The lac operon consists of one regulatory gene (the gene) and three structural genes ( , and a). The gene codes for the repressor of the lac operon. The z gene codes for -galactosidase, 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 -galactosides.

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