On the origin of the mitochondrial genetic code: first principles insight

The genetic code is a conspicuous universal feature of extant organisms and indicates a common ancestry of different forms of life on Earth. The origin of the genetic code is one of the most challenging problems in molecular evolution. Known variants can be mainly divided into mitochondrial and nuclear classes. Here we provide a new insight on the origin of the vertebrate mitochondrial genetic code: we show that its degeneracy distribution can be completely described by using a mathematical approach based on redundant number representation systems. The theoretical description allows a plausible formulation of a biological hypothesis about the origin of mitochondrial protein coding. Such a hypothesis is based on the symmetries of ancient chemical adaptors linking nucleic acids and amino acids and reconciles many theoretical and empirical findings. The results point to a primeval mitochondrial genetic code based on a special set of fourbase codons that we call tesserae (from the greek tessera = four). Tesserae exhibit many desirable mathematical properties mainly related to error detection capabilities. Our work shows that both mitochondrial and nuclear variants of the genetic code can be described through a unified mathematical framework. The results strongly suggest that the two classes of codes are connected by a symmetry breaking transformation and provide new firstprinciples insight relevant to the understanding of the evolutionary steps that led to extant genetic codes.