messenger rna messenger rna in molecular biology, messenger ribonucleic acid (mrna) is a single-stranded molecule of rna that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. the "life cycle" of an mrna in a eukaryotic cell. rna is transcribed in the nucleus; after processing, it is transported to the cytoplasm and translated by the ribosome. finally, the mrna is degraded. mrna is created during the process of transcription, where an enzyme (rna polymerase) converts the gene into primary transcript mrna (also known as pre-mrna). this pre-mrna usually still contains introns, regions that will not go on to code for the final amino acid sequence. these are removed in the process of rna splicing, leaving only exons, regions that will encode the protein. this exon sequence constitutes mature mrna. mature mrna is then read by the ribosome, and, utilising amino acids carried by transfer rna (trna), the ribosome creates the protein. this process is known as translation. all of these processes form part of the central dogma of molecular biology, which describes the flow of genetic information in a biological system. as in dna, genetic information in mrna is contained in the sequence of nucleotides, which are arranged into codons consisting of three ribonucleotides each. each codon codes for a specific amino acid, except the stop codons, which terminate protein synthesis. the translation of codons into amino acids requires two other types of rna: transfer rna, which recognizes the codon and provides the corresponding amino acid, and ribosomal rna (rrna), the central component of the ribosome's protein-manufacturing machinery. the concept of mrna was developed by sydney brenner and francis crick in 1960 during a conversation with françois jacob. in 1961, mrna was identified and described independently by one team consisting of brenner, jacob, and matthew meselson, and another team led by james watson. while analyzing the data in preparation for publication, jacob and jacques monod coined the name "messenger rna". synthesis rna polymerase transcribes a dna strand to form mrna the brief existence of an mrna molecule begins with transcription, and ultimately ends in degradation. during its life, an mrna molecule may also be processed, edited, and transported prior to translation. eukaryotic mrna molecules often require extensive processing and transport, while prokaryotic mrna molecules do not. a molecule of eukaryotic mrna and the proteins surrounding it are together called a messenger rnp. transcription transcription is when rna is copied from dna. during transcription, rna polymerase makes a copy of a gene from the dna to mrna as needed. this process differs slightly in eukaryotes and prokaryotes. one notable difference is that prokaryotic rna polymerase associates with dna-processing enzymes during transcription so that processing can proceed during transcription. therefore, this causes the new mrna strand to become double stranded by producing a complementary strand known as the trna strand, which when combined are unable to form structures from base-pairing. moreover, the template for mrna is the complementary strand of trna, which is identical in sequence to the anticodon sequence that the dna binds to. the short-lived, unprocessed or partially processed product is termed precursor mrna, or pre-mrna; once completely processed, it is termed mature mrna. eukaryotic pre-mrna processing dna gene is transcribed to pre-mrna, which is then processed to form a mature mrna, and then lastly translated by a ribosome to a protein processing of mrna differs greatly among eukaryotes, bacteria, and archaea. non-eukaryotic mrna is, in essence, mature upon transcription and requires no processing, except in rare cases. eukaryotic pre-mrna, however, requires several processing steps before its transport to the cytoplasm and its translation by the ribosome. splicing the extensive processing of eukaryotic pre-mrna that leads to the mature mrna is the rna splicing, a mechanism by which introns or outrons (non-coding regions) are removed and exons (coding regions) are joined. 5' cap addition 5' cap structure a 5' cap (also termed an rna cap, an rna 7-methylguanosine cap, or an rna m7g cap) is a modified guanine nucleotide that has been added to the "front" or 5' end of a eukaryotic messenger rna shortly after the start of transcription. the 5' cap consists of a terminal 7-methylguanosine residue that is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. its presence is critical for recognition by the ribosome and protection from rnases. cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. shortly after the start of transcription, the 5' end of the mrna being synthesized is bound by a cap-synthesizing complex associated with rna polymerase. this enzymatic complex catalyzes the chemical reactions that are required for mrna capping. synthesis proceeds as a multi-step biochemical reaction. editing in some instances, an mrna will be edited, changing the nucleotide composition of that mrna. an example in humans is the apolipoprotein b mrna, which is edited in some tissues, but not others. the editing creates an early stop codon, which, upon translation, produces a shorter protein. polyadenylation polyadenylation polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger rna molecule. in eukaryotic organisms most messenger rna (mrna) molecules are polyadenylated at the 3' end, but recent studies have shown that short stretches of uridine (oligouridylation) are also common. the poly(a) tail and the protein bound to it aid in protecting mrna from degradation by exonucleases. polyadenylation is also important for transcription termination, export of the mrna from the nucleus, and translation. mrna can also be polyadenylated in prokaryotic organisms, where poly(a) tails act to facilitate, rather than impede, exonucleolytic degradation. polyadenylation occurs during and/or immediately after transcription of dna into rna. after transcription has been terminated, the mrna chain is cleaved through the action of an endonuclease complex associated with rna polymerase. after the mrna has been cleaved, around 250 adenosine residues are added to the free 3' end at the cleavage site. this reaction is catalyzed by polyadenylate polymerase. just as in alternative splicing, there can be more than one polyadenylation variant of an mrna. polyadenylation site mutations also occur. the primary rna transcript of a gene is cleaved at the poly-a addition site, and 100–200 a's are added to the 3' end of the rna. if this site is altered, an abnormally long and unstable mrna construct will be formed. transport another difference between eukaryotes and prokaryotes is mrna transport. because eukaryotic transcription and translation is compartmentally separated, eukaryotic mrnas must be exported from the nucleus to the cytoplasm—a process that may be regulated by different signaling pathways. mature mrnas are recognized by their processed modifications and then exported through the nuclear pore by binding to the cap-binding proteins cbp20 and cbp80, as well as the transcription/export complex (trex). multiple mrna export pathways have been identified in eukaryotes. in spatially complex cells, some mrnas are transported to particular subcellular destinations. in mature neurons, certain mrna are transported from the soma to dendrites. one site of mrna translation is at polyribosomes selectively localized beneath synapses. the mrna for arc/arg3.1 is induced by synaptic activity and localizes selectively near active synapses based on signals generated by nmda receptors. other mrnas also move into dendrites in response to external stimuli, such as β-actin mrna. upon export from the nucleus, actin mrna associates with zbp1 and the 40s subunit. the complex is bound by a motor protein and is transported to the target location (neurite extension) along the cytoskeleton. eventually zbp1 is phosphorylated by src in order for translation to be initiated. in developing neurons, mrnas are also transported into growing axons and especially growth cones. many mrnas are marked with so-called "zip codes," which target their transport to a specific location. mrnas can also transfer between mammalian cells through structures called tunneling nanotubes. translation translation of mrna to protein because prokaryotic mrna does not need to be processed or transported, translation by the ribosome can begin immediately after the end of transcription. therefore, it can be said that prokaryotic translation is coupled to transcription and occurs co-transcriptionally. eukaryotic mrna that has been processed and transported to the cytoplasm (i.e., mature mrna) can then be translated by the ribosome. translation may occur at ribosomes free-floating in the cytoplasm, or directed to the endoplasmic reticulum by the signal recognition particle. therefore, unlike in prokaryotes, eukaryotic translation is not directly coupled to transcription. it is even possible in some contexts that reduced mrna levels are accompanied by increased protein levels, as has been observed for mrna/protein levels of eef1a1 in breast cancer. structure the structure of a mature eukaryotic mrna. a fully processed mrna includes a 5' cap, 5' utr, coding region, 3' utr, and poly(a) tail. coding regions coding regions are composed of codons, which are decoded and translated into proteins by the ribosome; in eukaryotes usually into one and in prokaryotes usually into several. coding regions begin with the start codon and end with a stop codon. in general, the start codon is an aug triplet and the stop codon is uag ("amber"), uaa ("ochre"), or uga ("opal"). the coding regions tend to be stabilised by internal base pairs; this impedes degradation. in addition to being protein-coding, portions of coding regions may serve as regulatory sequences in the pre-mrna as exonic splicing enhancers or exonic splicing silencers. untranslated regions universal structure of eukaryotic mrna, showing the structure of the 5' and 3' utrs. untranslated regions (utrs) are sections of the mrna before the start codon and after the stop codon that are not translated, termed the five prime untranslated region (5' utr) and three prime untranslated region (3' utr), respectively. these regions are transcribed with the coding region and thus are exonic as they are present in the mature mrna. several roles in gene expression have been attributed to the untranslated regions, including mrna stability, mrna localization, and translational efficiency. the ability of a utr to perform these functions depends on the sequence of the utr and can differ between mrnas. genetic variants in 3' utr have also been implicated in disease susceptibility because of the change in rna structure and protein translation. the stability of mrnas may be controlled by the 5' utr and/or 3' utr due to varying affinity for rna degrading enzymes called ribonucleases and for ancillary proteins that can promote or inhibit rna degradation. 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