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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 "triplets" or "''[[codon]]s")'' or ''triplets'', into [[amino acid]]s. The mappingNearly indicates,all forliving example,things that whenuse the sequencesame "adenine,genetic adeninecode, adenine"and isall encountered,use thesmall aminovariations acidof [[lysine]] should be producedit. When theThe code is followed repeatedly, creating many amino acids are created, and are strung together to forminto [[protein]]s.
In theThis process ofis called [[protein biosynthesis]]. First, a sub-sequence of [[DNA]] called a [[gene]] is first [[transcription|transcribed]] (copiedrewritten) into [[RNA]]. TheAn RNA is a sequence of repeating units (nucleotide bases). Each positionThere in the RNA may have four possible "values", signified by theare four types of basesbase: [[adenine]], [[guanine]], [[cytosine]] and uracil. This sequence of bases encodes a protein. A protein is a sequence of [[amino aciduracil]]s. There are twenty possible amino acids. The RNA is broken updivided into unitsgroups of three bases, called a '''codon'''codons. Each codon specifiesrepresents one amino acid. There are 64 possible codons. For example, the RNA sequence UUUAAACCC specifiescontains threethe codons (UUU, AAA and CCC), which each specifyof which specifies one amino acid. This So, this RNA sequence, then, encodes a protein sequence three amino acids inlong. length (asAs we will see, it encodes Phenylalanine-Lysine-Proline.). There are sixty-four possible codons.
NearlyThe all living things use the samestandard genetic code. The standard version is givenshown in the following tables,. which showTable 1 shows what amino acid each of the 4<sup>3</sup> = 64 possible codons specifyspecifies. (Table 1),2 andshows what codons specify each of the 20 standard 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. TheFor basesexample, inthe codon GAU encodes the tableamino below areacid [[adenineasparagine]], [[cytosine]](Asp), [[guanine]] and [[uracilcysteine]], which(Cys) areis usedencoded inby theUGU [[mRNA]];and inby theUGC. [[DNA]], [[thymine]] takes the place of uracil.
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="0" cellspacing="0" align=center bordercolor="#000000"> ▼
▲<table border= "1 " cellpadding= "0"3 cellspacing= "0 " align=center bordercolor="#000000">
<caption>
'''Table 1 :''' Codon table. This table illustratesshows the 64 possiblecodons and the amino acid each codon tripletsencodes.
</caption>
<tr><td rowspan=2 colspan=2></td><th colspan=4 border=0>2nd base</th></tr> ▼<tr>
<td rowspan=2 colspan=2></td>
▲<tr><td rowspan=2 colspan=2></td><th colspan=4 border=0>2nd base</th ></tr>
</tr>
<tr>
<th>U</th>
<th>G</th>
</tr>
<tr>
<th rowspan=4 border=0>1st <br>base</th>
<th>U</th>
<td>
UAU [[Tyrosine]]<br>
UAC [[Tyrosine]]<br>
UAA Ochre (''Stop'')<br>
UAG Amber (''Stop'')<br>
</td>
<td>
UGU [[Cysteine]]<br>
UGC [[Cysteine]]<br>
UGA Opal (''Stop'')<br>
UGG [[Tryptophan]]<br>
</td>
<th>A</th>
<td>
AUU [[Isoleucine]], ''Start''<br>
AUC [[Isoleucine]]<br>
AUA [[Isoleucine]]<br>
AUG [[Methionine]] ''Start'' <sup>1</sup>AUG [[Methionine]]<br>
</td>
<td>
GUC [[Valine]]<br>
GUA [[Valine]]<br>
GUG [[Valine]], ''Start''<br>
</td>
<td>
</tr>
</table>
<center>
<sup>1</sup>The AUG codon AUG both codes for methionine and serves as an initiation site;: the first AUG in an [[mRNA]]'s coding region will be the siteis where translation into protein begins.
</center>
<table border="1" cellpadding="0"3 cellspacing="0" align=center bordercolor="#000000">
<caption>
'''Table 2 :''' Reverse codon table. This table shows the 20 amino acids used in proteins, together withand the codons that codeencode each foramino themacid.
</caption>
<tr>
<tr>
<td align="center" valign="top">'''Arg'''</td>
<td valign="top">CGU, CGC, CGA, CGG, AGA, AGG</td>
<td widthalign=" 160center" valign="top"> AAA, AAG'''Lys'''</td> ▼AGG</td>
<td width="50" align="center" valign="top">'''Lys'''AAA, AAG</td>
▲<td width="160" valign="top">AAA, AAG</td>
</tr>
<tr>
<td valign="top">CAA, CAG</td>
<td align="center" valign="top">'''Ser'''</td>
<td valign="top">UCU, UCC, UCA, UCG, AGU,AGC</td>
AGC</td>
</tr>
<tr>
</tr>
<tr>
<td align="center" valign="top">'''STARTStart'''</td>
<td valign="top">AUG, GUG</td>
<td align="center" valign="top">'''STOPStop'''</td>
<td valign="top">UAG, UGA, UAA</td>
</tr>
</table>
<br>
In classical genetics, the STOPstop 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 orcodon START(start codon ,). but But unlike STOPstop 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]]s. The most notable start codon is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU , will also work. ▼
It is notable that the standard genetic code contains features which provide forof 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 onebase, which happens to be the base which is most often misread byin 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. ▼▲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]]s. The most notable start codon is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU, will also work.
▲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 onof the standard genetic code are found insidein [[mitochondrion|mitochondria]], energy-burning [[organelles]] that were probably derivedevolved from [[symbiotic]] [[bacteria]]. The [[ciliateCiliate]] [[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. One moreAnother variant is found in some species of the [[yeast]] ''[[Candida]]'', but interestingly not in all, where CUG codes for Serine. There areIn alsosome species of bacteria and [[archaea]], a few "non-standard" amino acids which are substituted for somestandard stop codons in some species of [[bacteria]] and [[archaea]]; UGA can code for [[selenocysteine]] and UAG can code for [[pyrrolysine]]. OtherThere may be other non-standard amino acids and codon interpretations maythat beare present but currentlynot unknownknown.
Despite these variations, the genetic codecodes used by all known forms of life on Earth displays aare very largesimilar. 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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