This is the case for most of the eukaryotic mRNAs. These polypeptides usually have a related function they often are the subunits composing a final complex protein and their coding sequence is grouped and regulated together in a regulatory region, containing a promoter and an operator. Most of the mRNA found in bacteria and archaea is polycistronic,  as is the human mitochondrial genome.
Other mechanisms for circularization exist, particularly in virus mRNA. Barley yellow dwarf virus has binding between mRNA segments on its 5' end and 3' end called kissing stem loops , circularizing the mRNA without any proteins involved. Template:Cite journal needed During genome replication the circularization acts to enhance genome replication speeds, cycling viral RNA-dependent RNA polymerase much the same as the ribosome is hypothesized to cycle.
Different mRNAs within the same cell have distinct lifetimes stabilities. In bacterial cells, individual mRNAs can survive from seconds to more than an hour. The limited lifetime of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs. There are many mechanisms that lead to the destruction of an mRNA, some of which are described below.
In general, in prokaryotes the lifetime of mRNA is much shorter than in eukaryotes.
Prokaryotes degrade messages by using a combination of ribonucleases, including endonucleases, 3' exonucleases, and 5' exonucleases. It was recently shown that bacteria also have a sort of 5' cap consisting of a triphosphate on the 5' end. Inside eukaryotic cells, there is a balance between the processes of translation and mRNA decay.
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Messages that are being actively translated are bound by ribosomes , the eukaryotic initiation factors eIF-4E and eIF-4G , and poly A -binding protein. The balance between translation and decay is reflected in the size and abundance of cytoplasmic structures known as P-bodies  The poly A tail of the mRNA is shortened by specialized exonucleases that are targeted to specific messenger RNAs by a combination of cis-regulatory sequences on the RNA and trans-acting RNA-binding proteins. Poly A tail removal is thought to disrupt the circular structure of the message and destabilize the cap binding complex.
The message is then subject to degradation by either the exosome complex or the decapping complex. In this way, translationally inactive messages can be destroyed quickly, while active messages remain intact. The mechanism by which translation stops and the message is handed-off to decay complexes is not understood in detail. The presence of AU-rich elements in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these sequences and stimulate poly A tail removal.
Loss of the poly A tail is thought to promote mRNA degradation by facilitating attack by both the exosome complex  and the decapping complex. Eukaryotic messages are subject to surveillance by nonsense mediated decay NMD , which checks for the presence of premature stop codons nonsense codons in the message. These can arise via incomplete splicing, V D J recombination in the adaptive immune system , mutations in DNA, transcription errors, leaky scanning by the ribosome causing a frame shift , and other causes.
Detection of a premature stop codon triggers mRNA degradation by 5' decapping, 3' poly A tail removal, or endonucleolytic cleavage. This complex contains an endonuclease that cleaves perfectly complementary messages to which the siRNA binds. The resulting mRNA fragments are then destroyed by exonucleases.
It is thought to be part of the innate immune system as a defense against double-stranded RNA viruses. The mechanism of action of miRNAs is the subject of active research. There are other ways by which messages can be degraded, including non-stop decay and silencing by Piwi-interacting RNA piRNA , among others.
Full length mRNA molecules have been proposed as therapeutics since the beginning of the biotech era but there was little traction until the s, when Moderna Therapeutics was founded and managed to raise almost a billion dollars in venture funding in its first three years. Theoretically, the administered mRNA sequence can cause a cell to make a protein, which in turn could directly treat a disease or could function as a vaccine ; more indirectly the protein could drive an endogenous stem cell to differentiate in a desired way.
The primary challenges of RNA therapy center on delivering the RNA to directed cells, more even than determining what sequence to deliver. From Wikipedia, the free encyclopedia. Main article: Transcription genetics. Main article: Post-transcriptional modification.
Main article: 5' cap. Main article: Polyadenylation. Main article: Translation genetics.
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Main article: Coding region. See also: Cistron. Main article: Nonsense mediated decay. Main article: siRNA. Main article: microRNA. Molecular Biology of the Cell. The Plant Journal.
Comparison of total and cytoplasmic mRNA reveals global regulation by nuclear retention and miRNAs
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Our Awards Booktopia's Charities. Are you sure you would like to remove these items from your wishlist? Info Print Cite. Submit Feedback. Thank you for your feedback. Alternative Titles: mRNA, messenger ribonucleic acid. Read More on This Topic. Learn More in these related Britannica articles:. Messenger RNA mRNA delivers the information encoded in one or more genes from the DNA to the ribosome, a specialized structure, or organelle, where that information is decoded into a protein.