Genetic code: Difference between revisions

Revision as of 02:36, 22 January 2003

Revision as of 20:20, 22 January 2003

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[[de:Genetischer Code]][[eo:Genetika Kodo]]

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

The '''genetic code''' is a translation table for how triplets of adjacent [[nucleotide|bases]], called ''[[codon|codons]]'', specify [[amino acid]]s in [[protein]] biosynthesis. In this process, the bases in the [[DNA]] from a gene are first copied into a molecule of [[mRNA]] during [[transcription]], certain sections of the [[mRNA]] are spliced out, and then amino acids are linked to it by molecules of [[tRNA]] during [[protein biosynthesis|translation]]. Some codons do not specify an amino acid, called STOP codons, which as a result end the translation process.

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.

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.

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.

Uracil is metabolically "cheaper" to produce than thymine is in terms of the chemical energy required, and so substituting it for thymine in RNA reduces the amount of metabolic energy required by living cells to produce RNA. However, there is a very good reason why uracil is not used in place of thymine in DNA; cytosine, one of the three other bases used in the genetic code, turns into uracil at a relatively high rate by a [[deamination]] reaction. All living cells contain [[enzyme]]s which detect uracil bases in DNA and replace them with cytosine, repairing the damage without causing a [[mutation]]. Since RNA is much shorter-lived than DNA is, and is not copied for purposes of heredity (except in a few rare and exceptional cases), mutations caused by cytosine deamination are not as important and so the use of uracil becomes an acceptable risk to take in exchange for its cheapness.

Three of the codons (UAA, UAG, UGA) do not encode amino acids, but are instead '''STOP 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.

<table border="1" cellpadding="0" cellspacing="0" align=center bordercolor="#000000">

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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>