Within your body, a huge amount of information is copied over and over again, reliably and predictably. Your life depends on it. Typos occur, but they are quickly corrected. Edits are made, but sparingly. Or, at least, that’s what we thought.

It starts with DNA. This famous molecule is a chain of four ‘bases’, denoted by the letters A, C, G and T. These four letters, in various combinations, contain instructions for building thousands of proteins, a workforce of molecular machines that keep you alive and well. But first, DNA has to be copied (or “transcribed”) into a related molecule called RNA. It too is made of four bases: A, C and G reprise their roles, but U stands in for