A codon is a sequence of three bases in DNA or RNA. A codon will code for one of the 20 commonally occuring amino acids, or for start or stop (TAA, TAG, TGA) codons which tell the transcription enzymes when to start and stop DNA transcription and translation. The start codon in humans is ATG, which codes for methionine, which is often removed after protein synthesis. The DNA and RNA codon system is almost completely universal, in that almost every organism uses the same codon to code for the same amino acid.
Here is a table of the 64 possible DNA codons, and the
amino acids they code for.
First T C A G Last
T Phe Ser Tyr Cys T
Phe Ser Tyr Cys C
Leu Ser Stop Stop A
Leu Ser Stop Trp G
C Leu Pro His Arg T
Leu Pro His Arg C
Leu Pro Gln Arg A
Leu Pro Gln Arg G
A Ile Thr Asn Ser T
Ile Thr Asn Ser C
Ile Thr Lys Arg A
Met Thr Lys Arg G
G Val Ala Asp Gly T
Val Ala Asp Gly C
Val Ala Glu Gly A
Val Ala Glu Gly G
A number of exceptions to this do exist: several organisms, most notably archeobacteria, use more than 20 amino acids, (they have 2 extra), and several organisms use different start and stop codons.