Genetic code: Difference between revisions

Revision as of 23:51, 23 January 2003

Revision as of 23:53, 23 January 2003

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The '''genetic code''' is the code used to [[translation|translate]] a sequence of [[RNA]] [[nucleotide]]s into [[protein]].

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-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.

The genetic code is '''redundant''', meaning that some amino acids are encoded by more than one codon, since there are twenty amino acids and sixty-four codons. This means that the same protein sequence may be encoded by multiple RNA sequences. Usually the first two positions in a codon are important, while the third position may "wobble", i.e., it is not usually as important in determining the correct amino acid.

Three of the codons (UAA, UAG, UGA) do not encode amino acids, but are instead '''STOPstop codons'''. These codons tell the [[translation]] machinery that it has reached the end of the protein sequence and it should stop translating. The stop codons are named "ochre", "amber" and "opal" respectively.

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.