DNA

DNA (Deoxyribonucleic acid) was the nucleic acid that served as the basis for carbon-based life. (TV: School Reunion) It was noted for its double-helix shape, at least in human societies.

The Kinda wore necklaces embossed with a design akin to the double-helix. This showed they had once been an advanced civilisation, (TV: Kinda) and possibly hinted that the double-helix design was a characteristic of humanoids.

In an alternate Paris of 1804, Garce and Eging commented on the idea of the double-helix, finding it ridiculous that two snakes coiled together were inside all humans. (PROSE: The Man in the Velvet Mask) While humans and Daleks (possibly also Kaleds) had two strands, Time Lords had three. They might have had one set of strands for each regeneration. (TV: The End of the World) According to the Eighth Doctor, he had four strands, but only three were visible to non-temporally aligned scanners (therefore part of his DNA was linked to the Time Vortex). (PROSE: Frontier Worlds)

River Song's part-Time Lord identity was visible from scanning her DNA as an infant. It is unclear if her DNA was double-helix, triple-helix, or something else. (TV: A Good Man Goes to War)