From the Primitive Atmosphere to the Prebiotic Soup to the Pre-RNA World

摘要

Organic compounds would have been produced in an earth's atmosphere that was reducing. The soup would contain amino and hydroxy acids, together with smaller amounts of purines and pyrimidines. The presence of sugars is less likely, although they can be produced by the formose reaction from formaldehyde. However, the prebiotic synthesis of RNA has not been demonstrated. One problem is that ribose is not produced selectively over other pentoses and hexoses, except under special conditions. The second problem is that ribose is unstable, with a half-life at pH7 and 100 C of 73 minutes (44 years at 0 C). Other sugars are similarly unstable. Another problem is that there is no efficient prebiotic synthesis of polyphosphates, nor the glycosidic bond of nucleosides. This suggests that there may have been an informational macromolecule that preceded RNA. The RNA world refers to the time when RNA carried both the genetic information and the catalytic activity, and was subsequently converted to the DNA/protein world when protein synthesis began. Preceeding the RNA world was the Pre-RNA world, where a backbone different from ribose phosphate was used, and the bases may have been different from adenine, uracil, guanine and cytosine. We have shown recently that cytosine and uracil can be synthesized efficiently under prebiotic conditions using a dried lagoon model instead of the usual dilute ocean hypothesis. In addition, we have shown that uracil adds formaldehyde efficiently to give 5-hydroxymethyl uracil, which in turn adds various nucleophiles to give uracil analogs of most of the amino acids that occur in proteins. The major problem is finding out what was the precursor to the ribose phosphate backbone. This will be the key to developing prebiotic self-replicating systems.