The concept of operon was first
proposed in 1961, by Jacob and Monad. An operon is a unit of prokaryotic gene
expression which includes coordinately regulated (structural)genes and control
elements which are recognized by regulatory gene product.
of art Operon
(i) Structural gene The fragment
of DNA which transcribe mRNA for polypeptide synthesis.
(ii) Promoter The
sequence of DNA where RNA polymerase binds and initiates transcription of
structural genes is called promoter.
(iii) Operator The
sequence of DNA adjacentto promoter where specific repressor protein binds is
that codes for the repressor protein that bind to the operator and suppresses
its activity as a result of which transcription will be switched off.
that prevents the repressor from binding to the operator, is called an inducer.
As a result transcription is switched on. It is a chemical of diverse nature like
metabolite, hormone substrate, etc.
Inducible Operon System
An inducible operon system is a
regulated unit of genetic material which is switched on in response to the
presence of a chemical, e.g., the lactose or lac-operon of E. coli.
The lactose operon Thelac
z, y, a genes aretranscribed from a lac transcription unit under the control of
a single promoter. They encodeenzyme required for the use of lactose as a carbon
source. The laci gene product, the lacrepressor, is expressed from a
separate transcription unit upstream from the operator.lac operon consists of
three structural genes(z, y and a), operator, promoter and aseparate regulatory
The three structural genes (a, y
and a) transcribe a polycistronic mRNA.
Gene z codes for (3-galactosidase (-gal) enzyme
which breaks lactose into galactose and glucose.
Gene y codes for permease, which
increases the permeability of the cell to lactose.
Gene a codes for enzyme
transacetylase, which catalyses the transacetylation of lactose in its active
When Lactose is Absent
(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 result prevents RNA polymerase to
bind to the operon.
(iii) The operon is switched
When Lactose is Present
(i) Lactose acts as an inducer
which binds to the repressor and forms an inactiverepressor.
(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, b-galactosidase,
(v) The lac operon is switched
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