Genetic code: Difference between revisions

Revision as of 07:08, 30 January 2004

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[[de:Genetischer Code]] [[eo:Genetika Kodo]] [[es:Codón]] [[ja:遺伝暗号]] [[nl:Genetische code]] [[pl:Kod genetyczny]]

The '''genetic code''' is a [[mapping]] that [[biological cell]]s use to [[translation (biology)|translate]] sequences of three [[nucleotide]] bases, called ''[[codon]]s'codons''' or '''triplets''', into [[amino acid]]s. Nearly all living things use the same genetic code, and all use small variations of it. The code is followed repeatedly, creating many amino acids strung together into [[protein]]s.

This process is called [[protein biosynthesis]]. First, a sub-sequence of [[DNA]] called a [[gene]] is [[transcription|transcribed]] (rewritten) into [[RNA]]. An RNA is a sequence of repeating nucleotide bases. There are four types of base: [[adenine]], [[guanine]], [[cytosine]] and [[uracil]]. RNA is divided into groups of three bases, called codons. Each codon represents one amino acid. There are 64 possible 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 encodesrepresents a protein sequence, three amino acids long. (AsIn we[[DNA]], willthe see,base it[[thymine]] encodestakes Phenylalanine-Lysine-Prolinethe place of uracil.)

The standard genetic code is shown in the following tables. [[#Table 1|Table 1]] shows what amino acid each of the 4³ = 64 codons specifies. [[#Table 2|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 encodesrepresents the amino acid [[asparagine]] (Asp), and [[cysteine]] (Cys) is encodedrepresented by UGU and by UGC.

<table 'id=Table 1' border=1 cellpadding=3 cellspacing=0 align=center>

The bases in the tables are [[adenine]], [[cytosine]], [[guanine]] and [[uracil]], which are used in the [[RNA]]. In [[DNA]], [[thymine]] takes the place of uracil.

<table border=1 cellpadding=3 cellspacing=0 align=center>

<caption>

'''Table 1Table1 :''' Codon table. This table shows the 64 codons and the amino acid each codon encodescodes for.

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</center>

<table id='Table 2' border=1 cellpadding=3 cellspacing=0 align=center>

<caption>

'''==== Table 2 :''' ==== Reverse codon table. This table shows the 20 amino acids used in proteins, and the codons that encodecode for each amino acid.

</caption>

<tr>

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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 [[ribosome]]s. 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 contains features of basic [[error correction]]. Many codons which differ by only one base encodecodes 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 [[mitochondrion|mitochondria]], energy-burning [[organelles]] that probably evolved from [[symbiotic]] [[bacteria]]. [[Ciliate]] [[protozoa]] also show some variation in the genetic code: UAG and often UAA code for Glutamine (a variant also found in some [[green alga]]e), 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 that are not known.