The genetic information in DNA is transcribed into messenger RNA (mRNA). mRNA, in turn, is used to specify the exact sequence of amino acids in a protein according to the genetic code. This process, which is called translation, occurs in protein-RNA complexes called ribosomes.

Special RNA molecules called transfer RNA (tRNA) actually perform the translation because they contain a three nucleotide sequence at one end, called an anticodon, that is complementary to the codon of the mRNA. At the other end of the tRNA, an amino acid (specific to the anticodon) is attached. The amino acid is covalently joined (via a peptide bond) to the growing polypeptide chain on the ribosome (See Figure 5.20).