Seminar

Mathematical Simulation of Early Life: The Evolution of an rror Minimizing Genetic Code,   Steven Massey,   UPR RĂ­o Piedras,  Wed, 11 April, 2012,   11:00 a.m. A-233

Abstract

The genetic code allows the translation of DNA gene sequences into functional proteins. It is comprised of 61 codons, each composed of three nucleotides (A, T, C or G). Each codon may code for one of the 20 amino acids present in proteins. The code has the remarkable statistical property of error minimization, whereby amino acids are assigned to codons so that when a mutation results in a change of amino acid, it is physicochemically similar to the original. This has the effect of minimizing the deleterious effects of mutations as the disruption to protein structure is reduced. Given that error minimization is a beneficial trait, it is widely assumed that it has arisen as a result of natural selection (natural selection results in the spread of beneficial traits throughout a population). This is difficult to test experimentally, due to the technical difficulties in reproducing genetic code evolution. However, it is amenable to mathematical simulation, using simple assumptions and avoiding specifics regarding the exact pathway of genetic code evolution. The assumptions used here are 1) that the genetic code expanded over time with the number of amino acids increasing to twenty; 2) that new amino acids were added to the code via gene duplication. When, these features are incorporated into the simulation it is found that the property of error minimization can "emerge", without the need to invoke natural selection. In addition, simulations show that in order to select an optimal code, numerous codon reassignments would have to occur, to effectively search "code space"; it is argued that in light of Crick's Frozen Accident theory that this is unlikely. Thus, the property of error minimization may be an example of a beneficial trait that has emerged, rather than been directly selected for.