In case of lactose presence,
(i) Lactose acts as an inducer which binds to the repressor and forms an inactive repressor.
(ii) The repressor fails to bind to the operator region.
(iii) The RNA polymerase binds to the operator and transcript lac mRNA.
(iv) Lac mRNA is polycistronic, i.e., produces all three enzymes, -galactosidase, permease and transacetylase.
(v) The lac operon is switched on.

In case of lactose absence,
(i) When lactose is absent, gene regulates and produces repressor mRNA.
(ii) The repressor protein binds to the operator region of the operon and as a result prevents RNA polymerase to bind to the operon.
(iii) The operon is switched off.

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