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Possible Progenitor of DNA Re-Created

A new molecule may be the re-creation of a relic from the first life on Earth. So-called TNA acts a lot like RNA, but TNA's four-carbon sugars would have formed more easily than five-carbon-sugar RNA. That means TNA might have been an early, self-replicating version of today's nucleotides.

Biologists suspect that life as we know it emerged from an "RNA world" in which RNA molecules could copy themselves and store genetic information. But where the first RNA came from is a mystery; it's hard to see how the chemicals on early Earth could have combined to form the complicated nucleotides that make up RNA.

Looking for possible progenitors, a team led by chemist Albert Eschenmoser of the Scripps Research Institute in La Jolla, California, and the Federal Institute of Technology in Zürich, Switzerland, and colleagues hunted for molecules similar to RNA that could be constructed from simpler pieces. Each of the nucleotides that make up RNA consists of a single sugar group made of five carbon atoms. So the team slimmed down the nucleotides by using sugars with four carbons, then hooked them up to form a molecule they call "TNA" (rather than, well, try to pronounce "(L)-_-(3->2) threofuranosyl oligonucleotides.")

The sugar alteration shortens the link between successive nucleotides by one atom, but a TNA molecule can still match its sequence with sequences of RNA and DNA, in addition to other molecules of TNA, the authors report in the 17 November issue of Science. Although Eschenmoser is hesitant to speculate that TNA is a precursor to RNA, such a base pairing would be necessary for any RNA ancestor to self-replicate and, at some point in evolution, pass its information to RNA.

It's easier to imagine the spontaneous formation of the nucleotides that make up TNA than those that make up RNA itself, says chemist Leslie Orgel of the Salk Institute for Biological Studies in La Jolla, California, in part because the sugars could be easily assembled by pairing identical two-carbon fragments. But nailing down exactly what preceded RNA will be difficult, Orgel says, because "it's like any history--one can't be absolutely sure since you weren't there."