title: The distribution of codons by classes of triplets in the sequence of genes.
abstract: According to a criterion of symmetry-asymmetry, the triplets of the genetic code can be divided into four classes. In the genes of viruses and human mitochondria, the frequency by which a codon is followed by a codon of the same class is higher than that theoretically estimated. This is the consequence of the fact that in an initial stage of evolution many codons were duplicated.
mesh_terms: D001483:Base Sequence; D003062:Codon / Q000145:classification*; D005814:Genes, Viral; D005815:Genetic Code; D006801:Humans; D008928:Mitochondria / Q000032:analysis; D012333:RNA, Messenger / Q000145:classification*
chemical_list: D003062:Codon; D012333:RNA, Messenger