lactose present/absent

• when lactose is absent 


  The regulatory gene is transcribed and translated to produce repressor protein

  the repressor protein binds to the operator

  RNA polyermase cannot bind to promoter

  transcription of genes does not take place

  no lactase enzyme produced
• when lactose present 


  The lactose binds to the second binding site of the repressor protein, distorting its shape so that it cannot bind to the operator

  RNA polymerase binds to the promoter region

  mRNA for 3 structural genes made

  lactase is produced and lactose can be broken down and used as energy