The genetic code

Nucleic and amino acids share a structure–function continuum

The genetic code has traditionally been regarded as an arbitrary encryption device for polypeptide synthesis. This creates a problem for origin-of-life theories: a pure encryption device should require preformed molecular machinery for interpreting and implementing the code, and would thus not exist in prebiotic environments. In fact, amino and nucleic acids exhibit significant structural and functional correspondences that seem likely to have been relevant to the evolution of primordial biomolecules.

With respect to the function of encoded amino acids, the most important codon component is the second base and the least critical the third, with the first occupying an intermediate position. The physical properties of amino acids are linked to the nucleotide sequence of codons: if the second nucleotide is A the encoded residue is hydrophilic (tending to sit on the outside of proteins), whereas if the second nucleotide is U/T, it is hydrophobic (tending to embed itself within the protein core). The survival of simple microorganisms such as bacteria depends more on the rapid mutability of their genomes rather than on conservation of functions: the strain will survive provided at least one organism resistant to environmental threats is available at any one time. Conversely, highly evolved multicellular organisms such as humans require stricter genomic conservation to maintain function.

These diverging paradigms of genetic evolution may be designated neutralist (primarily driven by random mutation) or selectionist (primarily driven by functional conservation).