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Regulation of Gene Expression - The Lac Operon

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CBSE: Class 12
CISCE: Class 12

Key Points: The Lac Operon

  • The operon model, proposed by Jacob and Monod (1961), explains coordinated regulation of gene expression in prokaryotes at the transcriptional level.
  • An operon consists of a promoter, operator, and a group of structural genes that are regulated together and transcribed as a single polycistronic mRNA.
  • In the lac operon of E. coli, three structural genes—z (β-galactosidase), y (permease), and a (transacetylase)—are involved in lactose metabolism.
  • The regulator gene (lac i) produces a repressor protein that binds to the operator and prevents transcription in the absence of lactose.
  • When lactose is present, it inactivates the repressor, allowing RNA polymerase to transcribe the structural genes (induction).
  • Catabolite repression occurs when glucose is present; lac operon expression is suppressed and later activated by CAP–cAMP when glucose is depleted.

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