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. (TV: Kinda)

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 had two strands, Time Lords had three. (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. (TV: A Good Man Goes to War)