The table below shows the possible number of codons depending on how many nucleotides are used each time.

The hypothesis of a three letter code must be tested.

Francis Crick, in 1961, provided experimental evidence for this hypothesis. Working with the bacterium Escherishia coli, he made additions and deletions of one or more nucleotides to specific genes. These changes caused the gene to be misread and resulted in abnormal phenotypes. But when three bases where added or substracted, the resulting phenotype was normal or almost normal.

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The code must be deciphered.

The code was eventually broken by the preparation of a length of nucleic acid, messenger RNA, in which one triplet code was repeated many times. Initially, this synthetic RNA was made of single nucleotides, producing structures like AAA-AAA-AAA-AAA-AAA which codes for a polypeptide made only of Lysine (Lys-Lys-Lys....), or CCC-CCC-CCCC (Proline: Pro-Pro-Pro-....), or UUU-UUU-UUU-UU (Phenylalanine; Phe-Phe-Phe...). Subsequently, synthetic RNAs with various known sequences of nucleotides were produced, and added to a cell-free enzymatic protein synthesis system, and the products analysed.

The genetic code

Ala: Alanine Arg: Arginine Asn: Asparagine Asp: Aspartic Acid Cys: Cysteine Gln: Glutamine Glu: Glutamic acid Gly: Glycine His: Histidine Ile: Isoleucine

Leu: Leucine Lys: Lysine Met: Methionine Phe: Phenylalanine Pro: Proline Ser: Serine Thr: Threonine Trp: Tryptophan Tyr: Tyrosine Val: Valine

The code is degenerate: more than one codon may code for the same amino acid.

The code is (almost) universal: any particular codon represents the same amino acid in bacteria, plants, fungi, or animals (small differences among bateria)

Only two amino acids are coded by a single codon: Methionine - ATG and Tryptophan - TGG.

There are three 'Stop' codons: they do not code for any amino acid, but when they are present, signal the end of a protein (TAA, TAG, TGA).