It has long been hypothesized that due to their complexity neither DNA nor RNA could have arisen spontaneously. Because of its simpler structure and ability to perform other functions beyond information storage it has been proposed that RNA preceded DNA. However, it is still too complex to have been formed in the prebiotic world. Therefore a variety xenonucleic acids are being studied as possible candidates for the transitional nucleic acid that could be a precursor to RNA and DNA. A recent study published in the Journal of the American Chemical Society has investigated threose nucleic acid (TNA).
Similar to DNA and RNA, TNA is composed of repeating nucleotide bases held together by a sugar phosphate backbone. However for each the sugar is distinct; being either deoxyribose, ribose or threose. TNA is also similar to DNA in that it can form a double helix but employs a 4 rather than 5 carbon sugar which could arise from two 2 carbon precursors. Despite this distinction TNA can still bind with complimentary DNA and RNA strands and therefore act as an alternative self-replicating entity.
The research group from Arizona State University used commercially available synthetic polymerases to transcribe DNA to TNA and reverse transcribe TNA to DNA with 90% accuracy. These results should allow for additional analysis of other xenonucleic acids and may indicate that TNA paved the way for RNA and subsequently DNA and the emergence of earthly life.