Difference between Operon and Cistron

What is an Operon?

The first operon was described in E.coli by François Jacob and Jacques Monod in the 1960s. They described a lac operon that is a regulatory system for the circulation and metabolism of lactose molecules.

An operon consists of structural genes, promoter sequence and an operator region. Inducer and repressor molecules are some extra elements that might be found in some of the operons.

Earlier, operons were believed to be functional in only the prokaryotic genome, but they have now been discovered in viral as well as eukaryotic genomes. The main advantage of an operon is that it can be turned on and off according to the requirement, and energy is also saved in this process.

Operons are mostly used in metabolic pathways such as synthesis of amino acids. One promoter controls a number of structural genes and a polycistronic mRNA is transcribed that can translate to separate proteins at a single time.

Difference between operon and cistron.

What is a Cistron?

A cistron is an alternative name used for genes. The word cistron was coined by Seymour Benzer when he performed his complementation cis/trans test by putting two genes together in a cytoplasm to study their interactions.

Now, a cistron can be defined as a structural gene that translates into a functional polypeptide. A DNA sequence or cistron is composed of two parts; exons and introns. Exons are the coding sequences and introns are the non-coding sequences. A cistron consists of one gene, and can hence be called monocistronic (as compared to operons which are polycistronic).

Operon vs Cistron




An operon is a regulatory system that consists of structural genes under the influence of a single promoter.

A cistron is a DNA sequence that codes for a protein. It is an alternative name for genes.

Number of Genes

An operon contains multiple structural genes (cistrons).

A cistron comprises one single gene.

Nature of mRNA

An operon produces a polycistronic mRNA.

A cistron produces a monocistronic mRNA in eukaryotes.


It is composed of structural genes, operator and promoter regions.

It is composed of exons and introns.


It saves energy by transcribing multiple proteins at once.

No such advantage.

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Frequently Asked Questions


What is the function of a cistron?

A cistron is a DNA sequence that codes for a functional polypeptide.


How many genes are in the lac operon?

There are three genes in the lac operon.


Why is UGA a stop codon?

UGA is a stop codon because there is no complementary t-RNA for the synthesis of amino acids.


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