Messenger RNA is an RNA molecule that carries genetic information from DNA or, in some organisms and biological systems, another nucleic acid template to the machinery responsible for protein synthesis. In cellular organisms, messenger RNA is transcribed from protein coding genes and contains coding information organised into codons that determine the amino acid sequence of a polypeptide.
In eukaryotic cells, newly synthesised messenger RNA commonly undergoes processing that includes addition of a five prime cap, removal of introns through splicing and formation of a three prime polyadenylated region. Mature messenger RNA is exported from the nucleus and can be translated by ribosomes in the cytoplasm or at specialised cellular locations. Messenger RNA therefore serves as a temporary information carrier connecting genome sequence with protein production.
Eukaryotic Messenger RNA is typically produced initially as a precursor transcript containing both exons and introns. Processing converts this precursor into a mature transcript capable of efficient translation and controlled stability. The five prime cap contributes to RNA stability, nuclear export and recognition by translation initiation machinery. Splicing removes introns and joins exons, while alternative splicing can generate multiple mature messenger RNA variants from a single gene.
Cleavage and polyadenylation produce the three prime end, with the polyadenylated tail contributing to stability, translation efficiency and interactions with RNA binding proteins. These processes mean that the final messenger RNA sequence may differ substantially from the original continuous DNA region. Regulation can occur at every stage, including transcription, processing, export, localisation, translation and degradation.
Messenger RNA has several structurally important regions. The five prime untranslated region lies before the coding sequence and can influence translation initiation, RNA stability and regulatory interactions. The coding sequence contains codons that specify the protein sequence according to the Genetic Code. The three prime untranslated region follows the coding sequence and can contain regulatory elements recognised by RNA binding proteins and small RNAs. The polyadenylated tail, where present, influences stability and interactions with proteins.
These regions do not all encode protein, but they can have substantial effects on how much protein is produced and when. The distinction between untranslated regions and coding sequence is therefore important because an RNA molecule can contain extensive regulatory information that is not translated into amino acids.
Messenger RNA is inherently dynamic. Unlike genomic DNA, which is generally maintained as a relatively stable information repository, individual messenger RNA molecules are produced and degraded continuously. RNA degradation involves multiple pathways and can determine how long a transcript remains available for translation. Transcript stability varies greatly among genes and cellular conditions. RNA degradation can begin through deadenylation, decapping, exonucleolytic digestion or other pathways.
Quality control mechanisms can identify defective messenger RNAs containing premature termination signals, stalled ribosomes or abnormal structures. The balance between transcription, processing, export, translation and degradation determines the abundance of a particular messenger RNA at a given time. Measuring messenger RNA abundance therefore provides information about gene expression but does not directly establish the amount or activity of the corresponding protein.
Alternative splicing gives Messenger RNA an important role in generating transcript diversity. A single gene can produce multiple messenger RNA isoforms by selecting different exon combinations, alternative splice sites or alternative transcription termination arrangements. Different isoforms can encode proteins with different domains, cellular localisations, regulatory properties or stability. Some alternatively processed transcripts may not produce proteins and instead participate in regulatory pathways or undergo degradation.
RNA editing can also alter messenger RNA sequence after transcription in certain organisms and cell types. These mechanisms demonstrate that the relationship between genomic DNA and protein sequence is not always a simple one to one correspondence. Post transcriptional regulation can substantially expand the functional output of a genome without requiring a corresponding increase in gene number.
Messenger RNA localisation provides another level of regulation. Some transcripts are transported to particular cellular regions where local translation produces proteins close to their sites of function. Localised messenger RNA is important in developing cells, neurons, embryos and other systems in which spatially restricted protein production is required. RNA binding proteins and regulatory RNA molecules can influence transport, stability and translation. Cellular stress can also alter messenger RNA organisation, translation and degradation.
In addition, messenger RNAs can associate with ribosomes in polysomes, allowing several ribosomes to translate the same transcript simultaneously. The functional lifetime and location of an RNA molecule are therefore as important as its nucleotide sequence. Messenger RNA biology is consequently a major component of post transcriptional gene regulation rather than simply an intermediate step between transcription and translation.
Messenger RNA has become important in biotechnology and medicine. Synthetic messenger RNA can be introduced into cells to produce selected proteins without permanently altering genomic DNA. Messenger RNA based approaches have been investigated for vaccination, protein replacement, transient gene expression and other therapeutic purposes. Successful synthetic RNA design can depend on nucleotide sequence, codon usage, untranslated regions, cap structure, polyadenylation and chemical modification.
Delivery systems are also critical because naked RNA is susceptible to degradation and may trigger innate immune responses. Laboratory techniques such as reverse transcription and quantitative amplification can measure messenger RNA abundance, while sequencing can characterise transcript sequences and isoforms. These applications illustrate how understanding the natural biology of messenger RNA directly informs modern biotechnology.
Messenger RNA should be distinguished from transfer RNA and ribosomal RNA because their primary functions differ. Messenger RNA provides the information template for protein synthesis, transfer RNA acts as the amino acid adaptor and ribosomal RNA forms a major structural and catalytic component of ribosomes. Messenger RNA should also be distinguished from DNA because RNA is chemically different, usually single stranded, uses uracil rather than thymine and is generally more transient in cellular function.
Not every RNA transcript is messenger RNA, and not every messenger RNA is necessarily translated efficiently. Viral systems can use RNA genomes or RNA molecules with specialised functions that do not fit the typical cellular messenger RNA model. The biological significance of Messenger RNA lies in its regulated movement of genetic information between transcriptional control and protein synthesis.
