Yet another question about the evolution of life, likely in the Archaen or Proterozoic eons. As I understand it, the genomes of all life - archaea, bacteria, eukaryotes - is based on DNA, even that of eukaryote organelles, such as plastids and mitochondria. Are there any counter-examples, in forms of life with us today (i.e. not fossils)? Viruses excluded. Is the DNA of all forms of life based on only A, T, C, and G? Or are there some other bases, used by some weird archaea, say? If so, details please!