  Due to the asymmetric nature of the nucleotides, the extant informational
biomolecule, DNA, is constrained to replicate unidirectionally on a template.
As a product of molecular evolution that sought to maximize replicative
potential, DNA's unidirectional replication poses a mystery since symmetric
bidirectional self-replicators obviously would replicate faster than
unidirectional self-replicators and hence would have been evolutionarily more
successful. Here we carefully examine the physico-chemical requirements for
evolutionarily successful primordial self-replicators and theoretically show
that at low monomer concentrations that possibly prevailed in the primordial
oceans, asymmetric unidirectional self-replicators would have an evolutionary
advantage over bidirectional self-replicators. The competing requirements of
low and high kinetic barriers for formation and long lifetime of inter-strand
bonds respectively are simultaneously satisfied through asymmetric kinetic
influence of inter-strand bonds, resulting in evolutionarily successful
unidirectional self-replicators.
