Question

Which of the following is required as inducer(s) for the expression of $lac$ operon?

• A. Lactose
• B. Galactose
• C. Glucose
• D. Lactose and galactose

Answer 1

The correct option is A Lactose

The lac operon
The lac operon is responsible for the transport and metabolism of lactose in bacteria like E. coli.
It comprises the regulatory gene (lac i) having its own promoter and three structural genes lac a, lac y, and lac z controlled by a promoter and operator.
Role of lactose in lac operon
Lactose is the inducer for lac operon. When lactose enters the bacterial cell, its rearranged form i.e., the allolactose binds with the lac repressor and inactivates it.
The lactose or allolactose acts as an inducer as the promoter region is available for RNA polymerase to bind to. Hence, it can progress downstream and allow the transcription of structural genes.
Role of glucose in lac operon
Presence of glucose in the medium leads to its utilisation by E. coli first. This keeps the lac operon inactivated till bacteria switch to lactose for energy source.
Significance of galactose
β-galactosidase is an enzyme coded by the gene lac z which hydrolyses lactose into glucose and galactose.
Final answer
(A) Lactose