DNA

From TARDIS Index File, the free Doctor Who reference.

DNA (Deoxyribonucleic acid) is the nucleic acid that serves as the basis for carbon-based life. The blueprint for a living organism, it codes for proteins. (DW: School Reunion)

While Humans have two strands, Time Lords have three strands, and can have one set of strands for each regeneration. (DW: The End of the World)