Class 12th Biology 2017 Set1 Outside Delhi Board Paper Solution
Write the different components of a lac-operon in E. coli. Explain its expression while in an ’open’ state.
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)
• Gene z codes for β-galactosidase
• Gene y codes for permease.
• Gene a codes for enzymes transacetylase.
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.
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