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, and could have 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)