All the biochemical reactions are controlled by enzymes. The synthesis of enzymes is controlled by genes. Whenever the cell needs a particular enzyme, the gene that controls a particular enzyme will be switched on. Then only the cell needed enzyme will be synthesized. If cell does need a particular enzyme, the gene that controls the synthesis of a particular enzyme will be switched off.

For example the enzyme ATPase is required by the cell. The gene that controls the synthesis of ATP ase will be switched on. Then the enzyme ATPase will be synthesized on Ribosomes. Now this ATPase metabolizes the ATP to liberate energy. If the cell does not need ATPase, the ATPase controlling gene will be switched off. Hence ATPase will not be formed.

The mechanism that controls the switch on or switch off of the genes is called gene regulation.


The process of switch on mechanism of gene, synthesis of a particular enzyme and metabolism of the substrate is called induction. The small molecules that participate in the induction are called inducers.

The process of switch off mechanism of gene is called repression.

Cell needs some enzymes always to perform its duties properly. The genes that control such enzymes are always in the state of expression or switch on. Such genes are called constitutive genes.


This concept was proposed by Jacob and Monod. They observed the regulation of gene in the Bacterium, Escherichia coli. Jacob and monod did many experiments and explained gene regulation.

i. Operon - definition:

A segment of DNA that contains regulator, promotor, operator and structural genes is called operon.

Regulator gene (i) + promotor gene (P) + Operator gene (O) + Structural gene = Operon.

All these genes lie serially on D.N.A. Regulator, promotor and operator genes together called control genes.

ii. Structural genes:

These control the synthesis of enzymes for the metabolism of a particular substrate.

For example, in E.coli the lactose is metabolised into glucose and galactose. This metabolism needs three enzymes. They are

I5 -galactosidase, Permease and transacetylase.The synthesis of these enzymes is under the control of three genes or cistrons. They are Lac'Z', Lac 'Y' and Lac 'A'. All these three genes lie serially. These structural genes are transcribed at a stretch. Hence the long polycistronic'm' R.N.A. is formed. The transcription or synthesis of'm' R.N.A. needs R.N.A. polymerase.

iii. Operator gene:

It is a switch to control or to switch on or to switch off the structural genes. When the operator gene is switched on, the transcription or 'm' RNA synthesis occurs from structural genes (When the operator gene is switch on, then only structural genes are switch on). When the operator gene is switched off, the structural genes that are controled by the above operator gene are switch off. As this gene operates the structural genes, it is called operator gene.

Working mechanism of operator gene:

The operator gene switch on or switch off is controled by a protein, repressor. When this protein binds to the operator gene, the switch (operator gene) is switched off. Hence the transcription does not occur from structural genes and finally enzymes are not synthesized the operator gene is switched off when the substrate (lactose) is not available.

When the lactose enters into the E.Coli by permease enzyme, it binds with repressor protein. When this happens, the repressor protein can not bind the operator gene. Hence operator gene is switched on. This causes the transcription of structural genes.

iv. Regulator gene:

This controls the operator gene. The regulator gene has the information for the synthesis of repressor protein. As the repressor protein regulates the operator gene to switch off or to switch on, the repressor protein controlling gene is called regulator gene.

v. Promotor gene:

This contains RNA polymerase. This enzyme is released when the operator gene is switched on. The RNA polymerase causes the synthesis of 'm' RNA from structural genes.

Operator gene

When the repressor protein binds the operator gene, the RNA polymerase is not released from the promotor gene. Hence transcription of structural genes does not occur.


vi. Feed back repression:

It is also explained by the same operon model in E.coli.

For example, the amino acid tryptophan is synthesized by the action of five enzymes.They are E, D, C, B and A.These enzymes synthesis is controlled by five genes. These are try-E, try-D, try-C, try-B, try-A. All these genes are arranged serially. All these five structural genes are controled by single controling unit.

The repressor protein synthesized by regulatory gene does not bind to the opeator gene by itself. Hence operator gene transfers RNA polymerase to the structural genes. Now polycystronic 'm' RNA is synthesized and finally tryptophan is synthesized.


However when tryptophan is added, it binds with repressor protein. Hence tryptophan is called co-repressor. The repressor-corepressor complex will bind to the operator gene. Hence transcription will not be occurred from structural genes.

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