Operon concept:-
1. The clusters of genes with related functions called as operon. OR The unit of genetic material that functions in a coordinated manner through operator, promoter & one or more than structural genes showing transcription is called as operon.
2. The concept was given by Jacob & Monad.

3. Components of operon:
(a) Structural genes:
1. Structural genes carry the information to produce enzymes. Therefore, they show transcription to produce m RNA. For e.g. the three structural genes of lac operon (z, y & a) produce a single polycistronic mRNA.

(b) Operator:
The sequence of DNA which is adjacent to promoter to which specific repressor protein attaches is called as operator.
The action of operator is under the regulation of repressor i.e. repressor is a switch.
(c) Promoter:
1. the sequence of DNA where RNA polymerase binds & starts transcription & controls the rate of synthesis of m-RNA.

(d) Regulator gene:
The protein repressor is coded by regulator gene.
When the repressor binds to the operator it is switched off terminating transcription. If the repressor is not allowed to bind to operator the switch is on & transcription continues. Due to these reasons, repressor is called as regulatory protein.
(e) Inducer or corepressor:
It binds to the repressor & by changing it configuration prevents the repressor from binding to the operator.

 

Lac operon.
1. If glucose is not available for cells they search for alternative source such as lactose.
If lactose is not available the repressor will attach to operator & block RNA polymerase.
Lactose acts as an inducer. If lactose is available it will remove the repressor from the operator, bind with repressor to form inducer- repressor complex for allowing synthesis of mRNA.
RNA polymerase will attach to promotor & may transcribe m-RNA if it is not blocked.
RNA polymerase first encounters 'lac z gene' responsible for producing galactosidase which hydrolyses or breaks the bond between glucose & galactose (Lactose is disaccharide).
RNA polymerase moves on to next gene i.e. lac Y which produces enzyme permease' responsible for carrying lactose inside the cell.

In the next step RNA polymerase moves onto lac gene which bring about synthesis of transacetylase.
Alpha-galactosidase, permease & transacetylase are the enzymes in metabolic pathway used to get energy from lactose.
After lactose is used up & level is decreased the repressor will attach to the operator blocking the production of all the three enzymes which will result in increase in amount of lactose.
When lactose level increases, the repressor is removed from the operator RNA polymerase starts synthesizing all the three enzymes resulting into decrease in level of lactose.