Messenger RNA

introduction image

Messenger RNA abbreviated as “m-RNA” and contained specific codons encoding particular amino acids, which is later processed into proteins (as amino acids are the monomer unit of proteins). Protein is an element that is required to develop the life forms, so a cell must translate specific proteins. Therefore for the proper growth and cell development, gene translation into protein is necessary.

Messenger RNA is a gene product formed as a result of DNA transcription, and functions as a “Guide” that contains all the information to start the process of translation.

Content: Messenger RNA

  1. Definition
  2. Synthesis
  3. Types
  4. Structure
  5. Functions
  6. Conclusion

Definition of Messenger RNA

Messenger RNA can define as an RNA sub-type that carries the specific codons corresponding to the DNA template and helps in the sequencing of amino acids to build a protein by the cooperation of transfer RNA and Ribosome. Therefore, m-RNA is a transcript of DNA that contains the information for the gene translation into protein. The formation of prokaryotic m-RNA is less complicated than the eukaryotic m-RNA. Messenger RNA is short, single-stranded and comprises a sugar-phosphate backbone.

Synthesis of m-RNA

In prokaryotes, messenger RNA synthesis occurs solely inside the cytoplasm as they lack a nucleus. In eukaryotes, messenger RNA synthesis occurs inside the nucleus by using the nucleotide sequence of a template DNA strand. Therefore, the messenger RNA transcribes from the template DNA through a biological process known as “Transcription”. The RNA polymerase-II initiates the process of mRNA transcription by using a substrate (nucleotide triphosphate) from the template DNA strand.

Types of mRNA

Based on mRNA synthesis, the DNA first, transcribes into pre- mRNA and later modifies into mature mRNA.

Precursor and mature mRNA

Pre- mRNA

It is an acronym for the term “Precursor mRNA”. Pre- mRNA can define as the primary transcript, which belongs to the group of heterogeneous nuclear mRNA. Precursor mRNA comprises both coding exon sequences and non-coding intron sequences. It also refers as “Immature mRNA”. During its processing, the unwanted sequences or introns splices out of the strand via two ways:

  • Splicing by the RNA’s catalytic activity.
  • Splicing by a multiprotein structure refers as “Spliceosome”.

Mature mRNA

The further modifications in the pre- mRNA converts it into a mature mRNA transcript. Therefore mature mRNA is derived after the maturation of pre- mRNA. Unlike pre- mRNA, it lacks introns. For the translation of amino acids into proteins, mature mRNA processes for 5’ capping and later tailing at the 3’ prime. After processing, the mature mRNA releases out from nucleus to cytosol. Then, t-RNA and ribosome associate, and decodes the information carried by mRNA to build proteins accordingly.

Based on protein expression, the mRNA can be of the following types:

Monocistronic and polycistronic mRNA

Monocistronic mRNA: It is a kind of mRNA common in eukaryotes, which carries the exon sequences that code for a single protein.
Bicistronic mRNA: It is a type of mRNA carries exon sequences that codes for the two protein.
Polycistronic mRNA: It is a kind of mRNA common in bacteria and bacteriophages, which carries the exon sequences that code for the multiple proteins.

Structure of Messenger RNA

The structure of mature mRNA includes the following elements:

structure of mRNA

Coding sequences

The coding sequences of mature mRNA contains triplet codons. In eukaryotes, the triplet codons code for specific amino acid and translate it into a single protein, whereas in prokaryotes these codons translate gene into multiple proteins by the assistance of tRNA and ribosome.

The coding region begins with the “Start codon” (AUG) and ends with the “Termination codon” (UAA, UAG and UGA). Internal base pairs maintain the coding region. Coding region also functions as the regulating sequence and exonic splicing enhancers and inhibitors in the precursor mRNA.

Untranslated region

There are two untranslated regions in mature mRNA, one at 5’ prime and the other at 3’ prime. The 5’ untranslated region is present in between the 5’ cap and start codon, whereas 3’ untranslated region is present in between the stop codon and 3’ tail. The untranslated region transcribes within the coding part. It performs the following functions like:

Stability of mRNA: Both 5’ and 3’ untranslated region regulates the mRNA stability, due to varying affinity for Ribonucleases and ancillary protein that can promote or inhibit RNA degradation.
Translational efficiency: 3’ or 5’ UTR may influence the transitional efficiency by controlling the ribosome’s ability to bind with the mRNA.
Localization of mRNA: This is regulated by 3’ UTR that contain sequences, which allow the transcript to be localized to the region for translation.

The untranslated region also contains some elements that regulate the mRNA:
SECIs element: It targets the protein to bind.
Riboswitches: It directly binds the small molecules.

Poly (A) tail

A mature mRNA comprises a polyadenylated tail (up to 150-250 adenine bases) after the 3’ untranslated region. The Poly (A) tail performs the following functions in mature mRNA:

  • 3′-tailing in eukaryotic mRNA helps in the transportation of mRNA residing in the nucleus to the cytoplasm.
  • The 3′ poly-A tailing protects the 3′-end of mature mRNA from degeneration.

5’-cap

A mature mRNA also comprises a 5’-cap before the 5’ untranslated region. The 7-methylguanosine cap attaches to the 5’ end via 5’-5’ phosphate linkage, and performs the following functions in mature mRNA:

  • The 5′-mRNA capping also protects the 5′-end of mature mRNA from degeneration.
  • 5′-capping in mRNA also directs the binding of the ribosome during protein synthesis.

Functions of Messenger RNA

The messenger RNA performs a functional role in the process of gene expression by participating in the following tasks:

  1. An mRNA contains the source of genetic information from the template DNA that directs the amino acid formation.
  2. It also contains multiple regulatory regions that determine the rate and flow of translation.
  3. An mRNA contains the information on how to connect the amino acids into a peptide chain to form the proteins.

Conclusion

Therefore mRNA is a sub-type of RNA that forms first by the enzymatic activity of RNA polymerase on the DNA strand. The messenger RNA transcribes from the 5′-3′ end and undergoes processing before translation. The transfer RNA in the cytosol contains one specific anticodon arm that decodes the coding sequence of mRNA. After appropriate sequencing of amino acids, the transfer RNA loads the amino acids onto the ribosome, that forms protein accordingly.

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