Define an operon, giving an example, explain an inducible operon.

Explanation

The concept of operon was first proposed in 1961, by Jacob and Monad. An operon is a unit of prokaryotic gene expression which indludes coordinately gene product.
Components of an Operon
(i) Structurel of an gene The fragment of DNA which transcribe mRNA for polypeptide synthesis.
(ii) Promoter The sequence of DNA where RNA polymerase binds and initiates transcriptin of structural genes is called promoter.
(iii) Operator The sequence of DNA adjacent to promoter where specific repressor protein binds is called operator.
(iv) Regulator gene The gene that codes for the repessor protein that binds to the operator and suppresses its acitvity as a result of which transcription will be switched off .
(v) Inducer The substrate 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 substrante, etc.
Inducible Operon System
An inducible operon system is a regulated unit of ganetic material which is switched on in response ot the presence of a chemical.e.g., the lactose or lac-operon of E.coli.
The lactose operon The lac z,y,a genes are transcribed from a lac transcription unit uner the control of a single promoter. They encode enzyme required for the use of lactose as a carbon source. The lac i gene product, the lac repressor, is expressed from a separate transcription unit upstream from the operator .
lac operon consists of three structural genes (z,y and a ),operator and a separate regulatory gene.
The three structural genes (a,y and a) transcribe a polycistronic mRNA.

Gene z codes for `bets`-galactossidase(`beta-gal`) enzyme which breaks lactose into galactose and glucose.
Gene y codes for permease, which incrreases the permeability of the cell to lactose .
Gene a codes for enzyme transacetyiase, which catalyses the transacetylation of lactose in
When Lactose is Absent
(i) When lactose is absent, i gene regulates and produces repressor mRNA which translate repression .
(ii) The operon is switched off.
when Lactose is Present
(i) Lactose acts as an inducer which binds to the repressor and an inactive repressor.
(ii)The repressor fails to bind to the oparator region .
(iii)The RNA polymerase binds to the operator and transcript lac mRNA.
(iv) lac mRNA is polycistronic,i.e., produces all three enzymes , `beta`-galactosidase, permease and transscatylase.
(v) The lac operon is switched on .