Genetic code

The genetic code is a translation table for how triplets of adjacent bases, called codons, specify amino acids in protein biosynthesis. In this process, the bases in the DNA from a gene are first copied into a molecule of mRNA during transcription, certain sections of the mRNA are spliced out, and then amino acids are linked to it by molecules of tRNA during translation. Some codons do not specify an amino acid, called STOP codons, which as a result end the translation process.

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 (Table 2). For instance, GAU codes for the amino acid Asp (asparagine?), and Cys (cysteine?) is coded for by the codons UGU and UGC. These are called forward and reverse codon tables, respectively. The bases in the table are adenine?, cytosine, guanine and uracil?, which are used in the mRNA; in the DNA, thymine? takes the place of uracil.

		2nd base
		U	C	A	G
1st base	U	UUU Phenylalanine? UUC Phenylalanine? UUA Leucine? UUG Leucine?	UCU Serine? UCC Serine? UCA Serine? UCG Serine?	UAU Tyrosine UAC Tyrosine UAA Stop UAG Stop	UGU Cysteine? UGC Cysteine? UGA 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

Table 1 : Codon table. This table illustrates the 64 possible codon triplets.
¹The AUG codon both codes for methionine and serves as an initiation site; the first AUG in an mRNA's coding region will be the site where translation into protein begins.

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

Table 2 : Reverse codon table. This table shows the 20 amino acids used in proteins, together with the codons that code for them.

In classical genetics, the STOP codons were given names - UAG was amber, UGA was opal, and UAA was ocher. These names were originally the names of the specific genes in which mutation of each of these stop codons was first detected. Translation starts with a chain initiation or START codon, but unlike STOP codons these are not sufficient by themselves to begin the process; nearby initiation sequences are also required to induce transcription into mRNA and binding by ribosomes. The most notable of these is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU, will also work.

Numerous variations on the standard genetic code are found inside mitochondria, energy-burning organelles that were probably derived from symbiotic bacteria. The Ciliophora or ciliate protozoa also show some variation in the genetic code: UAG and often UAA code for Glutamine, a variant also found in some green algae, or UGA codes for Cysteine. One more variant is found in some species of the yeast? Candida, but interestingly not in all, where CUG codes for Serine.

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