The genetic code is a mapping that biological cells use to translate sequences of three nucleotide bases, called codons or triplets, into amino acids. Nearly all living things use the same genetic code, called the standard genetic code, and all use small variations of it. The code is followed repeatedly, creating many amino acids strung together into a protein. This translation is the latter stage of protein biosynthesis. The first stage is transcription, where a sub-sequence of DNA called a gene is rewritten into an RNA. An RNA is a sequence of repeating nucleotide bases: adenine, guanine, cytosine and uracil. The RNA is divided into non-overlapping groups of three bases, called codons. Each codon is then translated to a particular amino acid. Thus a codon is said to code for that amino acid in the genetic code. There are 43 = 64 codons. For example, the RNA sequence UUUAAACCC contains the codons UUU, AAA and CCC, each of which specifies one amino acid. So, this RNA sequence represents a protein sequence, three amino acids long. (DNA is also sequence of nucleotide bases, but there thymine takes the place of uracil.) The standard genetic code is shown in the following tables. Table 1 shows what amino acid each of the 64 codons specifies. Table 2 shows what codons specify each of the 20 standard amino acids involved in translation. These are called forward and reverse codon tables, respectively. For example, the codon GAU represents the amino acid asparagine (Asp), and cysteine (Cys) is represented by UGU and by UGC.
|A||AUU Isoleucine, Start2
||GAU Aspartic acid
GAC Aspartic acid
2This is a start codon for prokaryotes only.
|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 codon (start codon). But unlike stop codons, these are not sufficient to begin the process; nearby initiation sequences are also required to induce transcription into mRNA and binding by ribosomes. The most notable start codon is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU, also work. It is notable that the standard genetic code has basic features of error correction. Many codons which differ by only one base codes for the same amino acid, and most often the base that differs is the last base, which happens to be the base which is most often misread in 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 of the standard genetic code are found in mitochondria, energy-burning organelles. Ciliate protozoa also have 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. Another variant is found in some species of the yeast candida, where CUG codes for Serine. In some species of bacteria and archaea, a few non-standard amino acids are substituted for standard stop codons; UGA can code for selenocysteine and UAG can code for pyrrolysine. There may be other non-standard amino acids and codon interpretations but are not known. Despite these variations, the genetic codes used by all known forms of life on Earth are very similar. 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.
*Online DNA → Amino Acid Converter