Observe the given picture and answer the following questions ?

Which among the following is  the substrate for the enzyme beta-galactosidase in lac operon ?

Lactose 

The correct answer is Option (3) -Lactose

The lac operon consists of one regulatory gene (the i gene – here the term i does not refer to inducer, rather it is derived from the word inhibitor) and three structural genes (z, y, and a). 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 a gene encodes a transacetylase. Hence, all the three gene products in lac operon are required for metabolism of lactose.Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon.