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So the question is if:

a deletion of a codon for the amino acid lysine (AAG) is more or less likely to cause nonfunctionality of the protein than:

  1. Insertion of an additional base (C) within the start codon for this protein

  2. Mutation of the stop codon (TAG) to an arginine codon (CGC)

My attempt at an answer:

  1. Not more likely. Since the insertion of the “C” base is done at the very start, the reading frame could be shifted and the sequence would remain the same after the first base. Since the most common start codon is “ATG”, the “C” would not affect it and the “reading” of the sequence would start after the “C” and at “ATG”

  2. More severe as the stop codon is what signals that the translation of the sequence has ended, so if the stop codon got modified then the sequence would continue into the untranslated region. More peptides would be added to the sequence which would only stop when the next stop codon is reached. If the next stop codon is relatively close and the length if similar to a normal protein then it would still be functioning. But most cases it would continue for a considerable length and cause it to be non-functional

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I think the question is fairly confusingly worded, so good job on your attempt based on that.

  1. If a C is inserted within the start codon, I presume that is referring to making a sequence that was originally ATGXXX and making it either ACTGXXX or ATCGXXX. Either insertion means the start codon will be missing completely. This is going to either cause no protein to be made whatsoever, or for the beginning of the protein to be severely truncated or completely frameshifted if there is another open reading frame created downstream. This is going to be magnitudes more serious than a deletion of 3 bases corresponding to a lysine.

  2. I think your answer here is fine - lots can happen with a missing stop codon. If there happens to be another stop codon in the same reading frame shortly downstream, then the effect could be not that severe, but if not it could result in a substantial additional peptide chain added to the end of the protein, which could cause all sorts of issues with folding, and the mRNA may not even be functional and be degraded.

Again, this question isn't worded in the best way, but I think the general principle it is trying to convey is that whole missing codons do a lot less damage to the structure of a protein than mutations that affect the reading frame or the stop/start codons, because they affect only a single amino acid. Of course, it could turn out that that amino acid is crucial to the structure of the protein, but the question is about what is most "likely."

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I would interpret "within the start codon" as meaning changing ATG to ACTG or ATCG or ATGC. All of which are going to destroy the start codon, which is likely to ruin the protein.

Whereas adding extra amino acids at the end might not affect the protein function of the rest of the protein. Protein functions tend to be based on domains, and adding more amino acids to the end might not disturb the structure, and therefore function, of the domains that belong to the protein.

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  • $\begingroup$ There can be nonstop decay if there is no downstream stop codon. $\endgroup$ – WYSIWYG Apr 20 '18 at 9:52

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