Jacob and Monod in 1961 first provided with a model of Lac operon. An operon is defined as several 4 genes situated in tandem, all controlled by a common structural gene having repressor, promoter and operator. The message produced by an operon is polycistronic because the information of all structural genes resides on a single mRNA molecule. The lac operon (lac refers to lactose) consists of one regulatory gene (i gene) and three structural genes (Z, Y and A).
The i gene codes for repressor of the operon. The Z genes codes for beta galactosidase which hydrolyses lactose into galactose and glucose. They Y gene codes for permease which increase permeability of cells to beta-galactosidase. A gene codes for transacetylase whose function is not definitely known. This shows that three genes are required for metabolism of lactose.
The above fact shows that presence of lactose switches on the operon for synthesis of enzyme beta - galactosidase inside cells. It therefore acts as inducer. The permease facilitate entry of lactose inside cell. The i gene synthesises repressor. The repressor protein binds with the operator to prevent transcribing operon by RNA polymerase. The repressor is then inactivated by the inducer (such as lactose). This in turn allows RNA polymerase access to promoter and then transcription starts.