Viroid-sized rings self-assemble from mononucleotides through wet-dry cycling: implications for the origin of life
It is possible that early life relied on RNA polymers that served as ribozyme-like catalysts and store genetic information1,2. 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 oligomers3. 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 template-directed synthesis may occur 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.