Mutations

ID #2132

While re-listening to the lecture and looking over my notes, I realized I was somewhat confused by a 3 base deletion. In class we assumed that the 3 bases that were deleted were all part of the same codon and thus only one protein would be deleted. Is it not possible for those 3 bases to actually be part of 2 separate codons? Because that would mean that in effect two amino acids are deleted and a new one is formed. And could the same idea be applied to a 3 base insertion?


This is a terrific question. You're right that we assumed in class that only one amino acid was affected, but you are absolutely correct that this isn't always the case when 3 bases are removed. If one base from one codon was removed, followed by the first two bases of the next codon (or two from the first and one from the second), then two amino acids could be affected. It still wouldn't be a frameshift by the strictest definition, but we'd still include it in this category for lack of a better category.

What makes your question so interesting and relevant is that very hypothetical situation you described, where the deletion is two bases from one codon and one base from the next (or previous) codon is exactly what's happening in the Cystic Fibrosis mutation I used as an example in class. The sequence of the mRNA for CFTR in that region is AUC AUC UUU GGU GUU, which is then translated in that region (starting with amino acid 506) as Ile-Ile-Phe-Gly-Val. The mutation actually deletes the bases bolded here: AUC AUC UUU GGU GUU. This leaves the following sequence in that region of the mRNA: AUC AUU GGU GUU, which removes the phenylalanine, but leaves the second of the two isoleucines unaffected because AUU (which is what's left in-frame after that deletion) is a different isoleucine codon! If this didn't happen to be the case, you would have removed the phenylalanine and swapped isoleucine for something else, but notice that the rest of the reading frame is unaffected. This is the true spirit of a frameshift mutation -- it affects everything downstream of the mutation.

And yes, an inclusion of three extra bases could throw off two different codons (or perhaps just one) based once again on degeneracy of the genetic code, but only that particular region would be affected.

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