The origin of the RNA world: A kinetic model

The aims of this paper are to propose, construct, and analyze microscopic k inetic models for the emergence of long chains of RNA from monomeric beta-D -ribonucleotide precursors in prebiotic circumstances. Our theory starts ou t from similar but more general chemical assumptions to those of Eigen,(1) namely, that catalytic replication can lead to a large population of long c hains. Ln particular, our models incorporate the possibility of (i) direct chain growth, (ii) template-assisted synthesis, and (iii) catalysis by RNA replicase ribozymes, all with varying degrees of efficiency. However, in ou r models the reaction mechanisms are kept "open"; we do not assume the exis tence of closed hypercycles which sustain a population of long chains. Rath er it is the feasibility of the initial emergence of a self-sustaining set of RNA chains from monomeric nucleotides which is our prime concern. Moreov er, we confront directly the central nonlinear features of the problem, whi ch have often been overlooked in previous studies. Our detailed microscopic kinetic models lead to kinetic equations which are generalisations of the Becker-Doring system for the stepwise growth of clusters or polymer chains; they lie within a general theoretical framework which has recently been su ccessfully applied to a wide range of complex chemical problems. In fact, t he most accurate model we consider has Becker-Doring aggregation terms, tog ether with a general Smoluchowski fragmentation term to model the competing hydrolysis of RNA polymer chains. We conclude that, starting from reasonab le initial conditions of monomeric nucleotide concentrations within a prebi otic soup and in an acceptable timescale, it is possible for a self-replica ting subset of polyribonucleotide chains to be selected, while less efficie nt replicators become extinct.