AbstractIn the cell the genetic information contained in a molecule of DNA is transferred to messenger RNA, which in turn acts as a template in the synthesis of protein. The nucleotide sequence in DNA is mirrored as a corresponding nucleotide sequence in the messenger RNA, the latter then determining the primary structure of the proteins (i.e. their amino acid sequence). In this way the language of the nucleic acids, which comprises four letters (the four nucleotide bases), is translated into that of the proteins, which comprises 20 letters (the 20 amino acids). It is supposed that each group of three consecutive nucleotide bases in the template RNA determines an amino acid. Hence the genetic code is a “triplet” code.Polyribonucleotides, which can be synthesized readily from the ribonucleoside‐5′‐diphosphates with the help of the enzyme polynucleotide phosphorylase, can replace natural template RNA in a cell‐free system similar to that used in protein synthesis in which the amino acids are polymerised into polypeptide chains. Since the nature of the amino acids incorporated into the polypeptide chains depends on the base composition of the synthetic polynucleotide, it has been possible to discover the base compositions of a number of the triplets corresponding to the various amino acids. In theEscherichia colisystem homopolynucleotides such as polyadenylic acid, polycytidylic acid and polyuridylic acid lead to the incorporation of lysine, proline and phenylalanine. Under these conditions polysine, polyproline and polyphenylalanine, respectively, are formed. Copolynucleotides, which contain two or more different nucleotide bases, direct several different amino acids into the polypeptide chain; for example, polypeptides made in the presence of polyadenylocytidylic acid contain lysine, isoleucine, asparagine, leucine, tyrosine and phenylalanine.A correlation has been made between the base compositions of the individual synthetic polynucleotides which were added to the cell‐freeE. colisystem and the nature and quantity of the amino acids incorporated into the polypeptide chain. In this way the base compositions of 46 of the 68 possible triplets of template RNA, corresponding to the 20 amino acids, have so far been established. Probably all the “meaningful” triplets are not yet known. In some instances it is found that a certain amino acid is coded for by several different triplets; a code with such a property is termed a “degenerate” code. With few exceptions, the actual sequence of bases within a given triplet is not at present known; only the base composition has been determined. There are indications that the genetic code is universal or, in other words, that there is only one code