Operon: The genetic switch in prokaryotes

Being economic and conservation of resources are some of the key strategies by which nature has a regulatory control over the living systems. When and in what quantities the product of any gene are required, that is also under the control system.

You know guys! From the experimental evidence, it is now clear that the genes do not operate to produce enzymes all the time. Each gene is more or less inactive. They got activation signal only when their product is needed.

Gene expression is the mechanism at molecular level by which a gene is able to express itself in the phenotype of an organism. Different gene s in an organism are meant for the synthesis of different proteins.

The protein synthesis rate is regulated by genetic apparatus and environmental factors according to the need of a cell. The regulation of gene expression in bacteria is called.

OPD-1Operon system

In prokaryotes (Bacteria), the synthesis of enzyme is regulated in such a way that the enzymes are produced only when the substrates for the enzymes are present.

Operon concept

Operon is a regulated cluster of adjacent structural genes (genes that code for a polypeptide) with related functions with a single promoter region, so an RNA polymerase will transcribe all structural genes on an all-or-none basis.

As a result of transcription of this polycistronic mRNA with coding sequences for all enzymes in metabolic pathway.

Jacob and Monod were the first who have given the concept of operon while working on E.coli.

There are basically two different types of operon on the basis of the mechanism of regulation:

Inducible operon and repressible operon

Inducible operon:

This type of operon system regulates genetic material which remain switched off normally but it becomes operational in the presence of inducer. For example- Lac operon of E. coli

OPD-2Lac operon of E. coli have four components-

  • Regulatory gene: Synthesizes the repressor that attach with the operator gene and block the passage of RNA polymerase.
  • Promotor gene: Provides site for attachment to RNA polymerase and from the initiating point for the transcription.
  • Operator gene: It controls the activities of structural gene and provides the passage to the RNA polymerase enzymes.
  • Structural gene: It have three structural genes:
  1. Cistron-Z, codes for ß-galactosidase (hydrolyses lactose into glucose and galactose)
  2. Cistron-Y, codes for permease (increase the entry of lactose inside the bacterial cell)
  3. Cistron-X, codes for transacetylase (transfer of an acetyl group from one molecule to another)

The Lactose act as an inducer and whenever it combines with repressor, the repressor detached from the operator gene and allow RNA polymerase to get its passage and reached to the structural genes and the transcription started.

Repressible operon:

This type of operon remains active and synthesize product, but when product cross the limit value, operon becomes inactive. For example: Trp operon (Tryptophan operon)


Like that of Lac operon, Trp operon also consist of four components:

  • Structural gene: Bacteria (E. coli) synthesizes tryptophan from few precursor molecules which are catalyzed by a specific enzyme (convert chorismic acid to tryptophan). For enzymes, five genes are responsible for coding and they are designated trp E, trp D, trp C, trp B and trp A that lies together on a chromosome. The reason for their adjacent arrangement is that a single “ON-OFF switch” can control all of them.
  • Operator gene:  Adjacent to structural gene, this gene controls the activity of structural genes and provide the passage to the RNA polymerase enzyme.
  • Promotor gene: Promoter sequences are DNA sequences that define where transcription of a gene by RNA polymerase begins. This gene is typically located upstream or at 5’ end of the transcription initiation site. Here RNA polymerase and some essential transcription factors bind.
  • Regulatory gene: It controls the operator gene in cooperation with a substance called The regulator gene codes for a protein called aporepressor.

So guys, you must have developed some idea behind the gene regulation in prokaryotes If you want to learn more in details about other important topics as well, just jump into the link.

I think you remember me. Yes, you are right! I am Aabid Hussain, your biology educator.

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See you with again with another amazing topics about life. Till then, keep smiling and stay healthy.

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