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5.3.1.2. Messenger RNA

Messenger RNA

The messenger RNA (mRNA) is single stranded, linear basically. It only constitutes 2-5% of the total RNA.

The function of mRNA is to take the information from the DNA, which is in the nucleus, and take it to the cytoplasm, where the ribosomes are where the proteins will be synthesized with the amino acids provided by the tRNAs.

mRNA is formed from a strand of DNA in a process called transcription. With the complementary nitrogenous bases, a template is created with the genetic information necessary for protein synthesis. The size of the mRNA depends on the size of the protein for which it carries information. After performing their function, protein synthesisribonuclease enzymes destroy it to prevent unnecessary protein production. When the synthesis of a particular protein is needed again, new mRNA will be created.

The information contained in the mRNA is presented in a sequence of nitrogenous bases, grouped in triplets or codons, each of which determines the binding of a certain amino acid .

The mRNA has a different structure in prokaryotes  and in eukaryotes.

By The original uploader was Sverdrup at English Wikipedia (Transferred from en.wikipedia to Commons.) [Public domain], via Wikimedia Commons

mARN in eukaryotes

The messenger RNA obtained after transcription is known as primary transcribed RNA or precursor RNA or pre-RNA, which in most cases is not released from the transcription complex in a fully active form, but must undergo modifications before exerting its function (RNA processing or maturation). Among these modifications are the elimination of fragments ( splicing ), the addition of other non-coding in the DNA and the covalent modification of certain nitrogenous bases.

Eukaryotic mRNA is formed from primary transcribed RNA (pre-mRNA), which is made up of two types of alternating segments:

  • Exons, segments with information.
  • Introns, segments without information that will be deleted and do not appear in the mRNA.

The maturation of the RNA primary transcript to mRNA occurs in the nucleus, and is in the process when lost introns.

El pre-ARNmensajero pierdo los intrones en su proceso de maduración

By Qef [Public domain], via Wikimedia Commons

Eukaryotic mRNA has a cap at its 5' end, formed by a nucleotide derived from guanine, which gives stability to the mRNA and allows access to the ribosome for protein synthesis. Then there is a segment without information, followed by another segment with information that usually begins with the bases A-U-G.

At the 3' end, about 200 adenine nucleotides appear, the so-called poly-A "tail", which stabilizes the molecule against exonuclease enzymes.

Eukaryotic mRNA is monocistronic, that is, it only contains information to synthesize one polypeptide chain.

mRNA in prokaryotes

The transcription and translation processes are carried out in a similar way as in eukaryotic cells. The fundamental difference is that, in prokaryotes, the messenger RNA does not undergo a maturation process and, therefore, no cap or tail is added to it or introns are removed. Also, it does not have to leave the nucleus as in eukaryotes, because in prokaryotic cells there is no defined nucleus.

The mRNA in prokaryotes is polycistronic, it contains separate information to synthesize different proteins.

Fundamental ideas about messenger RNA

Messenger RNA:

  • MRNA is to take the information from the DNA (transcription), which is in the nucleus, and take it to the cytoplasm, where the ribosomes will carry out the  protein synthesis with the amino acids provided by the tRNAs.
  • Differences of mRNA in prokaryotic and eukaryotic cells :
    • In eukaryotic cells :
      • mRNA needs maturation. Following transcription is obtained RNA primary transcript or precursor RNA or pre-mRNA, comprises:
        • Exons: segments with information.
        • Introns: segments with no information that are suppressed and do not appear in mature mRNA.
      • MRNA is monocistronic.
    • In prokaryotic cells:
      • mRNA does not need maturation.
      • mRNA is polycistronic.