Question

Answer the following question.
Write the different components of a lac-operon in E. coli. Explain its expression while in an ’open’ state.

Answer 1

The lac operon (Inducible operon)Operon : 
The concept of the operon was first proposed in 1961, by Jacob and Monod

Components of an operon : 

1. Structural genes:
   The fragment of DNA which transcribe mRNA for polypeptide synthesis.
2. Promoter:
   The sequence of DNA where RNA polymerase binds and initiates transcription.
3. Operator:
   The sequence of DNA adjacent to the promoter is called OPERATOR.
4. Regulator gene:
   It is the gene that codes for repressor protein which binds to the operator due to which operon is switched ‘‘off’’
5. Inducer:
   Lactose is an inducer that helps in switching ‘‘on’’ of the operon. Lac operon consists of there structural genes (z, y, a), operator (o), promoter (p), regulatory gene (i).

a)

b)

• Gene z codes for β - galactosidase
• Gene y codes for permease.
• Gene codes for enzymes transacetylase.

When lactose is absent:-
When lactose is absent, i.e. the gene produces repressor protein.
This repressor protein binds to the 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 the operator.
• Now the RNA polymerase binds to the operator and transcribes lac mRNA.
• Lac mRNA is polycistronic i.e. produces all three enzymes β −galactosidase, permease, and transacetylase.
• The lac operon is switched on.