: Operon is a group of genes that are clustered m the chromosome and can be controlled in a coordinated way. These are the regulated unit of genetic material. An operon contains operator gene, regulator gene, represser, structural genes and promoter gene. In operon concept, the operator gene combines with represser protein produced by the regulator gene to switch off structural gene transcription. Regulator gene functions as represser and controls chemical synthesis by inhibiting transcription of mRNA. In E. coli two types of operons are recognized, the lac operon and tryptophan operon.