How did RNA increase in size in a prebiotic scenario?

What is it about?

It is generally considered that if an RNA World ever existed that it would be driven by an RNA capable of RNA replication. Whether such a catalytic RNA could emerge in an RNA World or not, there would need to be prior routes to increasing complexity in order to produce it. It is hypothesized here that increasing sequence variety, if not complexity, can in fact readily emerge in response to a dynamic equilibrium between synthesis and degradation. A model system in which T4 RNA ligase catalyzes synthesis and Benzonase catalyzes degradation was constructed. An initial 20-mer served as a seed and was subjected to 180 minutes of simultaneous ligation and degradation. The seed RNA rapidly disappeared and was replaced by an increasing number and variety of both larger and smaller variants. The resulting RNA products generally lack point mutations but instead reflect additions and subtractions of fragments of the original RNAs. The system unequivocally demonstrates that if such equilibrium were established in a prebiotic world it would result in significant exploration of RNA sequence space and possibly increased complexity. It remains to be seen if the variety of products produced is affected by the presence of small peptide oligomers.

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Why is it important?

A system of simultaneous RNA ligation and degradation is achievable. In a prebiotic world, the starting sequence would almost certainly be a mixture. This work follows the consequences of the offspring of one initial sequence lacking significant amounts of secondary structure so as to minimize any problems that might arise due to structure. From an origins perspective, the present system demonstrates that simultaneous ligation and degradation can rapidly produce RNAs of increasing variety in the absence of a replicase.

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