Did the Pre-RNA World Rest Upon DNA Molecules?
The isolation of a DNA sequence that catalyzes the ligation of oligodeoxynucleotides via the formation of 3' - 5' phosphodiester linkage significance in selection experiments has been reported. Ball recently used this to discuss the possibility that natural DNA molecules may have formed in the primitive Earth leading to the origin of life. As noted by Ferris and Usher, if metabolic pathways evolved backwards, it could be argued that the biosynthesis of 2-deoxyribose from ribose suggests that RNA came from DNA. As summarized elsewhere, there are several properties of deoxyribose which could be interpreted to support the possibility that DNA-like molecules arose prior to the RNA world. For example, 2-deoxyribose is slightly more soluble than ribose (which may have been an advantage in a drying pool scenario), may have been more reactive under possible prebiotic conditions (it forms a nucleoside approx. 150 times faster than ribose with the alternative base urazole at 25 C), while it decomposes in solution (approximately 2.6 times more slowly than ribose at 100 C). Other advantages of DNA over RNA are that it has one fewer chiral center, has a greater stability at the 8.2 pH value of the current oceans, and does not has the 2'5' and 3'5' ambiguity in polymerizations. Yet, there is strong molecular biological and biochemical evidence that RNA was featured in the biology well before the last common ancestor. The presence of sugar acids, including both ribo- and deoxysugar acids, in the 4.6 Ga old Murchison meteorite suggest that both may have been available in the primitive Earth, derived from the accretion of extraterrestrial sources and/or from endogenous processes involving formaldehyde and its derivatives. However, the abiotic synthesis of deoxyribose, ribose, and other sugars from glyceraldehyde and acetaldehyde under alkaline conditions is inefficient and unespecific. Although sugars are labile compounds, the role of cyanamide or borate minerals in the stabilization of the cyclic forms of ribose and other pentoses has recently been demonstrated. Nonetheless, the assumption either RNA or DNA was the first genetic material needs to be supplemented by laboratory models demonstrating that the prebiotic synthesis of activated beta-D-(deoxy)ribonucleotides and their polymers was feasible. As of today such evidence is lacking, and there is no convincing synthesis of any nucleotide, since all model experiments produce complex mixtures of products in which there is no preferential synthesis of chiral D-nucleotides. This strongly suggests that both DNA and RNA may have been preceded by pairing structures much simpler than extant nucleic acids. It is doubtful that DNA molecules, or indeed other (de0xy)ribofuranoid oligonucleotides formed the basis of these as yet undescribed pre-RNA worlds.