Symmetrical-asymmetrical codons and hydrophobic-hydrophilic amino acids.

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.