Content deleted Content added
link to conversion page |
adding a paragraph explaining why uracil is used in RNA instead of thymine |
||
Line 2:
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.
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.
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.
<table border="1" cellpadding="0" cellspacing="0" align=center bordercolor="#000000">
| |||