Nature and Evolution of Early Replicons

RNA and protein molecules were found to be both templates for replication and specific catalysts for biochemical reactions. RNA molecules, although very difficult to obtain via plausible synthetic pathways under prebiotic conditions, are the only candidates for early replicons. Only they are obligatory templates for replication which can conserve mutations and propagate them to forthcoming generations. RNA based catalysts, called ribozymes, act with high efficiency and specificity on all classes of reactions involved in the interconversion of RNA molecules such as cleavage and template assisted ligation. The idea of an RNA world was conceived for a plausible prebiotic scenario of RNA molecules operating upon each other and constituting thereby a functional molecular organization. A theoretical account on molecular replication making precise the conditions under which one observes parabolic or exponential growth is presented. Exponential growth is observed in a protein assisted RNA world where plus-minus-(±)-duplex formation is avoided by the action of an RNA replicase. Error propagation to forthcoming generations is analyzed in absence of selective neutral mutants as well as for predefined degrees of neutrality. A model of evolution is proposed that allows to deal explicitly with phenotypes. To appear in Origin and Evolution of Viruses,edited by E. Domingo, R. Webster, and J. Holland, Academic Press, forthcoming.