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Lac-operon Concept

The concept of Operon was first proposed in 1961, by Jacob and Monod...

The lac Operon

The concept of Operon was first proposed in 1961, by Jacob and Monod. An Operon is a unit of prokaryotic gene expression which includes coordinately regulated (structural) genes and control elements which are recognised by regulatory gene product.

Concept of an Operon
  1. Structural gene: The fragment of DNA which transcribe mRNA for polypeptide synthesis.

  2. Promoter: The sequence of DNA where RNA polymerase binds, and initiates transcription of structural genes is called promoter.

  3. Operator: The sequence of DNA adjacent to the promoter where specific repressor protein binds is called operator.

  4. Regulator genes: The gene that codes for the repressor protein that binds to the operator and suppresses its activity as a result of which transcription will be switched off.

  5. Inducer: The substrate that prevents the repressor from binding to the operator, is called an inducer. As a result, transcription is switched on. It is a chemical of diverse nature like metabolite, hormone substrate etc...

The concept of lactose Operon (lac Operon)
  • It consists of regulatory genes, and structural genes lac zya.

  • Control genes consists of promoter. Between promoter and structural genes is the operator.

  • Regulatory gene lac I codes for repressor, which binds to operator and switches off of lac-Operon.

  • When glucose not available, lactose available Lactose enters E.coli and binds inducer and forms inducer repressor complex that frees the operator.

  • RNA polymerase will transcribe the structural gene produce polycistronic mRNA now lac Operon is switched


  • Polycistronic mRNA is translated into three enzymes;

    beta galactosidase - by lac Z,

    betagalactosidepermease - by lac Y, and

    betagalactosidetransacetylase - by lac A

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