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The '''genetic code''' is a mapping that [[biological cell]]s use to [[translation (biology)|"translate"]] sequences of three [[nucleotide]] bases (called "triplets" or "[[codon]]s") into [[amino acid]]s. The mapping indicates, for example, that when the sequence "adenine, adenine, adenine" is encountered, the amino acid [[lysine]] should be produced. When the code is followed repeatedly, many amino acids are created, and are strung together to form [[protein]]s.
In the process of [[protein biosynthesis]], a sequence of DNA called a [[gene]] is first [[transcription|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 acid]]s. 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
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 4<sup>3</sup> = 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.
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