Jacob and Menol, in 1961 proposed operon model to explain the genetic control of enzymes in procaryotes. According to Operon Model, an operon (a piece of DNA molecule) contains a repressor gene, a promoter gene, an operator gene and structural genes. Structural genes are responsible for the formation of enzymes (proteins) the repressor gene transcribes a mRNA, which gets translated to form a repressor protein, in order to stop the synthesis of the enzymes. This repressor protein binds to the operator gene and block the transcription of structural genes. There by stopping the synthesis of the enzymes, to induce the synthesis of the enzymes, the substrate acts as an ‘inducer.’
The inducer combines with the repressor protein and form a repressor inducer complex which cannot bind to the operator gene. In this way, the blockage at the operator gene was removed and the structural gene again transcribe mRNA for the synthesis of enzymes. In this vein of that and more than describe the availability of the substrate and uses the synthesis of the enzymes, whereas, accumulation of the end product of a metabolic pathway repressor the enzymes synthesis. Example, of inducer operon is lac-operon in which lactose function as inducer and example of repressor operon is tryptophane operon in which tryptophane function as repressor.