Abstract
It is possible that early life relied on RNA polymers that served as ribozyme-like catalysts and for storing genetic information. The source of such polymers is uncertain, but previous investigations reported that wet–dry cycles simulating prebiotic hot springs provide sufficient energy to drive condensation reactions of mononucleotides to form oligomers. The aim of the study reported here was to visualize the products by atomic force microscopy. In addition to globular oligomers, ring-like structures ranging from 10–200 nm in diameter, with an average around 30–40 nm, were abundant, particularly when nucleotides capable of base pairing were present. The thickness of the rings was consistent with single stranded products, but some had thicknesses indicating base pair stacking. Others had more complex structures in the form of short polymer attachments and pairing of rings. These observations suggest the possibility that base-pairing may promote polymerization during wet–dry cycling followed by solvation of the rings. We conclude that RNA-like rings and structures could have been synthesized non-enzymatically on the prebiotic Earth, with sizes sufficient to fold into ribozymes and genetic molecules required for life to begin.
Highlights
Our current understanding of the early Earth suggests that life originated approximately four billion years ago [1,2,3], and that early stages of life depended on polymers resembling RNA [4,5].It has been proposed that organic compounds from geochemical sources and meteoritic infall [6]likely accumulated on volcanic land masses emerging from a global ocean [7]
More recently it has been reported that ribonucleotides can be synthesized from simple organic compounds in conditions that incorporate a wet-dry cycle [14,15]
We have reported a series of experimental results supporting the conclusion that such conditions can drive polymerization of mononucleotides: Life 2020, 10, 321; doi:10.3390/life10120321
Summary
Our current understanding of the early Earth suggests that life originated approximately four billion years ago [1,2,3], and that early stages of life depended on polymers resembling RNA [4,5]. When tested by nanopore analysis with the alpha-hemolysin pore, the oligomers produce blockade signals virtually identical to those caused by single stranded RNA molecules. This demonstrates that at least some of the products are linear polyanionic strands that impede ionic currents as they are driven through the nanopore by an applied voltage [16,17,18]. If a 1:1 mole ratio of AMP and UMP is exposed to wet–dry cycling, the oligomeric products exhibit hyperchromicity, but the products from AMP alone do not [17] This result is consistent with hairpin structures forming in random sequence linear polymers of RNA composed of monomers capable of Watson-Crick base pairing. We employed atomic force microscopy (AFM) to visualize the products at single molecule resolution
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