What does DNA carry?

Genetic instructions, written in a four-letter alphabet, read in groups of three. What makes the molecule work is its shape: two strands whose bases pair only in fixed ways, so each strand carries the full information needed to rebuild the other. Watson and Crick's paper ends with a famously understated line noting that the pairing immediately suggests a copying mechanism.

The scale is odd in both directions. Every cell holds about two metres of DNA folded into a nucleus a few micrometres across, and all of it in your body laid end to end would stretch to the Sun and back many times. Yet only around two per cent codes for proteins - the rest regulates, or is inert, or is the remains of ancient viruses, of which roughly eight per cent of the human genome is made.

It also survives. DNA has been recovered from horse bone frozen for 700,000 years and, in fragments, from two-million-year-old sediment in Greenland, which is how an entire lost ecosystem was reconstructed without a single fossil. That durability is what makes ancient DNA a historical source: it settled that the Plague of Justinian was Yersinia pestis, and that every non-African human carries Neanderthal ancestry.