Sequence of structural genes of $lac$ operon is

• A. a, y, z
• B. y, z, a
• C. a, z, y
• D. z, y, a

Answer 1

The correct option is D z, y, a

The $lac$ 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.
Structural genes
A lac operon has three structural genes (lac z, lac y and lac a). The structural genes code for specific enzymes: lac z codes β-galactosidase, lac y codes permease and lac a codes transacetylase.
These enzymes perform specific functions:
$\bullet$β-galactosidase: Breaks lactose into glucose and galactose.
$\bullet$Permease: Helps in the entry of lactose from outside into the cell.
$\bullet$Transacetylase: Transfers the acetyl group from acetyl CoA to β-galactosidase.
Final answer
(B) z, y, a