MRNA

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Messenger RNA (mRNA) is an important form of RNA, both in the body and in the lab. Physiologically, it is used as the coding template for proteins, i.e. it directly transcribes the sequence of nucleotides from the DNA template (in a process known as transcription), forming a second complementary strand which is later processed by rRNA and tRNA (participates in a process known as translation) to form a new DNA strand.

There are two forms of mRNA that can be found in eukaryotic cells: pre-mRNA and mature mRNA.

Pre mRNA is the exact copy of the DNA sequence, containing introns and exons, that is made during translation. For the mRNA to be usable in transcription it must first be spliced by a large enzyme complex called spliceosome which removes the introns (non-coding regions of DNA) from the pre mRNA. Pre mRNA cannot be used for recombination techniques with prokaryotes as prokaryotes are unable to process the introns in pre mRNA.

Mature mRNA is the product when pre mRNA undergoes processing. This involves a process called splicing which removes the non-coding introns. Processing also encompasses joining the coding exons together, an addition of a CAP at the 5' end and adding a poly A tail to the 3' end, to form a new strand with an uninterrupted sequence, which is then translated into amino acids and proteins. Mature mRNA is required for protein cloning in the lab as it does not require any further processing by the prokaryotes.

In the cell, mRNA has a half-life which dictates how long until it degrades naturally, or by enzymatic interactions. The stability of the mRNA can be affected by a number of factors including: length of the poly-A tail, or other proteins bound to it which prevent the action of degrading enzymes.

mRNA can also be used to make single stranded cDNA (copy DNA). An enzyme called Reverse transcriptase is used to convert mRNA into cDNA. mRNA uses the nucleotide uracil, instead of thymine, which is found in DNA.

mRNA can be degraded via by a main endonuclease (enzyme). Exosomes are the principle 3'-5' exonuclease and comprises of 3 main subunits; RRP6 subunit is only found in the nucleus and is 'preferentially associated with structured targets[1]. The RRP44 subunit also works from the 3'-5' end. Finally, the core subunit is involved in RNA binding and acts as a helicase by unwinding the RNA.

References

  1. Claudia Schneider, Grzegorz Kudla, Wiebke Wlotzka, Alex Tuck, David Tollervey1; Mol Cell. 2012 Nov 9; 48(3-3): 422–433
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