Operon: History , Types And Process

Operon: History , Types And Process



The Number Of Genes Estimated To Be Present In The E.Coli Chromosome Is 4,397. Many Genes With Related Functions Are Closely Grouped On The Chromosomes. Such Clusters Of Genes With Related Functions Are Called Operon And Transcribe A Single mRNA.

  • An Operon Is A Unit Of Bacterial Gene Expression And Regulation That Includes Structural Genes And Their Control Element (Promoters, Operator And Regulator)
  • An Operon Is A Part Of Genetic (Or DNA) Material.
  • Operon System Is More Common In Prokaryotes.



F. Jacob And J. Monad(1961) Has Proposed A Model In Order To Explain The Process Of Induction And Repression In E. Coli. The Model Is Known As The Operon Model.

According To Them ‘’Operon Is A Group Of The Controller (Promoter, Operator, Regulator) And Structural Genes Which Regulate Catabolism Genetically.



It Contains the Following Gene-


  • The Structural Gene Is Nucleotide Sequence, Which Actually Synthesis Of mRNA.
  • An mRNA Controls Metabolic Activity By Forming Protein Or Enzyme On The Ribosome.
  • An Operon Has 1 Or More Structural Gene.


  • Regulator Gene Is Not Themselves Parts Of Operon, Produce The Repressor Protein That Interacts With The Operator Locus.
  • Repressor Protein Binds To The Operator Gene And Suppresses Or Shuts Off The Synthesis Of mRNA From The Operator Gene.


  • Promoter Genes Are Signal Sequences In DNA That Start RNA Synthesis.
  • Promoter Gene Is The Part Of DNA Which Regulates The Activity Of RNA-Polymerase And Initiates The Transcription Of mRNA.
  • The Gene Promotor Is Continuous With The Gene Of The Operator.


  • It Is The Part Of The Operon(DNA) Which Controls The Activity Of One Or More Structural Genes And In Which Repressor Protein Is Binded.
  • Operator Genes Are Short Regulating Sequences That In Many Genes Are Positioned Between The Promoters And The Structural Genes.
  • The Operator Is Located At The Beginning Of The Operon.
  • Operator Bind Specific DNA Binding Protein Called Repressor.
  • Many Operons Contain More Than One Operator Sequence.







Such Enzyme Whose Synthesis Can Be Induced By Adding The Substrate Is Known As Inducible Enzyme And Genetic System Responsible For The Synthesis Of Such An Enzyme Is Called Inducible System.

Lac OPERON IN E.Coli:- It Consists Following Gene-



Genes Which Actually Synthesis mRNAs, An Mrna Controls Metabolic Activity By Forming Protein Or Enzyme On The Ribosome.

Lac-Operon Of E.Coli Consists Of Three Structural Genes.


  • Lac Z GENE:-  It Codes For The Enzyme ß-Galactosidase, Which Is Responsible For Hydrolysis Of Disaccharide(Lactose) Into Glucose And Galactose.


  • Lac Y GENE:- It Codes For The Enzyme ß-Galactoside Permease, Which Regulates The Entry Of ß-Galactosides Into Bacteria.


  • Lac A GENE:- It Codes For The Enzyme ß-Galactoside Transacetylase.

These 3 Genes Required For The Metabolism Of Lactose.


  • OPERATOR GENE:- Operator Gene Of The Lac Operon Is Made Of Only 27 Base Pairs Only. It Is Switched Off By Presence Repressor And On By Its Absence.


  • REGULATOR GENE:- Produce Specific Protein Called Repressor.


  • PROMOTER GENE:- Initiates The Transcription Of mRNA.

Process Of Lac Operon:-

  1. Negative Control Or In The Absence Of Inducer-
  2. First Of All, mRNA Is Synthesized By Regulatory Gene.
  3. After That Repressor Is Synthesized From A Regulatory Gene.


  • In The Absence Of Inducer That Is Lactose, This Repressor Is Bound To The Operator.
  1. RNA Polymerase Binds The Promoter So Strongly That Is Unable To Leave The Promoter & Carry On Transcription, This Is Only Due To Binding Of The Operator By Repressor, So Transcription Does Not Occur.
  2. The Regulation Of Lac-Operon By Repressor Is Called Negative Control.


  • Positive Control Or In The Presence Of Inducer:-
  1. When The Repressor Is Synthesized From A Regulatory Gene, Due To Presence Of Inducer. It Binds With Inducer.
  2. Catabolic Activator Protein (CAP) Is An Activator.


  • It Exerts The Control In Lac-Operon In Its Absence. The Promoter Gene Cannot Be Recognized By The RNA Polymerase.
  1. Its Gene Is Located Away From The Operon And It Activates Lac-Operon Only When Glucose Is Absent.
  2. The RNA Polymerase Is Recognized By The Promoter Gene Passing Through The Operator Gene And Catalyzing The Transcription Of mRNAs Over The Structural Genome.


  • The mRNA Into The Cytoplasm & Form A Particular Enzyme.


  • These Produced Enzymes Perform Their Function As ß-Galactosidase Break Lactose Into Glucose And Galactose.


  • Regulation Of Lac-Operon By Inducer Is Called Positive Control.


REPRESSIBLE SYSTEM:- Such Enzymes Whose Synthesis Can Be Checked By The Addition Of End Product Are Known As Repressible Enzymes, And The System Is Known As A Repressible System.


STRUCTURAL GENE:- Trp Operon Has 5 Structural Genes They Are-

Trpe, Trpd, Trpc, Trpb, & Trpa. For 5 Steps Of Tryptophan Synthesis, They Form An Enzyme.


OPERATOR GENE:- It Controls The Functioning Of The Structural Gene.


PROMOTER GENE:- It Is A Site Where RNA Polymerase Is Initially Binding. It Means RNA Polymerase Moves Over Structural Gene And From It When Operator Gene Is Function & Help To Formation Of Tryptophan.


REGULATOR GENE:– It Forms A Proteinaceous Component That Can Block The Operator Gene Activity. This Proteinaceous Component Is Aporepressor.


APOREPRESSOR:- It Is A Proteinaceous Substance That Is Synthesized From The Regulatory Gene. Aporepressor Is A Repressor Element That Blocks The Operator Gene’s Role, But This Repressor Is Inactive.


Process Of Trp Operon:-

  1. Positive Control:
  2. This Repressible System Is Always Inactive Form By Itself.
  3. Aporepressor That Is Produced From The Regulatory Gene, Is In Inactive Form It Can’t Able To Bound Operator Gene For Inhibit The Process.
  4. Negative Control:-
  5. The End Product Of Trp-Operon Structural Gene That Is Nonproteinaceous Co-Repressor.
  6. The Tryptophan That Is Synthesized From The Structural Gene Act As A Co-Repressor When It Becomes Available From Outside Source.


When Co-Repressor Bind With Aporepressor For Becoming The Inactive State Of Repressor.

Then This Apporepressor Is Inactive Form And Make It Repressor That Blocks The Operator Gene.



After Binding The Operator Gene, The Structural Gene Now Stops Transcription, This Phenomenon Is Feedback Repression.



The Information About Operon Structure Leads To A Better Estimation Of Gene Expression Levels.

Mutation: Definition, Types, Application