NCERT · Class 12 · Biology · Molecular Basis of InheritanceWhat is the lac operon? Explain its structure, genes involved, and its regulation in the presence and absence of an inducer (lactose).

Introduction to Lac Operon\nThe lac operon is a classic model of gene regulation proposed by François Jacob and Jacques Monod in 1961 in bacteria like Escherichia coli. An operon is a cluster of genes with a single promoter that are transcribed together, allowing coordinated regulation of metabolic pathways, specifically lactose metabolism.

Structure and Components of Lac Operon\nThe lac operon consists of the following structural and regulatory elements:

• Regulator Gene (i gene): Codes for the repressor protein, which binds to the operator region and inhibits transcription.
• Promoter Gene (p gene): The binding site for RNA polymerase to initiate transcription of the structural genes.
• Operator Gene (o gene): Serves as the binding site for the lac repressor protein; located adjacent to the promoter.
• Structural Genes:
  • lacZ gene: Encodes the enzyme beta-galactosidase ($\beta$-galactosidase), which hydrolyzes lactose into glucose and galactose.
  • lacY gene: Encodes the enzyme permease, which increases the permeability of the cell membrane to lactose.
  • lacA gene: Encodes transacetylase, an enzyme that transfers an acetyl group to beta-galactosides.

Regulation Mechanism

• Regulation in the Absence of Inducer (Lac Off): When lactose is absent from the medium, the i gene actively synthesizes the repressor protein. This repressor protein binds tightly to the operator gene (o), blocking RNA polymerase from moving down the structural genes. Thus, transcription is turned off, and enzymes are not produced.
• Regulation in the Presence of Inducer (Lac On): When lactose (acting as an inducer) is added to the medium, it enters the cell and binds to the repressor protein. This causes a conformational change in the repressor, rendering it inactive and unable to bind to the operator region. Consequently, the operator is free, RNA polymerase binds to the promoter, and the structural genes (lacZ, lacY, lacA) are transcribed to produce the necessary enzymes for lactose utilization.