The lac operon (Inducible operon) Operon : - The concept of operon was first proposed in 1961, by Jacob and Monod Components of an operon : - (i) Structural genes : - The fragment of DNA which transcribe mRNA for polypeptide synthesis.
(ii) Promoter : - The sequence of DNA where RNA polymerase binds and initiates transcription.
(iii) Operator : - The sequence of DNA adjacent to promoter is called OPERATER.
(iv) Regulator gene : - It is the gene that codes for repressor protein which binds to operator due to which operon is switched ‘‘off’’
(v) Inducer : - Lactose is inducer which helps in switching ‘‘on’’ of operon. Lac operon consists of there structural genes (z, y, a), operator (o), promoter (p), regulatory gene (i)
When lactose is absent:- When lactose is absent, i.e. gene produces repressor protein. This repressor protein binds to operator and as a result prevents RNA polymerase to bind to operon. The operon is switched off. When lactose is present : -
• Lactose act as inducer which binds to the repressor and forms inactive repressor.
• The repressor cannot bind to operator.
• Now the RNA polymerase binds to operator and transcribes lac mRNA.
• Lac mRNA is polycistronic i.e. produces all three enzymes β−galactosidase, permease and trans-acetylase.
• The lac operon is switched on.