In the process of protein biosynthesis, a sequence of DNA called a gene is first transcribed (copied) into RNA. The RNA is a sequence of repeating units (nucleotide bases). Each position in the RNA may have four possible "values", signified by the four types of bases: adenine, guanine, cytosine and uracil. This sequence of bases encodes a protein. A protein is a sequence of amino acids. There are twenty possible amino acids. The RNA is broken up into units of three, called a codon. 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 (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 below are adenine, cytosine, guanine and uracil, which are used in the mRNA; in the DNA, thymine takes the place of uracil.
UAA Ochre Stop
UAG Amber Stop
UGA Opal Stop
GAU Aspartic acid
GAC Aspartic acid
GAA Glutamic acid
GAG Glutamic acid
|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.
It is notable that the standard genetic code contains features which provide for basic forms of error correction. Many codons which differ by only one base still encode the same amino acid and most often the single base that differs is the last one, which happens to be the base which is most often misread by the translation process. Furthermore, amino acids which tend to occur more frequently in proteins on average tend to have more codons which code for them.
Numerous variations on the standard genetic code are found inside mitochondria, energy-burning organelles that were probably derived from symbiotic bacteria. The 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. There are also a few "non-standard" amino acids which are substituted for some stop codons in some species of bacteria and archaea; UGA can code for selenocysteine and UAG can code for pyrrolysine. Other non-standard amino acids and codon interpretations may be present but currently unknown.
Despite these variations, the genetic code used by all known life on Earth displays a very large degree of similarity. Since there are many possible genetic codes that are thought to have similar utility to the one used by Earth life, the theory of evolution suggests that the genetic code was established very early in the history of life.