Observe the given picture and answer the following questions.
Which of the following is the substrate for the enzyme beta-galactosidase in the lac operon?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Lactose
The z gene of the lac operon codes for beta-galactosidase. This enzyme hydrolyses the disaccharide lactose into its monomers, galactose and glucose. The molecule the enzyme acts on is its substrate, so the substrate is lactose. The same lactose also switches the operon on, which is why it is called the inducer.
Option (1) Glucose — Incorrect. Glucose is one of the two products of lactose hydrolysis. It is formed by the enzyme, not acted upon by it. Glucose cannot act as an inducer of the lac operon either.
Option (2) Galactose — Incorrect. Galactose is the other product of the reaction. Like glucose, it is released when lactose is split and cannot induce the operon.
Option (3) Lactose — Correct. Lactose is the disaccharide that beta-galactosidase breaks down. It is the substrate of the enzyme and also regulates switching on and off of the operon.
NCERT Extract:
The Lac Operon – “… The i gene codes for the repressor of the lac operon. The z gene codes for beta-galactosidase (β-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. The y gene codes for permease, which increases permeability of the cell to β-galactosides.”
The Lac Operon – “Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon. Hence, it is termed as inducer.”