The genetic code

The standard codon table, written as mRNA codons

Sixty-four codons of three bases encode the 20 amino acids and the end of translation. Read a codon by its first base (left), second base (top) and third base (right). Codons are written as mRNA, with U in place of T. AUG codes for methionine and is the usual start codon. UAA, UAG and UGA are the stop codons and encode no amino acid. Most amino acids have more than one codon; only methionine and tryptophan have one.

Practice the 20 amino acids Practice codons Practice pathways

First base	Second base				Third base
First base	U	C	A	G	Third base
U	UUU Phe	UCU Ser	UAU Tyr	UGU Cys	U
	UUC Phe	UCC Ser	UAC Tyr	UGC Cys	C
	UUA Leu	UCA Ser	UAA Stop	UGA Stop	A
	UUG Leu	UCG Ser	UAG Stop	UGG Trp	G
C	CUU Leu	CCU Pro	CAU His	CGU Arg	U
	CUC Leu	CCC Pro	CAC His	CGC Arg	C
	CUA Leu	CCA Pro	CAA Gln	CGA Arg	A
	CUG Leu	CCG Pro	CAG Gln	CGG Arg	G
A	AUU Ile	ACU Thr	AAU Asn	AGU Ser	U
	AUC Ile	ACC Thr	AAC Asn	AGC Ser	C
	AUA Ile	ACA Thr	AAA Lys	AGA Arg	A
	AUG Met	ACG Thr	AAG Lys	AGG Arg	G
G	GUU Val	GCU Ala	GAU Asp	GGU Gly	U
	GUC Val	GCC Ala	GAC Asp	GGC Gly	C
	GUA Val	GCA Ala	GAA Glu	GGA Gly	A
	GUG Val	GCG Ala	GAG Glu	GGG Gly	G

The order U, C, A, G is the usual textbook order. Some organelles and organisms use small variations of this code. How the code may have grown.