NCERT · Class 12 · Biology · Molecular Basis of InheritanceDescribe the concept of the Lac Operon and explain its regulation in the presence and absence of an inducer (lactose).

Introduction to Lac Operon\nThe lac operon is a genetic unit found in bacteria like Escherichia coli, regulating the metabolism of lactose. It was discovered by François Jacob and Jacques Monod. An operon consists of a promoter, operator, and structural genes that function together under the control of a single promoter.

Components of Lac Operon

• Regulatory Gene ($i$ gene): Codes for the repressor protein that binds to the operator region.
• Promoter Gene ($p$ gene): The site where RNA polymerase binds to initiate transcription of structural genes.
• Operator Gene ($o$ gene): Acts as a switch; it controls the transcription of structural genes by interacting with the repressor protein.
• Structural Genes:
  • $z$ gene: Codes for $\beta$-galactosidase, which hydrolyzes lactose into glucose and galactose.
  • $y$ gene: Codes for permease, which increases the permeability of the cell to $\beta$-galactosides.
  • $a$ gene: Codes for transacetylase, transferring an acetyl group to $\beta$-galactoside.

Regulation Mechanism

• In the Absence of Inducer (Lac Absent / Switched Off):

  • The regulatory gene ($i$) continuously transcribes mRNA to synthesize the repressor protein.
  • This active repressor protein binds tightly to the operator region ($o$) of the operon.
  • By binding to the operator, it blocks the movement of RNA polymerase along the DNA template.
  • Consequently, transcription of structural genes ($z, y, a$) does not occur, and lactose-metabolizing enzymes are not produced.

• In the Presence of Inducer (Lac Present / Switched On):

  • When lactose (or its isomer, allolactose) enters the bacterial cell, it acts as an inducer.
  • The inducer binds to the repressor protein, causing a conformational change that inactivates the repressor.
  • The inactive repressor can no longer bind to the operator region.
  • RNA polymerase is now free to bind to the promoter and moves down to transcribe the structural genes ($z, y, a$).
  • Enzymes are synthesized, allowing the cell to metabolize lactose efficiently.

Significance\nThis operon is a classic example of negative gene regulation and substrate induction, demonstrating how prokaryotes efficiently conserve energy by expressing genes only when their products are needed.