Any evidence that some organisms use bases other than A T C & G?

[Nereid]

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!

 

[Monique]

U (urasil) replaces thymidine in RNA, but this doesn't answer your question. I'm not sure about a genetic code based on a divergent system, but on a higher level there is: some organisms have alternative codon usage (http://www.genetics.org/cgi/content/full/170/2/831).

This article might be of interest to you:
http://www.ncbi.nlm.nih.gov/pubmed/14746991

 

[Flatland]

I actually have an addition to that question. Would it ever be possible for new base pairs to evolve?

 

[Monique]

If the change is gradual, why not. Here is an interesting article where new base pairs were engineered: http://www.newscientist.com/article/dn13252-artificial-letters-added-to-lifes-alphabet.html

 

[Nereid]

U (urasil) replaces thymidine in RNA, but this doesn't answer your question. I'm not sure about a genetic code based on a divergent system, but on a higher level there is: some organisms have alternative codon usage (http://www.genetics.org/cgi/content/full/170/2/831).

This article might be of interest to you:
http://www.ncbi.nlm.nih.gov/pubmed/14746991

Thanks Monique.

I knew about U in RNA, and that, at a higher level, codon usage does not follow a single, universal, scheme.

I'm interested in the fact that, in 3+billion years of evolution, just one basic DNA code (ATCG) seems to be used. It would seem exceedingly unlikely that, had there been alternatives in the history of life*, we would be able to find any evidence of them.

On the other hand, with 'synthetic life' almost within reach, it may be possible - several decades or more from now - to determine if any such alternatives could have been, or could be!, the basis for life on an Earth-like planet.

* not counting pre-DNA life