How to read the codon table
A codon is three consecutive bases of messenger RNA, and each codon tells the ribosome which amino acid to add next. The table lists all 64 codons. Find the first base in the left-hand column, the second base along the top, and the third base as one of the four lines inside the cell. TTC, for example, is in the T row, the T column, third line down for C: phenylalanine. Tap any codon to highlight every codon for the same amino acid, and switch to RNA to see the codons with U instead of T, which is how they appear in mRNA. To translate a whole sequence use the DNA to protein translator.
Start and stop codons
ATG (AUG in mRNA) is the start codon and codes for methionine, so nearly every newly made protein begins with Met, which is often removed later. TAA, TAG and TGA(UAA, UAG, UGA) are the stop codons, historically called ochre, amber and opal. They code for no amino acid; when the ribosome reaches one, a release factor binds and the protein is released. Both are shown in colour in the table.
Codons for each amino acid
Degeneracy and the wobble position
Sixty-one codons encode twenty amino acids, so most amino acids have several codons: leucine, serine and arginine have six each, methionine and tryptophan only one. The redundancy sits mostly in the third position, where a change of base often leaves the amino acid unchanged. Such changes are called synonymous or silent mutations. This is also why a single tRNA can read more than one codon, by pairing loosely, or wobbling, at the third position. The pattern has practical consequences: codon optimisation for expression, the design of degenerate primers, and the reason two DNA sequences can differ substantially yet encode the same protein. Compare codon preferences with the codon usage tool.
Variant genetic codes
The standard table is nearly universal, but not quite. Mitochondria, chloroplasts and a handful of nuclear lineages reassign a few codons. The genetic code menu above the table follows the NCBI numbering used by GenBank and BLAST; choosing one redraws the table.
| Code | Differences from the standard code |
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| 2 · Vertebrate mitochondrial | TGA = Trp, ATA = Met, AGA and AGG = stop |
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| 3 · Yeast mitochondrial | TGA = Trp, ATA = Met, CTN = Thr |
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| 4 · Mold and protozoan mitochondrial, Mycoplasma | TGA = Trp |
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| 5 · Invertebrate mitochondrial | TGA = Trp, ATA = Met, AGA and AGG = Ser |
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| 6 · Ciliate nuclear | TAA and TAG = Gln |
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| 9 · Echinoderm and flatworm mitochondrial | TGA = Trp, AAA = Asn, AGA and AGG = Ser |
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| 11 · Bacterial and plant plastid | Same translation; extra start codons GTG and TTG |
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| 13 · Ascidian mitochondrial | TGA = Trp, ATA = Met, AGA and AGG = Gly |
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Amino acid codes
Each amino acid has a three-letter abbreviation and a one-letter code, and the table shows both. The one-letter code is what sequence databases and alignment tools use. Theamino acid table lists the properties behind the letters, and the one-letter to three-letter converter switches a sequence between the two.
Frequently asked questions
How many codons are there?
Sixty-four. Four bases in three positions give 4 × 4 × 4 combinations. Sixty-one of them code for the twenty standard amino acids and three, TAA, TAG and TGA, are stop codons that end translation.
What is the start codon?
ATG, which codes for methionine, so almost every protein begins with Met. In the mRNA it reads AUG. Bacteria also start some genes at GTG or TTG, which are then read as methionine by the initiator tRNA.
What are the stop codons?
TAA (ochre), TAG (amber) and TGA (opal). In mRNA they are UAA, UAG and UGA. They code for no amino acid; a release factor recognises them and the finished protein is released.
Why do most amino acids have more than one codon?
The code is degenerate: 61 codons for 20 amino acids. Leucine, serine and arginine each have six codons, most others have two or four, and only methionine and tryptophan have one. The redundancy is mostly in the third position, the wobble position, which is why many single-base changes there are silent.
Is the codon table the same in all organisms?
The standard table applies to nuclear genes of nearly every organism and to bacteria, but mitochondria and a few lineages reassign codons. Vertebrate mitochondria read TGA as tryptophan and AGA and AGG as stops, and ciliates read TAA and TAG as glutamine. Switch the genetic code above the table to see them.