Image: after Science

DNA and RNA are the molecules that evolution chose to store all genetic information. But at the time when life began, the prebiotic soup may have contained other information molecules, which were ultimately superceded by present day nucleic acids. In last week¿s Science, Albert Eschenmoser and colleagues from La Jolla and Zurich show that a molecule they call TNA could have been one such alternative.

TNA, like DNA and RNA, has a sugar phosphate backbone that carries information as a sequence of four different bases (A,T,C and G for short). But instead of ribose (as in RNA) or deoxyribose (as in DNA), both five-carbon sugars, TNA contains threose, a four-carbon sugar (see illustration above). In TNA, the phosphate link between sugars occurs between the third carbon and the second carbon of neighboring threoses (3¿-2¿)--not between the third carbon and the fifth carbon (3¿-5¿) of adjacent sugars, as is the case in DNA and RNA.

The researchers chemically synthesized TNA in a number of steps and found that complementary TNA molecules pair up to Watson-Crick double helices. Moreover, they form double helices with complementary RNAs and DNAs. This ability is important because it shows that TNA could serve as a template to reproduce (replicate) itself. Scientists generally assume that life started in an "RNA world." But because four-carbon sugars like threose can be made from two identical two-carbon units, TNA or related polymers maybe developed more easily. Thus, TNA and its relatives could have been the precursors of RNA on primitive Earth.

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