Rewiring the keyboard evolvability of the genetic code

The genetic code evolved in two distinct phases. First, the 'canonical' cod e emerged before the last universal ancestor; subsequently this code diverg ed in numerous nuclear and organelle lineages. Here, we examine the distrib ution and causes of these secondary deviations from the canonical genetic c ode. The majority of non-standard codes arise from alterations in the tRNA, with most occurring by post-transcriptional modifications, such as base mo dification or RNA editing, rather than by substitutions within tRNA anticod ons.