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* it signals the start of translation  
* it codes for the incorporation of the amino acid methionine (Met) into the growing polypeptide chain  
The genetic code can be expressed as either RNA codons or DNA codons. RNA codons occur in messenger RNA (mRNA) and are the codons that are actually "read" during the synthesis of polypeptides (the process called translation). But each mRNA molecule acquires its sequence of nucleotides by transcription from the corresponding gene. Because DNA sequencing has become so rapid and because most genes are now being discovered at the level of DNA before they are discovered as mRNA or as a protein product, it is extremely useful to have a table of codons expressed as DNA. So here are both.  
Note that for each table, the left-hand column gives the first nucleotide of the codon, the 4 middle columns give the second nucleotide, and the last column gives the third nucleotide.  
The RNA Codons  
Second nucleotide U	C	A	G  
U UUU Phenylalanine (Phe)	UCU Serine (Ser)	UAU Tyrosine (Tyr)	UGU Cysteine (Cys)	U  
UUC Phe	UCC Ser	UAC Tyr	UGC Cys	C  
UUA Leucine (Leu)	UCA Ser UAA STOP	UGA STOP	A  
UUG Leu	UCG Ser UAG STOP	UGG Tryptophan (Trp)	G  
C CUU Leucine (Leu)	CCU Proline (Pro)	CAU Histidine (His)	CGU Arginine (Arg)	U  
CUC Leu	CCC Pro	CAC His	CGC Arg C  
CUA Leu	CCA Pro	CAA Glutamine (Gln)	CGA Arg A  
CUG Leu	CCG Pro	CAG Gln	CGG Arg G  
A AUU Isoleucine (Ile)	ACU Threonine (Thr)	AAU Asparagine (Asn)	AGU Serine (Ser)	U  
AUC Ile	ACC Thr	AAC Asn	AGC Ser C  
AUA Ile	ACA Thr AAA Lysine (Lys)	AGA Arginine (Arg)	A  
AUG Methionine (Met) or START	ACG Thr	AAG Lys	AGG Arg G  
G GUU Valine Val	GCU Alanine (Ala)	GAU Aspartic acid (Asp)	GGU Glycine (Gly)	U  
GUC (Val)	GCC Ala	GAC Asp	GGC Gly	C  
GUA Val	GCA Ala	GAA Glutamic acid (Glu)	GGA Gly	A  
GUG Val	GCG Ala	GAG Glu	GGG Gly	G  
The DNA Codons  
These are the codons as they are read on the sense (5' to 3') strand of DNA. Except that the nucleotide thymidine (T) is found in place of uridine (U), they read the same as RNA codons. However, mRNA is actually synthesized using the antisense strand of DNA (3' to 5') as the template. [Discussion]  
This table could well be called the Rosetta Stone of life.  
The Genetic Code (DNA)  
TTT	Phe	TCT	Ser	TAT	Tyr	TGT	Cys  
TTC	Phe	TCC	Ser	TAC	Tyr	TGC	Cys  
TTA	Leu	TCA	Ser	TAA	STOP	TGA	STOP  
TTG	Leu	TCG	Ser	TAG	STOP	TGG	Trp  
CTT	Leu	CCT	Pro	CAT	His	CGT	Arg  
CTC	Leu	CCC	Pro	CAC	His	CGC	Arg  
CTA	Leu	CCA	Pro	CAA	Gln	CGA	Arg  
CTG	Leu	CCG	Pro	CAG	Gln	CGG	Arg  
ATT	Ile	ACT	Thr	AAT	Asn	AGT	Ser  
ATC	Ile	ACC	Thr	AAC	Asn	AGC	Ser  
ATA	Ile	ACA	Thr	AAA	Lys	AGA	Arg  
ATG	Met*	ACG	Thr	AAG	Lys	AGG	Arg  
GTT	Val	GCT	Ala	GAT	Asp	GGT	Gly  
GTC	Val	GCC	Ala	GAC	Asp	GGC	Gly  
GTA	Val	GCA	Ala	GAA	Glu	GGA	Gly  
GTG	Val	GCG	Ala	GAG	Glu	GGG	Gly  
*When within gene; at beginning of gene, ATG signals start of translation.  
Codon Bias  
All but two of the amino acids (Met and Trp) can be encoded by from 2 to 6 different codons. However, the genome of most organisms reveals that certain codons are preferred over others. In humans, for example, alanine is encoded by GCC four times as often as by GCG. Why should this be? The answer is uncertain but probably reflects a greater translation efficiency by the translation apparatus (e.g., ribosomes) for certain codons over their synonyms.  
Codon bias even extends to pairs of codons: wherever a human protein contains the amino acids Ala-Glu, the gene encoding those amino acids is seven times as likely to use the codons GCAGAG rather than the synonymous GCCGAA.  
Codon bias is exploited by the biotechnology industry to improve the yield of the desired product. The ability to manipulate codon bias may also usher in a era of safer vaccines. Link to a discussion.  
Exceptions to the Code  
The genetic code is almost universal. The same codons are assigned to the same amino acids and to the same START and STOP signals in the vast majority of genes in animals, plants, and microorganisms. However, some exceptions have been found. Most of these involve assigning one or two of the three STOP codons to an amino acid instead.  
Mitochondrial genes  
When mitochondrial mRNA from animals or microorganisms (but not from plants) is placed in a test tube with the cytosolic protein-synthesizing machinery (amino acids, enzymes, tRNAs, ribosomes) it fails to be translated into a protein.  
The reason: these mitochondria use UGA to encode tryptophan (Trp) rather than as a chain terminator. When translated by cytosolic machinery, synthesis stops where Trp should have been inserted.  
In addition, most  
* animal mitochondria use AUA for methionine not isoleucine and  
* all vertebrate mitochondria use AGA and AGG as chain terminators.  
* Yeast mitochondria assign all codons beginning with CU to threonine instead of leucine (which is still encoded by UUA and UUG as it is in cytosolic mRNA).