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. (TV: Evolution of the Daleks)

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)

A sentient cell culture known as the Culture was endangered by an infection of viruses which inserted their DNA into the Culture cells, turning them into nurseries for incubation of more invaders. (COMIC: Culture Shock)