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<tr><td rowspan=2 colspan=2></td><th colspan=4 border=0>2nd base</th></tr>
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▲'''Table 1 :''' Codon table. This table illustrates the 64 possible codon triplets.<br>
<sup>1</sup>The 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.
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'''Table 2 :''' Reverse codon table. This table shows the 20 amino acids used in proteins, together with the codons that code for them.
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<td align="center" valign="top">'''Ala'''</td>
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▲'''Table 2 :''' Reverse codon table. This table shows the 20 amino acids used in proteins, together with the codons that code for them.</center>
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 [[ribosome
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 [[mitochondrion|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. 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.
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