Continuous nonenzymatic cross-replication of DNA strands with activated DNA oligonucleotides.

Continuous enzyme-free replication of oligonucleotides is central for open-ended evolution experiments that mimic the origin of life. Here, we studied a reaction system, whereby two 24mer DNA templates cross-catalyzed each other's synthesis from four 12mer DNA fragments, two of which were activated with the condensing agent 1-ethyl-3-(3-dimethylamino-propyl)carbodiimide (EDC). We circumvented the problem of product inhibition by melting the stable product duplexes for their reuse as templates in the following ligation step. The system reproduced itself through ligation/melting cycles and survived exponential dilution. We quantified EDC-induced side reactions in a detailed kinetic model. The model allowed us to analyze the effects of various reaction rates on the system's kinetics and confirmed maximal replication under the chosen conditions. The presented system enables us to study nonenzymatic open-ended evolution experiments starting from diverse sequence pools.