The operon model is proposed by F. Jacob and J. Monod in 1961. This model explain the genetic basis of enzyme induction and repression. The structural gene, operator gene and promoter gene are collectively called operon. Operator gene controls the activity of structural gene taking part in the synthesis of proteins and the activity of operator gene is under the control of regulator gene which synthesizes represser molecules, which get attached to operator gene and thus checking mRNA synthesis (ie, transcription is checked) and hence, no protein synthesis occurs.