messenger RNA (mRNA)
/ MES-en-jer AR-en-AY /
Your DNA stays locked in the nucleus, but proteins are built out in the main body of the cell, sometimes far away. Something has to carry the gene's instructions from the archive to the construction site. Messenger RNA is that courier. It is the RNA copy of a gene that physically delivers the protein-building instructions from DNA to the cell's protein factories.
An mRNA is a single strand of RNA whose letter sequence spells out, in three-letter words called codons, the order of amino acids for one protein. In eukaryotes it is the finished, processed product of transcription: a primary transcript that has been capped at the front, given a poly-A tail at the back, and had its introns spliced out. The middle stretch carries the actual code, flanked by untranslated regions at each end that help control how it is handled. Once mature, the mRNA is exported from the nucleus to be read by ribosomes.
The word messenger captures its whole purpose: it is a temporary, expendable go-between, not a permanent record. A typical mRNA is read by ribosomes some number of times and then deliberately broken down, which lets the cell adjust protein output on the fly. This disposability is exactly what makes mRNA useful in medicine — mRNA vaccines simply supply the cell with a short-lived message to make one chosen protein, then let the cell destroy it as usual.
An mRNA vaccine is just a man-made messenger RNA: it carries the instructions for one viral protein, your ribosomes read it to make that protein, your immune system learns the shape, and then your cells degrade the mRNA — it never enters or changes your DNA.
An mRNA vaccine supplies a short-lived message; it never enters or alters your DNA.
mRNA is only one type of RNA — the kind that codes for protein. Many RNAs (tRNA, rRNA, microRNA) are never translated into protein at all; they do their jobs as RNA.