in Molecular Basis of Inheritance by

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  • Operon is a unit of genetic material functions as a single transcription unit.
  • It contains cluster of genes which are under control of a single regulatory signal or promoter.
  • It comprises: Operator, Promoter and one or more structural genes which are transcribed together into a polycistronic mRNA.
  • The operon model first introduced by Jacob, Monod and Wollman during their genetic and biochemical studies on lactose requiring mutations on E-coli.
  • There are two types of operon: (i) anabolic operon and (ii) catabolic operon
  1. Anabolic Operon are those who direct the synthesis of enzymes required for the Anabolic process.
  2. Catabolic Operon are those who direct the synthesis of enzymes required for the Catabolic process.
  • Lac operon is the best example of the Catabolic operon as it synthesize the enzymes for the catabolism of lactose.
    •  Lac operon consists of: a regulatory gene, an operator, a promoter and structural Genes
    • The regulatory gene codes for a regulatory protein e.g.: The lac repressor, encoded by the lacI gene.
    • The operator is the region of DNA of the operon that is the binding site for the regulatory protein.
    • The promoter is the DNA sequence of the operon recognized by DNA-dependent RNA polymerase. The lac operon encodes enzymes necessary for lactose metabolism, including β-galactosidase, β-galactoside permease, and β-galactoside transacetylase.