Question

Why does the lac operon shut down some time after the addition of lactose in the medium where E.coli was growing? Why low level expression of lac operon is always required?

Answer 1

• The Lactose operon is negatively regulated.
• When beta-galactosidase is synthesized enough, it breaks down Lactose into glucose and galactose.
  
• Lactose is removed from the repressor and the repressor binds to the operator inhibiting the lactose structural gene expression.
  
• So, the lactose operon is turned off.
  

• Low level of lactose operon expression allows synthesis of Permease enzyme from the structural genes to allow lactose to enter the cell when it is present in the medium.