Define an operon. giving an example, exp | Class 12 Biology Chapter Molecular Basis of Inheritance, Molecular Basis of Inheritance NCERT Solutions

The concept of operon was first proposed by Jacob and Monad, in 1961. An operon is a unit of prokaryotic gene expression which includes coordinately regulated or structural genes and control elements which are recognized by regulatory gene product.

Components of an operon:

1. Structural gene When lactose is added to the E.coli, the structural genes produces mRNAs, which in turn, synthesize specific polypeptides on the ribosomes (translation).
2. Promoter The sequence of DNA where RNA polymerase binds and initiates transcription of structural genes is called promoter.
3. Operator The sequence of DNA adjacent to promoter where specific repressor protein binds is called operator.
4. Regulator gene The gene that codes for the repressor protein which gets bind to the operator and suppresses its activity as a final result by which transcription will get switched off.
5. Inducer It is a chemical substrate which inactivates the repressor by the latter’s structure and many other processes are in turn induces in terms of this. It is a chemical of diverse of diverse nature like metabolite, hormone substrate etc.

Inducible operon system:

An inducible operon system is a regulatory unit of genetic material which remains switched on in response to the presence of a chemical signal, e.g., the lactose or lac operon of E.coli.

The lactose operon: the lac Z, Y and A genes are transcribed from a lac transcription unit under the control of a single promoter. They encode enzyme required for the use of lactose as a carbon source. The lac I gene product and the lac repressor are expressed from a separated transcription unit that is upstream from the operator.

Lac operon consists of three structural genes (Z, Y and A), operator, promoter and a separate regulatory gene. The three structural genes (Z, Y and A) transcribe polycistronic mRNA.

1. Gene z codes for β- galactosidase enzyme which breaks lactose into galactose and glucose
2. Gene y codes for permease, which increases the permeability of the cell to lactose.
3. Gene a codes for enzyme transacetylase that catalyses the transacetylation of lactose in the active form.   

1. When lactose is absent

• When lactose is absent, I gene regulates and produces repressor mRNA which translate repression.
• The repressor protein binds to the operator region of the operon and as a result prevent RNA polymerase to bind the operon.
• The operon is switched off.

2. When lactose is present

• Lactose acts as an inducer which binds to the repressor and forms an inactive repressor.
• The repressor fails to bind the operator region.
• The RNA polymerase binds to the operator and transcript lac mRNA.
• lac mRNA is polycistronic, i.e. produces all three enzymes, β-galactosidase, permease and transacetylase.
• The lac operon is switched on.