Glossary Term

codon
A codon is a sequence of three bases in RNA. Each codon specifies one amino acid. For example, the RNA sequence UUUAAACCC specifies three codons (UUU-AAA-CCC), which each specify one amino acid. This RNA sequence, then, encodes a protein sequence three amino acids in length (as we will see, it encodes Phenylalanine-Lysine-Proline). There are sixty-four possible codons.

Nearly all living things use the same genetic code. The standard version is given in the following tables, which show what amino acid each of the 4³ = 64 possible codons specify (Table 1), and what codons specify each of the 20 amino acids involved in translation. For instance, GAU codes for the amino acid Asp (asparagine), and Cys (cysteine) is coded for by the codons UGU and UGC. The bases in the table below are adenine, cytosine, guanine and uracil, which are used in the mRNA; in the DNA, thymine takes the place of uracil.

**Table 1 :** Codon table. This table illustrates the 64 possible codon triplets.
		U	C	A	G
		2nd base
1st base	U	UUU Phenylalanine UUC Phenylalanine UUA Leucine UUG Leucine	UCU Serine UCC Serine UCA Serine UCG Serine	UAU Tyrosine UAC Tyrosine UAA Ochre Stop UAG Amber Stop	UGU Cysteine UGC Cysteine UGA Opal Stop UGG Tryptophan
	C	CUU Leucine CUC Leucine CUA Leucine CUG Leucine	CCU Proline CCC Proline CCA Proline CCG Proline	CAU Histidine CAC Histidine CAA Glutamine CAG Glutamine	CGU Arginine CGC Arginine CGA Arginine CGG Arginine
	A	AUU Isoleucine AUC Isoleucine AUA Isoleucine ¹AUG Methionine	ACU Threonine ACC Threonine ACA Threonine ACG Threonine	AAU Asparagine AAC Asparagine AAA Lysine AAG Lysine	AGU Serine AGC Serine AGA Arginine AGG Arginine
	G	GUU Valine GUC Valine GUA Valine GUG Valine	GCU Alanine GCC Alanine GCA Alanine GCG Alanine	GAU Aspartic acid GAC Aspartic acid GAA Glutamic acid GAG Glutamic acid	GGU Glycine GGC Glycine GGA Glycine GGG Glycine