The RNA World

A living system must be able to replicate its genetic material and be capable of evolving. Proteins are necessary for DNA replication, but most proteins are synthesized on RNA templates that themselves were synthesized on DNA templates.

It has been hypothesized that RNA molecules capable of self-replication arose prebiotically by random condensation of mononucleotides into small polymers. The active sites of most modern proteins and catalytic RNAs constitute relatively small segments of the polymers to which they belong. The smaller primitive RNA replicase polymers, formed abiotically, would probably have only weak catalytic activity, and would have been subject to error-prone replication. But such a molecule might have been able to use itself or other RNA molecules as a template for polymerizing RNA nucleotides. The many errors made during replication of the early RNA replicase would create a pool of genetic diversity on which natural selection could act to favor those molecules that were able to replicate faster and/or have greater accuracy. One problem, however, is that no replicase can copy its own active site. It is thus necessary to propose that a minimum of two RNA replicases were synthesized at nearly the same time from the "primordial soup" of precursors. A primitive type of cell containing an RNA genome, called the eugenote, is hypothesized to have evolved from the progenote population.

RNAmolecules were probably the primordial genome/enzyme molecules of primitive living systems. Ribose sugars are easier to synthesize under simulated primordial conditions than deoxyribose sugars. The DNA precursors of all extant cells are produced by reduction of RNAnucleoside diphosphates by the highly conserved protein enzyme ribonucleoside diphosphate reductase. This enzyme appears in all modern

Gradually, proteins took over many of the catalytic functions originally performed by RNAmolecules. This would have allowed for greater flexibility in the sequences since there are 20 amino acids and only 4 ribonucleotides. Also, three-dimensional shapes in RNA molecules would require a complementary sequence elsewhere on the strand to form hydrogen bonds.

Early life systems that could make a variety of useful proteins would tend to have a selective advantage over those that had a more restrictive repertoire. Selection would thus promote the early protoribosomes, tRNAs, and tRNA synthetases to diversify. This process is envisioned to have produced a set of peptide-specific ribosomes, each with a different internal guide sequence serving as an mRNA sequence. A primitive genetic code would thus become established as sets of tRNA synthetases and peptide-specific protoribosomes evolved.

Notes
ssDNA molecules that cut RNA molecules can be evolved through artificial selection in cell-free systems.