Brief Notes on the Start and Stop Codons

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The start or initiation codon in most eases is AUG which normally codcs for amino acid methionine but when AUG is present as initiation codon, it codcs for formylatcd methionine. Rarely GUG serves as initiation or start codon. As start codon GUG codes for methionine where as it normally codcs for valine.

Three codons namely, ochre (UAA), amber (UAG) and opal (UGA) arc stop codons or non-sense codons as they do not code for any amino acid. The translation is terminated at that point where on mRXA any one of these three codons is encountered.

UGA or Opal has several roles; some of which are l. termination codon in universal genetic code, 2. Tryptophan codon in mitochondria, 3. an efficiently read tryptophan codon in B.subtilus and E.coli, 4. Cysteine codon in Euplotes octocarinatus and selenocysteine (SeCys) codon in E.coli, mammals, higher plants and fungi.

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The specification of an amino acid by a codon is mediated by a tRXA. It may be recalled that tRXA has anticodon which matches in base complementarity to a particular codon. So a particular tRXA is meant for a particular codon and that tRXA is specific to a particular amino acid. Hence it is obvious that for twenty different amino acids there must be atleast twenty different tRXAs.

In fact there are about forty five different tRXAs for twenty different amino acids, (more than one for each aminoacid) these tRXAs function as adapter molecules. It is important to note that, excluding the three non-sense codons there are 61 valid amino acid codons. Then about 45 anticodons on 45 tRXA molecules can match 61 codons. This is possible, because some tRXA molecules can recognize more than one codons as the third base on tRXA anticodon allows some “wobble”.

Dr. F.H.C. Crick in 1965 proposed the wobble hypothesis. He discovered that if U is present at the first position of anticodon, it can pair with either A or G present at third position of codon. Similar is the case with G or I (Ionosine, a modified base in tRXA) found in anticodon. Thus the wobbling allows economy of the number of tRXA molecules and several codons meant for same amino acid are recognised by the same tRXA. In wobbling U as first base in anticodon pairs with A/G as third base in codon, G pairs with U/C and I with U,C,and A.

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