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,
(v) The lac operon is switched
In case of lactose absence
(i) When lactose is absent, i
gene regulates and produces repressor mRNA which translate repression.
(ii) The repressor protein binds
to the operator region of the operon and as a resultprevents RNA polymerase to
bind to the operon.
(iii) The operon is switched
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