Difference Between Monocistronic and Polycistronic mRNA

The main difference between monocistronic and polycistronic mRNA is that the monocistronic mRNA produces a single protein while polycistronic mRNA produces several proteins that are functionally-related. Furthermore, eukaryotes have monocistronic mRNA while prokaryotes have polycistronic mRNA.

Monocistronic and polycistronic mRNA are two types of mRNA molecules, which can be decoded into polypeptide sequences. Most mRNA are monocistronic while fewer are polycistronic.

Key Areas Covered

1. What is Monocistronic mRNA
     – Definition, Structure, Translation
2. What is Polycistronic mRNA
     – Definition, Structure, Translation
3. What are the Similarities Between Monocistronic and Polycistronic mRNA
     – Outline of Common Features
4. What is the Difference Between Monocistronic and Polycistronic mRNA
     – Comparison of Key Differences

Key Terms

Cistron, Monocistronic mRNA, Operon, Polycistronic mRNA, Post-Transcriptional Modifications

What is Monocistronic mRNA

Monocistronic mRNA refers to eukaryotic mRNA that consists of a single cistron. Therefore, it can produce a single protein. The nascent transcripts of genes are called the pre-mRNA. Pre-mRNA and other nuclear RNA are collectively called heterogeneous nuclear RNA (hnRNA). They associate with proteins to form heterogeneous nuclear ribonucleoprotein (hnRNP) inside the nucleus. Some hnRNA undergoes post-transcriptional modifications in order to become mature RNA, which travels to the cytoplasm for translation.

Figure 1: Monocistronic and Polycistronic mRNA

The addition of the 5’ cap is the initial stage of post-transcriptional modifications. It is catalyzed by a dimeric capping enzyme associated with the CTD (phosphorylated carboxyl-terminal tail domain) of RNA polymerase II. Another post-transcriptional modification is 3’ polyadenylation, which covalently adds a polyadenylyl moiety at the 3’ end. Poly-A tail promotes the export of the mRNA molecule into the cytoplasm while protecting it from degradation. RNA splicing is the other event in the post-transcriptional modifications. During this, the introns are removed and exons are spliced together.

What is Polycistronic mRNA

Polycistronic mRNA refers to prokaryotic mRNA consisting of two or more cistrons. The mRNA produced by chloroplasts and mitochondria are also polycistronic. In prokaryotes, functionally-related genes assemble in groups in such a way that all proteins can be transcribed at once when needed. Lac operon is one such famous operon. lacZ, lacY, and lacA are the three genes in the Lac operon, which encode the beta-galactosidase, beta-galactoside permease, and beta-galactoside transacetylase enzymes respectively. All enzymes are involved in the lactose metabolism.

Figure 2: Lac Operon

An operon that produces polycistronic mRNA consists of a leader and a trailer sequence. The leader sequence is the first gene followed by an intercistronic region and the sequence of the second gene. The trailer sequence is the last gene. Transcription of the operon is regulated by a single promoter. However, each cistron is flanked by transcription initiation site and transcription termination site.

Similarities Between Monocistronic and Polycistronic mRNA

• Monocistronic and polycistronic mRNA are two types of mRNA that can be translated into proteins.
• Untranslated regions flank the protein coding region in both mRNA.
• Both are transcribed under a single promoter.

Difference Between Monocistronic and Polycistronic mRNA

Definition

Monocistronic mRNA refers to mRNA with a single cistron while polycistronic mRNA refers to mRNA with two or more cistrons.

Found in

Monocistronic mRNA occurs in eukaryotes while polycistronic mRNA occurs in prokaryotes.

Transcribed from

The transcription of a gene produces monocistronic mRNA while the transcription of an operon produces a polycistronic mRNA.

Amount

Most mRNA are monocistronic while a fewer is polycistronic.

Open Reading Frames

Monocistronic mRNA consists of a single open reading frame while polycistronic mRNA consists of several open reading frames.

Translation Start Site/Translation Termination Site

Monocistronic mRNA has a single translation start site/translation termination site while polycistronic mRNA has multiple translation start sites/translation termination sites.

Number of Proteins

Monocistronic mRNA can produce a single protein while polycistronic mRNA can produce several proteins.

Relationship

Monocistronic mRNA are not related to each other while polycistronic mRNA is functionally-related since it is produced by the transcription of an operon.

Post-Transcriptional Modifications

Monocistronic mRNA undergoes post-transcriptional modifications while polycistronic mRNA does not undergo post-transcriptional modifications.

Conclusion

Monocistronic mRNA consists of a single cistron while polycistronic mRNA consists of more than two cistrons. Most eukaryotic genes produce monocistronic mRNA while the prokaryotic operons produce polycistronic mRNA. The main difference between monocistronic and polycistronic mRNA is the number of cistrons in the mRNA.

Reference:

1. Lodish, Harvey. “Processing of Eukaryotic MRNA.” Advances in Pediatrics., U.S. National Library of Medicine, 1 Jan. 1970, Available Here
2. “Transcription: Transcription Products.” Phillip E. McClean — Plant Sciences – NDSU Agriculture and Extension, Available Here

Image Courtesy:

1. “Lac operon1” (Public Domain) via Commons Wikimedia