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- Modelling evolution on design-by-contract predicts an origin of Life through an abiotic double-stranded RNA world

The evolution of life and the origins of DNA and RNA as the carriers of information are still a mystery. It has been proposed that the DNA world was preceded by an RNA world in which RNA fulfilled a role both as the information carrier and as the catalyst of early chemical processes [1,2]. The general idea is that from a pool of random strings of RNA, ribozymes would emerge with a primitive RNA polymerase activity and, in this way, RNA could provide its own replication machinery [3-8]. The transition from a ssRNA world to the dsDNA world is thought to have arisen by a reverse transcriptase activity that copied an RNA template to dsDNA, possibly by a dsRNA intermediate [9,10]. However, the abiotic availability of strands of RNA that are long enough to function as a ribozyme or exhibit replicase activity is uncertain. There are also no indications that ribozymes with polymerase activity have ever existed since the only naturally occurring ribozymes do not perform polymerization reactions [11,12]. The transition from ssRNA as the informational carrier to dsDNA is also mechanistically difficult since in order to maintain existing catalytic function, the concomitant evolution of a transcription system based on dsDNA is needed. Therefore, the fundamental molecular mechanisms that underlie the origin of life are still unknown.

The potential of RNA to perform as both an information carrier and as a catalytic molecule, is at the basis of the RNA world hypothesis. However, it is difficult to imagine ssRNA to have a dual function since the catalytic properties of RNA depend on the three-dimensional structure, while the informational capacity would require a simple linear structure that can be replicated [13]. The folding of an RNA molecule would prevent its own replication, while for replication a folded ribozyme would be necessary. Therefore, a dual function for the informational and the catalytic functions of ssRNA seem to be mutual exclusive on a biochemical mechanistic basis. From a system perspective, a dual function in a single entity would create a dependency that would reduce evolvability of the system, since each adaptation of one function could negatively affect the other function and would prevent an independent evolution. This is for instance apparent in the transition from ssRNA to dsDNA where the double-stranded form would completely prevent function until the development of a complex transcription machinery and would therefore not be based on the fundamental principle of functional continuity in evolution. Thus, a scenario that is not dependent on a dual function of ssRNA could be a starting point for alternative mechanisms for the origin of Life.

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