The lac operon consists of one regulatory gene (the i gene) and three structural genes , and ). The i gene codes for the repressor of the lac operon. The gene codes for beta-galactosidase (b-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. The gene codes for permease, which increases permeability of the cell to b-galactosides. The a gene encodes a transacetylase. Hence, all the three gene products in lac operon are required for metabolism of lactose.

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