Operons are segments of genetic material which function as regulated units that can be switched on and switched off. Operon model was given by Jacob and Monod. Lac operon system is an example of inducible operon system i.e., it regulated genetic material which remains switched off normally but becomes operational in the presence of an inducer i.e., lactose. In lac operon system gene lac-1 codes for represser, lac-z for β - galactosidase , lac-y for permease and lac-a for transacetylase.