genetic code

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• National Center for Biotechnology Information - Genetics, Trinucleotide
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• The University of Hawaiʻi Pressbooks - Biology - The Genetic Code
• Projects at Harvard - The Genetic Code and Translation
• BCcampus Open Publishing - Human Biology - Genetic Code
• American Chemical Society - Deciphering the Genetic Code

While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.

Our editors will review what you’ve submitted and determine whether to revise the article.

• National Center for Biotechnology Information - Genetics, Trinucleotide
• Biology LibreTexts - Genetic Code
• Khan Academy - The genetic code
• The University of Hawaiʻi Pressbooks - Biology - The Genetic Code
• Projects at Harvard - The Genetic Code and Translation
• BCcampus Open Publishing - Human Biology - Genetic Code
• American Chemical Society - Deciphering the Genetic Code

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genetic code, the sequence of nucleotides in deoxyribonucleic acid ( DNA) and ribonucleic acid ( RNA) that determines the amino acid sequence of proteins. Though the linear sequence of nucleotides in DNA contains the information for protein sequences, proteins are not made directly from DNA. Instead, a messenger RNA (mRNA) molecule is synthesized from the DNA and directs the formation of the protein. RNA is composed of four nucleotides: adenine (A), guanine (G), cytosine (C), and uracil (U). Three adjacent nucleotides constitute a unit known as the codon, which codes for an amino acid. For example, the sequence AUG is a codon that specifies the amino acid methionine. There are 64 possible codons, three of which do not code for amino acids but indicate the end of a protein. The remaining 61 codons specify the 20 amino acids that make up proteins. The AUG codon, in addition to coding for methionine, is found at the beginning of every mRNA and indicates the start of a protein. Methionine and tryptophan are the only two amino acids that are coded for by just a single codon (AUG and UGG, respectively). The other 18 amino acids are coded for by two to six codons. Because most of the 20 amino acids are coded for by more than one codon, the code is called degenerate.

The genetic code, once thought to be identical in all forms of life, has been found to diverge slightly in certain organisms and in the mitochondria of some eukaryotes. Nevertheless, these differences are rare, and the genetic code is identical in almost all species, with the same codons specifying the same amino acids. The deciphering of the genetic code was accomplished by American biochemists Marshall W. Nirenberg, Robert W. Holley, and Har Gobind Khorana in the early 1960s.

Nucleotide triplets (codons) specifying different amino acids are shown in the table.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers.