Gene regulation -models



  • Gene regulation is an important aspect of Molecular Biology
  • It is the genetic control of gene regulation
  • Deals with gene action and its control
  • Genes coding for  a particular protein need not be active all the time
  • They need to transcribe m-RNA only when a particular protein is required by the organism
  • At other the gene could be switched off or kept away from transcription
  • When needed the gene could be switched on or transcription could be resumed
  • This is called regulation of gene action and many models have been proposed to explain
  • Common among these is the Battery model for  Eukaryotes  and
  • The Operon Model  that is appropriate for Prokayotes

Gene Battery Model

  • Proposed By Roy John Britten and Eric. H. Davidson in 1969
  • Explains regulation of Protein synthesis in eukaryotes
  • The model proposes – four classes of gene sequences for control of gene action in  genomes of Eukarytoic multicellular organisms 
  • They are
    • 1. Integrator gene
    • 2. Producer gene
    • 3. Receptor site and
    • 4. Sensor site
  • Integrator gene: synthesizes activator for initiation of transcription
  • Integrator needs to be activated by the sensor site
  • Sensor site in turn is  activated by internal/external stimuli
  • Producer gene is responsible for the production of the gene product i.e. Protein
  • Receptor site is the site of binding of activator m-RNA to start the transcription
  • This model is applicable to Eukaryotic genomes with split gene and is able to explain the presence of repetitive DNA
  • Lacks experimental proof  hence not applied widely
  • But in the modern context many of the molecular biology puzzles are taken into account in the model proposed

Operon Model 

  • Proposed by Francois Jacob and Jacques Monad in 1961
  • Explains gene regulation in Prokaryotic genomes where transcription and translation are simultaneous
  • Can also be applied to monocistronic m-RNA in Eukaryotes
  • Operons also found  in viruses like bacteriophages
  • Postulates the presence of  a single Promoter gene that controls a gene cluster for a particular pathway

General Structure of  Operon

  • An operon essentially consists of  two main Components  1.  Regulatory Genes  also called control genes and 2. Structural genes
  • Regulatory Genes – generally present upstream of the structural genes and are involved in controlling transcription of structural genes.
  • Regulatory genes may include an Inducer, Promoter and an Operator each having a special function
  • Promoter is the site where the RNA polymerase binds to initiate transcription.
  • Inducer is a sequence that is responsible for the synthesis of  repressor protein that controls transcription
  • Operator is a site after the promoter where the  repressor binds to stop transcription
  • Operator site has to be free of  repressor to initiate transcription of structural genes
  • Several operons in E.coli have been studied and based on mode of regulation 2 important types of operons have been recognized

(Diagram provided in the last page as a scanned file)


  • First studied in Escherichia coli
  • Explains the control of genes involved in the uptake and utilization of Lactose by  E.coli
  • Described by Jacob and Monad and has been experimentally proved based on the gene products of regulator and structural genes
  • There are two parts
  • Regulatory or Control genes
  • Structural genes
  • Regulatory genes are three regions commonly mentioned as i,p,o expanded as inducer, promoter and operator respectively
  • Inducer gene need not necessarily be situated at the operon region.
  • It can also be remotely present somewhere upstream  of the Operon
  • Inducer is responsible for producing an repressor  protein that reversibly binds to Lactose
  • Promoter is the region on the DNA to which the RNA polymerase enzyme binds to bring about transcription of the structural genes
  • Operator is a part of the promoter to which  Lactose binds when lactose levels fall and so transcription is stopped
  • Structural genes – In the Lac operon there are three  genes-Lac Z, Lac Y and Lac A
  • Lac Z gene produces an enzyme called β-Galactosidase that breaks down Lactose to Glucose and Galactose
  • Lac Y gene produces an enzyme called β-Galactoside permease that helps in uptake of Galactose into the cells
  • Lac A gene  produces an enzyme called Galactoside acetyl transferase that helps in the assimilation of galactose by the cells

Mechanism of action of Lac Operon

  • Lac Operon controls uptake and utilization of lactose
  • Should be switched on when lactose is present
  • Should be switched off when lactose is absent

When lactose is  PRESENT in the medium the following events take place:

  • Inducer produces an i-m-RNA that produces a repressor protein which can bind at the operator region.
  • Lactose binds to repressor  and prevents it from binding to operator
  • RNA polymerase binds to Promoter and Transcription takes place
  • Enzymes are formed
  • Operon is switched  On

When lactose is  ABSENT  in the medium the following events take place:

  • The repressor protein is not bound to Lactose
  • Repressor binds to operator
  • RNA polymerase is not able to move along the DNA
  • Transcription is stopped
  • Enzymes are not produced
  • Operon is switched Off
Dr. Mujeera Fathima

Dr. Mujeera Fathima

Dedicated, Passionate teacher and Associate Professor of Botany specializing in Plant Physiology and Plant Anatomy with research interest in Ethnobotany and Traditional knowledge possessing a teaching experience of 28 years.

About Me

Dr. Mujeera Fathima

Dedicated, Passionate teacher and Associate Professor of Botany specializing in Plant Physiology and Plant Anatomy with research interest in Ethnobotany and Traditional knowledge possessing a teaching experience of 28 years. Officiated as one of the co-authors of the XI standard Botany and Bio-Botany textbook (2005) and XI standard Botany and Bio-Botany textbook (2015) as Domain expert for Tamilnadu State Higher Secondary Board of Education. A certified soft skills trainer and counselor holding a Doctorate in Botany and a M.Sc. in Psychology