messenger RNA

Messenger RNA, usually shortened to mRNA, is the working copy of a gene. A cell's DNA sits in the nucleus and stays there. When the cell needs a particular protein, it transcribes the relevant stretch of DNA into a single-stranded length of RNA, and that copy travels out into the cytoplasm, the watery interior of the cell. There the protein-making machinery reads the copy three bases at a time, and each of those three-base groups specifies one amino acid to add to a growing chain. The chain folds into the finished protein. The US National Human Genome Research Institute describes mRNA as the translated form of DNA that the machinery can recognise and use, which makes it the link between the code and the living cell that code builds.

The copy is deliberately short-lived. A cell makes mRNA when it needs a protein and breaks it down afterwards, which is why mRNA is a set of instructions rather than a permanent change: it never enters the nucleus and never becomes part of a cell's DNA.

That property is what medicines built on mRNA exploit. Rather than delivering a finished protein made in eggs, in cultured cells or in a fermentation tank, an mRNA vaccine delivers the instructions and lets the recipient's own cells make the protein for a short while. The immune system meets the protein, recognises it as foreign and learns it. Changing which protein the vaccine teaches means changing the sequence written into the mRNA, not rebuilding the manufacturing process.