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.
Subject(s)
Codon/classification , RNA, Messenger/classification , Base Sequence , Genes, Viral , Genetic Code , Humans , Mitochondria/analysisABSTRACT
The symmetrical codons present in the first and third positions two purine (P) or two pyrimidine (p) bases (e.g. PNP or pNp), while the asymmetrical codons present in the first and third positions a purine and a pyrimidine base (e.g. PNp or pNP). The percentages of symmetrical and asymmetrical codons specifying hydrophobic and hydrophilic amino acids are similar in the genes of viruses and vertebrates, which utilise the "universal" genetic code, but they are different for the genes of human mitochondria and yeast mitochondria, which utilize two specific genetic codes.
Subject(s)
Amino Acids/genetics , Codon/genetics , Nucleic Acid Conformation , RNA, Messenger/genetics , Base Sequence , Genes, Fungal , Genes, Viral , Humans , Mitochondria/ultrastructure , Yeasts/geneticsABSTRACT
The genetic code of mammalian mitochondria presents some symmetric in the sequences of certain codons and in the relationships existing between the codons and aminoacids. These symmetries suggest the classification of codons in four classes and indicate that the evolution of the genetic code developed in four stages.
