Messenger ribonucleic acid

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Definition

noun

plural: messenger ribonucleic acids

mes•sen•ger ri•bo•nu•cle•ic ac•id, [ˈmɛ.sɪn.dʒəɹ raɪboʊnjuːkliːɪk ˈæsɪd]


A ribonucleic acid RNA that carries the genetic code for a specific protein, and conveys this code from the DNA to the ribosome


Details

Overview

Ribonucleic acid (RNA) is a nucleic acid, which similar to deoxyribonucleic acid (DNA) is made up of linear chains of monomeric nucleotides. Each nucleotide component, in turn, is made up of phosphoric acid, sugar, and nitrogenous base. RNA differs from DNA in generally being single-stranded and with a ribose sugar component as opposed to DNA's deoxyribose sugar. Another marked difference is the uracil of RNA in place of thymine in DNA. Thus, adenine complementary base pairs with uracil in RNAs. One of the main functions of RNA is for protein synthesis. There are three major types of RNA involved in this process: (1) messenger RNA (mRNA), (2) transfer RNA (tRNA), and (3) ribosomal RNA (rRNA). During protein synthesis, messenger RNA carries the DNA transcript for specific amino acids for translation.


Structure

mRNA is a single strand RNA that consists of nucleotides copied from a segment of DNA. Just as the other RNAs, it has uracil in place of thymine. mRNA begins at being a precursor mRNA, i.e. an immature mRNA molecule that is transcribed by the RNA polymerase and goes through further processing to become a functional mature mRNA. A mature eukaryotic mRNA structure would have the following components:


mRNA components Features Function
5' cap A cap is added on the 5'end of mRNA. In eukaryotes, the cap may be a guanine nucleotide. In bacteria, it may be NAD+, NADH, or 3'-dephospho-coenzyme A. Regulation of nuclear export; prevents degradation by exonucleases; promotes translation and 5' proximal intron excision
5' untranslated region (UTR) It is the region in mRNA that is directly upstream from the initiation codon. It is not translated in many organisms. But when translated, it produces a protein product that is involved in the regulation of translation of the main coding sequence. When untranslated, forms a complex secondary structure that regulates translation
coding region It is the main coding sequence derived from a portion of DNA or RNA coding for a particular protein. It begins with a start codon at 5'end and a stop codon at the 3' end. It carries the code for the synthesis of a particular protein. The code is in the form of a codon that is decoded and translated by the ribosome
3'-UTR It is part of mRNA situated after the coding region, particularly after the stop codon. It often contains regulatory regions that have effects on gene expression. Binding of microRNAs to specific sites in 3'-UTR may lead to a decrease in gene expression (i.e. by inhibiting translation or by causing transcript degradation)


Binding of repressor proteins to silencer regions in 3'-UTR could lead to the inhibition of mRNA expression.

poly(A) tail A long stretch of adenine nucleotides added to the "tail" or 3' end via polyadenylation Protects mRNA from degradation by exonucleases in the cytoplasm, and also helps in transcription termination, export of the mRNA from the nucleus, and translation


Biological function

mRNAs are essential for their role in protein synthesis. They carry the genetic code from the DNA (or RNA) for a particular protein. In eukaryotes, the mRNA carries the genetic information from the nucleus to the site of protein synthesis (ribosome) in the cytoplasm to be translated. In non-eukaryotes such as bacteria, the mRNA is produced and translated in the cytoplasm of the bacterial cell.


Common biological reactions

mRNA synthesis

In prokaryotes such as bacteria, the mRNA is produced and translated in the cytoplasm of the bacterial cell. The prokaryotic mRNA is produced by splicing a large primary transcript from a DNA sequence. Protein synthesis starts even while the mRNA is still being synthesized. The resulting mRNA is essentially mature upon transcription and requires no extensive processing.


In eukaryotes, the mRNA is produced in the nucleus (during transcription). This mRNA needs to be processed extensively to become mature. This extensive processing includes the addition of a 5' cap at the 5' end and a sequence of adenylate groups at the 3' end, the poly a tail, as well as the removal of any introns and the splicing together of exons. When the pre-mRNA has been completely processed, it is now called a "mature mRNA", which will then be transported for translation into the cytoplasm from the nucleus through the nuclear pore.


Translation

Protein synthesis is comprised of two processes, i.e. transcription and translation. Transcription is the process in which mRNA is produced based on a DNA template. The mRNA moves from the nucleus to the cytoplasm to reach the site of protein synthesis -- the ribosomes. In the ribosome, translation occurs. It is when the amino acids transported by the tRNAs will be joined together in a particular order as specified by the genetic code as copied by the mRNA from a DNA segment.


Degradation

The mRNAs of prokaryotes are usually very short lived (from seconds to more than an hour). In contrast, the eukaryotic mRNA has relatively longer lifespan, e.g. mammalian mRNA can live from several minutes to days. The degradation of mRNAs takes place via various mechanisms. In prokaryotes, mRNAs are degraded by the action of ribonucleases (e.g. endonucleases, 3' exonucleases, and 5' exonucleases). In eukaryotes, the removal of poly(A) tail is presumed to cause disruption of the mRNA and thereby become open to degradation by the exosome complex or by the de-capping complex. Other forms of mRNA degradation entails AU-rich element decay, nonsense mediated decay, small interfering RNAs, and microRNAs.


Supplementary

Abbreviation(s)

  • messenger RNA
  • mRNA

Synonyms

  • informational RNA
  • template RNA

Derived term(s)



Further reading

See also



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