DNA

DNA (Deoxyribonucleic acid) was the nucleic acid that served as the basis for carbon-based life. (DW: School Reunion)

The Kinda wore necklaces embossed with a design akin to the double-helix of DNA. (DW: Kinda)

While humans had two strands, Time Lords had three. They might have had one set of strands for each regeneration. (DW: 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). (EDA: Frontier Worlds)

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