CHAPTER 1 - Early Replicons: Origin and Evolution

Abstract

RNA and protein molecules have been 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 for 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 of molecular replication making precise the conditions under which one observes parabolic, exponential, or hyperbolic 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 the absence of selective by neutral mutants as well as for predefined degrees of neutrality. The concept of an error threshold for sufficiently precise replication and survival of populations derived from the theory of molecular quasispecies is discussed. Computer simulations are used to model the interplay between adaptive evolution and random drift. A model of evolution is proposed that allows for explicit handling of phenotypes.