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.
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|
|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|
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.