Explain how structural genes of lac operon start transcription on addition of lactose.

When lactose is added in the culture medium of E.coli, Lactose enters in E.coli. Lactose behaves as an inducer and repressor bind to lactose. When repressor bound to lactose, it undergoes conformational change due to which it's a binding site to operator undergo change and repressor leave operator. Now the operator is free. RNA polymerase moves over structural genes and transcribed polycistronic mRNA which translates three enzymes :

1) Beta-galactosidase from gene Z.

2) Permease from gene Y.

3) Transacetylase from A.

• Beta-galactosidase convert lactose into glucose and galactose which are carbon and energy source.

• Permease makes E.coli permeable for lactose.

• Transacetylase takes part in other reaction.

The condition of the operon in which operator is free is switch on condition.