DNA codes

Despite the variations that exist, the genetic codes used by all known forms of life on Earth are very similar. Since there are many possible genetic codes that are thought to have similar utility to the one used by Earth life, the theory of evolution suggests that the genetic code was established very early in the history of life, with phylogenetic analysis of transfer RNA suggests that tRNA molecules evolved before the present set of aminoacyl-tRNA synthetases

The genetic code is not a random assignment of codons to amino acids.[10] For example, amino acids that share the same biosynthetic pathway tend to have the same first base in their codons, and amino acids with similar physical properties tend to have similar codons There are three themes running through the many theories that seek to explain the evolution of the genetic code (and hence the origin of these patterns).One is illustrated by recent aptamer experiments which show that some amino acids have a selective chemical affinity for the base triplets that code for them. This suggests that the current, complex translation mechanism involving tRNA and associated enzymes may be a later development, and that originally, protein sequences were directly templated on base sequences. Another is that the standard genetic code that we see today grew from a simpler, earlier code through a process of "biosynthetic expansion". Here the idea is that primordial life 'discovered' new amino acids (e.g. as by-products of metabolism) and later back-incorporated some of these into the machinery of genetic coding. Although much circumstantial evidence has been found to suggest that fewer different amino acids were used in the past than today,[16] precise and detailed hypotheses about exactly which amino acids entered the code in exactly what order has proved far more controversial.A third theory is that natural selection has led to codon assignments of the genetic code that minimize the effects of mutations