Operon model and the lac operon

23/06/2020 Vinod 0 Comments

Operon :-

Operon consists of a coordinated group of genes which are transcribed togather and regulate a metabolic pathway as a unit.

  • Operon model were first proposed by F. Jacob and J. Monod in 1961
  • Each operon has its specific repressor.

Example – lac operator is present only in lac operon and it interacts specifically with lac repressor only.

The lac operon :-

The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli (E. Coli) and many other interic bacteria.

fig. The lac operon.
  • Lac operon consists of following genes – 1. Regulatory gene :- A regulatory gene is a gene involved in controllig the expression of one or more other genes. Regulatory sequences, which encode regulatory genes, are often at the five prime end (5′) to the start site of transcription of the gene they regulate.
  • The lac operon consists of one regulatory gene (i gene -here ‘ i ‘ is inhibitor). The i gene codes for the repressor of the lac operon, repressor binds to the operator region and prevent RNA polymerase from transcribing the operon.

2. Promotor gene :- A promotor gene is a regulatory region of DNA located towards the 5′ region.

They starts transcription of structural gene and control the formation of mRNA.

3. Operator gene :- it is located towards the first structural gene.

  • It helps in initiation process of transcription with regulator molecule.

Or inhibits the process of transcription with the help of repressor molecule.

4. Structural genes :- there are of three types – A. Z lac gene :- The z lac gene codes for beta- galactosidase, which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose.

B. Y lac gene :- The y lac gene codes for permease, which increases permeability of the cell of beta galactosidases.

C. a lac gene :- The a lac gene encodes to transacetylase.

Hence, all the three gene procucts in lac operon are required for metabolism of lactose.

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