Difference Between Operon and Cistron

The key difference between operon and cistron is that operon is a functional DNA unit present in prokaryotes and consists of several genes that are regulated by a single promoter and an operator while cistron is a term used to refer a gene, which is the functional unit of heredity that code for a protein.

Gene is a functional unit of heredity. It is a segment of DNA that consists of genetic information in order to synthesize a protein. Prokaryotes have several genes clustered together under one promoter and one operator. It is known as an operon. Eukaryotes have single genes operating under a single promoter. Cistron is another term that refers to a gene.

1. Overview and Key Difference
2. What is an Operon
3. What is a Cistron
4. Similarities Between Operon and Cistron
5. Side by Side Comparison – Operon vs Cistron in Tabular Form
6. Summary

What is an Operon?

Prokaryotes (bacteria and archaea) primarily have operons. An operon consists of a cluster of genes working under a common promoter and a common operator. As the operon consists of multiple genes, it gives rise to a polycistronic mRNA upon completion of transcription. The operon is regulated by repressors and inducers. Thus, operons can be mainly classified as inducible operons and repressible operons. The inducible Lac operon and the repressible Trp operon are the two main operons studied in prokaryotes. In fact, the structure of an operon is typically studied with respect to the lac operon.

Figure 01: An Operon

The lac operon is composed of a promoter, operator and three genes known as Lac Z, Lac Y and Lac A. These three genes code for three enzymes which are involved in lactose metabolism in microbes. Lac Z codes for Beta-galactosidase, Lac Y codes for Beta – galactoside permease and Lac A codes for Beta – galactoside transacetylase. All three enzymes help in the degradation and transportation of lactose. In the presence of lactose, the compound allolactose is formed; it binds to the lac repressor, allowing RNA polymerase action to proceed and result in the transcription of the genes. In the absence of lactose, the lac repressor is bound to the operator, thereby blocking the activity of RNA polymerase. As a result, no mRNA is synthesized. Thus, the lac operon acts as an inducible operon, where the operon is functional when the substrate lactose is present.

In comparison, the trp operon is a repressible operon. Trp operon codes for five enzymes required in the synthesis of tryptophan which is an essential amino acid. The activity of trp operon is active all the time. When there is an excess of tryptophan, the operon is inhibited. At that time it works as a repressible operon. This will result in the inhibition of tryptophan production until a homeostatic condition is reached.

What is a Cistron?

Cistron is another term used to refer to a structural gene. Cistron is a section of DNA that carries genetic instruction to make a protein. Therefore, cistron encodes for a protein. Cistron transcribes into an mRNA and then translates into a protein. This two-step complex process is known as gene expression. The name “cistron” was given in early bacterial genetics since it was originally defined experimentally as a genetic complementation unit by using the cis/trans test. The term cistron was coined by Seymour Benzer.

Figure 02: Cistron

Prokaryotic operons are polycistronic. It means an operon has several cistrons or genes. A cistron has introns (noncoding sequences) and exons (coding sequences). The number of introns and number of exons, as well as the length of those sequences, vary among the genes. Hence, genes have different sizes. Moreover, genes have a unique position on a chromosome.

What are the Similarities Between Operon and Cistron?

Operon has a cluster of cistrons, so operons are polycistronic.
They have genetic instruction to make proteins.
Both are functional units of heredity.
They function under one promoter.
Moreover, they transcribe and translate into proteins.

What is the Difference Between Operon and Cistron?

An operon is a cluster of several genes that works under one promoter and one operator, but cistron is another term used to refer to a gene. So, this is the key difference between operon and cistron. Furthermore, the operon transcribes into a polycistronic mRNA while the cistron transcribes into a monocistronic mRNA. Thus, this is another important difference between operon and cistron. Moreover, operon produces several proteins, while cistron produces a single protein.

Below infographic lists the differences between operon and cistron in tabular form.

Summary – Operon vs Cistron

An operon is a cluster of genes regulated by a common promoter and operator. They are found in bacteria and archaea. On the other hand, cistron is an alternative name for a gene. Operons are polycistronic. They give a polycistronic mRNA which gives several proteins. But, cistron gives monocistronic mRNA, which translates into a single protein. Thus, this summarizes the difference between operon and cistron.

Reference:

1. “Cistron.” ScienceDirect Topics, Available here.
2. “Operon.” Biology Dictionary, 17 Apr. 2019, Available here.

Image Courtesy:

1. “Operon 1” By User:Bensaccount – Own work (Public Domain) via Commons Wikimedia
2. “Gene Intron Exon nb” By Smedlib – Own work (CC BY-SA 4.0) via Commons Wikimedia