AAMC 11: Protein synthesis question

[geeyouknit]

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Hey, can anyone help with this question?
I don't get how changing a codon at the beginning affects a carboxy terminal near the end of synthesis?

Thanks.

 

[PiBond]

Adding a nucleotide will change the reading frame and effect everything downstream from the site of addition. Proteins are made N --> C so the carboxyl terminal will be different.

One additional nucleotide won't add/delete an amino acid since they're coded by codons of 3 nucleotides. Eliminate C and D. Since proteins are made amino terminal to carboxy terminal we can eliminate B.

 

[geeyouknit]

Adding a nucleotide will change the reading frame and effect everything downstream from the site of addition. Proteins are made N --> C so the carboxyl terminal will be different.

One additional nucleotide won't add/delete an amino acid since they're coded by codons of 3 nucleotides. Eliminate C and D. Since proteins are made amino terminal to carboxy terminal we can eliminate B.

Maybe I'm not getting the definition of "terminal", but wouldn't adding a nucleotide at the very beginning change the beginning amino acid as well?
If everything is shifted down, aren't 2/3 AA of the first codon also different?

 

[notbobtrustme]

Maybe I'm not getting the definition of "terminal", but wouldn't adding a nucleotide at the very beginning change the beginning amino acid as well?
If everything is shifted down, aren't 2/3 AA of the first codon also different?

When you add/delete a nucleotide, you change all the AA downstream of the insertion/deletion, unless it happens to be 3 nucleotides in length. In that case, you either add or subtract a single AA. Terminal simply means one end of the protein (eg N terminus means towards the free NH3 group, C terminus means towards the free COOH group). If you only make a point mutation, then only that amino acid may be changed depending on where that change was made. For example, a lot of codons are redundant in the 3rd spot, so it's quite common to have a base pair change at the 3rd codon, but no corresponding change in AA sequence.

I can't read the question exactly because OpenOffice is being a bitch-hole, so hopefully my explanation clarifies the information you are looking for.

 

[PiBond]

Maybe I'm not getting the definition of "terminal", but wouldn't adding a nucleotide at the very beginning change the beginning amino acid as well?
If everything is shifted down, aren't 2/3 AA of the first codon also different?

Because a protein is synthesized it must still have its same start codon AUG. Therefore it's implied that the addition occurred downstream of the start codon and will thus change the C-terminus.