04 Molecular Basis of Inheritance - part 11 - Operon concept - Lac operon

04 Molecular Basis of Inheritance - part 11 - Operon concept - Lac operon


Operon concept : 
  • It is a transcriptional control mechanism of gene regulation. 
  • Francois Jacob and Jacques Monod (1961) explained that metabolic pathways are regulated as a unit.
  • For example in E.coli, when lactose sugar is provided to the culture medium, cell induces production of three enzymes necessary for digestion of lactose. 
  • The enzymes are :
  1. b-galactosidase : Digests lactose into galactose and glucose.
  2. b-galactoside permease : Permits lactose molecules to enter into the cell.
  3. Transacetylase (b-Galactoside acetyltransferease) : Transfers an acetyl group from acetyl CO-A to galactoside. 
  • Synthesis of these three enzymes, is controlled by a long segment of DNA known as Operon. 
  • It consists of an operator site O and three structural genes Z, Y and A .
  • The action of structural genes is regulated by operator site with the help of a repressor protein. 
  • Repressor protein is produced by the action of gene i (inhibitor) known as regulator gene. 
  • The gene expression depends on whether operator is switched on or switched off. 
  • If the operator is switched on, the three genes z, y and a are transcribed by RNA Polymerase into a single m-RNA. 
  • Each structural gene is generally known as cistron and the transcribed long m-RNA covering various cistrons is known as Polycistronic. 
  • Switching on or switching off of the operator is achieved (acomplished) by a protein called repressor. 
  • When this protein is attached to the operator and blocks it, the switch is turned off and structural genes are not expressed. 
Lac operon : 
  • Lactose or lac operon of E.coli is inducible operon. 
  • The operon is switched on when a chemical inducer- lactose is present in the medium.
  • Lac operon consists of following components : 
  1. Regulator gene (repressor gene) 
  2. Promoter gene 
  3. Operator gene 
  4. Structural genes 
  5. Inducer - It is not a component of operon. 
1. Regulator gene : 
  • This gene controls the operator gene in cooperation with an inducer present in the cytoplasm. 
  • Regulator gene preceeds the promoter gene. 
  • It may not be present immidiately adjacent to operator gene. 
  • Regulator gene produces a protein called repressor protein. 
  • Repressor binds with operator gene and represses (stops) its action. It is called regulator protein. 
2. Promoter gene : 
  • This gene preceeds the operator gene. 
  • It is present adjacent to operator gene. 
  • The promoter gene marks the site at which the RNA Polymerase enzyme binds. 
  • When the operator gene is turned on, the enzyme moves over the operator gene and transcription of structural genes starts. 
  • Promoter gene base sequence determines which strand of DNA acts a template. 
3. Operator gene : 
  • It preceeds the structural genes. 
  • This controls the functioning of structural genes. 
  • It lies adjacent to the Structural genes. 
  • When operator gene is turned on by an inducer, the Structural genes produce m-RNA. 
  • Operator gene is turned off by a product of repressor gene. 
4. Structural gene : 
  • When lactose is added to the E.coli culture, the structural genes catalyse (produce) m-RNA which in turn produces polypeptides, on the ribosomes. 
  • The polypeptides formed, act as enzymes to caltalyse lactose in the cell. 
  • There are 3 structural genes in the sequence lac-Z, lac-Y and lac-A. 
  • Enzymes produced are - 
  1. lac-Z- b-galactosidase
  2. lac-Y- b-galactoside permease and 
  3. lac-A - transacetylase 
5. Inducer : 
  • It is a chemical in the cytoplasm (allolactose) which inactivates the repressor. 
  • When lac operon is switched on, then inducer joins with repressor protein preventing the binding of repressor to the operator gene. 
  • So the Operator gene is free and now enzyme RNA polymerase can move from promoter to structural genes via operator gene.
Role of lactose : 
  • A few molecules of lactose enter into the cell by an enzyme permease.
  • A small amount of this enzyme is present even when operon is switched off. 
  • A few molecules of lactose, act as inducer and bind to repressor. This repressor ­ inducer complex fails to join with the operator gene, which is then turned on. 
  • Structural genes produce all enzymes. Thus, lactose acts as an inducer of its own break down. 
  • When the inducer level falls, the operator is blocked again by repressor. So structural genes are repressed/ inactivated again. This is negative feedback.

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