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Answer the Following Question. Write the Different Components of a Lac-operon in E. Coli. Explain Its Expression While in an ’Open’ State. - Biology

Answer in Brief

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

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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).



  • 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.
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