Question

What is an operon? What are the main component of an operon in prokaryotic cell?

Answer 1

1. Definition of operon –

Operon is a functional unit of transcription and genetic regulation containing a cluster of genes under the control of a single promoter. Operons are found both in eukaryotic cells and prokaryotic cells, and also in viruses. Operons are basically comprised of genes which code for functionally related protein, and this feature allows protein synthesis to occur according to the need of the cell which in turn allows the cell to conserve energy. Expression of eukaryotic operons lead to formation of monocistronic mRNAs, whereas the expression of operons in prokaryotic cells generally leads to the generation of polycistronic mRNAs. Lac-operon, Trp-operon are the examples of operon found in prokaryotic cells.

2. Components of operon found in prokaryotic cell –

The 3 main and basic components of a operon are promoter, operator, structural genes. Though not always included within the operons, but regulatory genes and regulatory molecules e.g. inducer and co-repressors and activators also play significant roles in proper functioning of operons.

Promoter – The promoter is a particular DNA sequence which is recognised by RNA polymerase, and after recognising the promoter RNA polymerase starts the transcription. So, promoter basically enables the transcription of the genes included within a operon. In an operon, the expression of a number of functionally related genes are controlled by a single promoter.

Operator – Operator is an important component of a prokaryotic operon which regulates the transaction of genes within a operon. Operator is basically a segment of DNA included within a operon where the repressor binds and supress the transcription of the genes.

Structural Genes – Structural genes are a cluster of genes within an operon that are functionally related to each other in some or other way and regated by a single promoter of the operon. Transcription of structural genes within a prokaryotic operon ultimately leads to the formation of polycistronic mRNAs.

Regulatory Genes – Regulatory genes are an important part of the operon that constantly express and code for repressor protein.

Inducers – Inducers are small molecules that either activate or repress the transcription of the genes depending upon the availability of the substrate or the in accordance to the need of the cell. Inducers can displace the repressor protein from the operator site of the operon.

Repressors – Repressors supress the transcription of the genes in response to the external stimulus and similar to the repressors co-repressors are proteins that can bind to the repressor to allow its binding to the operator site.

Activators – The function of activators is to activate the transcription of the genes in accordance to the signal provided by external stimulus. Activators generally bind to the promoter site.

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