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frame shift and nonsense - There is nothing particularly...

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frame shift and nonsense By nonsense is meant the premature insertion of a stop codon into the gene sequence. This might be by a single nucleotide change as in the figure above where the codon CAG which encodes glutamine (Q) has mutated to the stop codon TAG (UAG in the mRNA of course). Alternatively it can be as a consequence of a deletion or insertion of a number of nucleotides not divisible by three which shifts the reading frame and by chance will usually quickly lead to a stop codon. In the figure above, the same C nucleotide has been deleted in the last line leading to frameshift and a quick termination. The tuberous sclerosis gene TSC1 contains a direct repeat of four nucleotides, AAAGAAAG. Four independent mutations have been identified in which one repeat has been lost by deletion of AAAG. This leads to frameshift and premature chain termination. triplet deletion
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Unformatted text preview: There is nothing particularly special about a triplet deletion which removes exactly one amino acid from the polypeptide (and which may change one amino acid at the mutation site). However, I include it because the most common mutation in cystic fibrosis is deltaF508 (i.e. deletion of amino acid number 508 (a phenylalanine, F)). splice site Frameshifts can also come about by mutations which interfere with mRNA splicing. The beginning and end of each intron in a gene are defined by conserved DNA sequences. If a nucleotide in one of the highly conserved positions is mutated then the site will no longer function with predictable consequences for the mature mRNA and the coded protein product. There are many examples of such mutations, for instance, some beta thalassemia mutations in the beta globin gene are caused by splice junction mutations....
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