CytoTour › Codon chart

Codon chart: the genetic code, interactive

mRNA is read in triplets called codons. There are 64 codons: 61 specify the 20 common amino acids and 3 are stop codons. Click any codon below, translate your own sequence, or look up every codon for an amino acid.

The standard codon chart (mRNA)

First base on the left, second base across the top, third base on the right.start codon (AUG)stop codons

1st	2nd base: U	2nd base: C	2nd base: A	2nd base: G	3rd
U					UCAG
C					UCAG
A					UCAG
G					UCAG

Find the codons for an amino acid

Translate mRNA or DNA into amino acids

The sequence is Start at the first AUG

Examples:

How to read a codon chart

Write the codon 5′→3′, for example GUG. The chart is for mRNA, so it uses U, not T.
Find the row for the first base (G), then the column for the second base (U).
In that box, take the line for the third base (G): GUG codes for valine (Val).

Starting from DNA? The mRNA is complementary to the template strand and almost identical to the coding strand — every T is replaced by U. RNA polymerase reads the template 3′→5′ and builds the mRNA 5′→3′. The translator above does this conversion for you.

Start and stop codons

AUG codes for methionine and is also the start codon: it sets the reading frame. From there, each set of three bases specifies the next amino acid.
UAA, UAG and UGA are stop (nonsense) codons. They code for no amino acid; when one reaches the ribosome, translation ends.
The ribosome reads mRNA 5′→3′ and builds the protein from its N terminus to its C terminus.

Why 64 codons for 20 amino acids?

Four bases taken three at a time give 4 × 4 × 4 = 64 codons, but proteins are built from only 20 common amino acids. So many amino acids have more than one codon — the code is degenerate. Leucine, serine and arginine have six codons each; methionine and tryptophan have just one. Each tRNA carries a specific amino acid, and its anticodon base-pairs with one or more codons for that amino acid.

Nearly all living organisms use this same genetic code — evidence for common ancestry.

Example: one letter in β-globin

Human β-globin, the beta subunit of hemoglobin, begins AUG GUG CAU CUG ACU CCU GAG GAG → Met-Val-His-Leu-Thr-Pro-Glu-Glu. In sickle-cell hemoglobin, a single base change (a point mutation) turns codon 7 from GAG (glutamate) into GUG (valine). Try both with the example buttons above.

See translation happen in 3D. CytoTour's Translation chapter follows this β-globin mRNA through a ribosome built from real structural data: tRNAs arrive at the A site, peptide bonds form, and the chain grows codon by codon.

Every step, with pictures: transcription and translation, step by step.

See the Teacher plan Free: tour an animal cell in 3D

All codons by amino acid

Amino acid	3-letter	1-letter	Codons (mRNA)
Alanine	Ala	A	GCU, GCC, GCA, GCG
Arginine	Arg	R	CGU, CGC, CGA, CGG, AGA, AGG
Asparagine	Asn	N	AAU, AAC
Aspartic acid (aspartate)	Asp	D	GAU, GAC
Cysteine	Cys	C	UGU, UGC
Glutamic acid (glutamate)	Glu	E	GAA, GAG
Glutamine	Gln	Q	CAA, CAG
Glycine	Gly	G	GGU, GGC, GGA, GGG
Histidine	His	H	CAU, CAC
Isoleucine	Ile	I	AUU, AUC, AUA
Leucine	Leu	L	UUA, UUG, CUU, CUC, CUA, CUG
Lysine	Lys	K	AAA, AAG
Methionine	Met	M	AUG
Phenylalanine	Phe	F	UUU, UUC
Proline	Pro	P	CCU, CCC, CCA, CCG
Serine	Ser	S	UCU, UCC, UCA, UCG, AGU, AGC
Threonine	Thr	T	ACU, ACC, ACA, ACG
Tryptophan	Trp	W	UGG
Tyrosine	Tyr	Y	UAU, UAC
Valine	Val	V	GUU, GUC, GUA, GUG
Stop	—	—	UAA, UAG, UGA

Sources

NCBI, The Genetic Codes — Table 1, the standard code — ncbi.nlm.nih.gov
IUPAC-IUB JCBN, Nomenclature and symbolism for amino acids — iupac.qmul.ac.uk
OpenStax, Biology 2e, §15.1 The Genetic Code and §15.5 Ribosomes and Protein Synthesis — openstax.org
College Board, AP Biology Course and Exam Description, Topics 6.4 and 6.7
β-globin sequence: Ensembl HBB-201 and UniProt P68871; sickle-cell variant rs334 (Ensembl VEP)