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Induction of the Lac Operon


Illustration of the method by which the lac operon is transcribed.

Gene transcription can be switched on and off by gene regulation proteins. The lac operon in E.Coli is an example of that dual control. Glucose and lactose levels control the initiation of transcription of the lac operon, i.e. whether the lac operon is switched "ON" or "OFF".

  • In an E. Coli cell growing in the absence of lactose, a repressor protein binds to the operator, preventing RNA polymerase from transcribing the lac operon's genes. The operon is OFF.
  • When the inducer, lactose, is added, it binds to the repressor and changes the repressor's shape so as to eliminate binding to the operator. As long as the operator remains free of the repressor, RNA polymerase that recognizes the promoter can transcribe the operon's structural genes into mRNA. The operon is ON.

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