3: The lac operon

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An operon is a length of DNA, made up of structural genes and control sites. The structural genes code for proteins. The control sites are the operator and promoter region, which are both genes.

  • E. coli normally respires glucose but can also use lactose as a respiratory substrate. 
  • At first, E.coli can't metabolise the lactose as it only has small amounts of the enzymes beta-galactosidase (which catalyses the hydrolysis of lactose to glucose and galactose) and lactose permease (which transports lactose into the cell). 
  • A few minutes after lactose is introduced into the medium surrounding the E.coli, the bacteria increases it's rate of synthesis of these two enzymes.
  • Lactose triggers the production of the enzymes, and is called an inducer. 

The lac operon

  • The regulator gene (I): This is not part of the operon and is some distance away from it. It is important as it produces the repressor protein.
  • The control sites (P and O): P is the promoter region. It's a length of DNA to which the enzyme RNA polymerase binds to in order to begin the transcription of the structural genes.
  • The structural genes (Z and Y): Z codes for beta galactosidase.


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