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