Aamc 11

[impact2d]

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I took the famed ebola passage and got everything right except the last question. Unfortunately even after reading the explanation I don't quite understand what is going on.

Two of the answers say an extra Amino acid of some sort, so I can now eliminate those because I know one necleotide shouldn't be able to code for an extra AA. I get this part.

The two choices left say a change will occur at the Carboxyl Terminus or a Change will Occur at the Amino Terminus.

During translations, If the mRNA is transcribed 3 Bases at a time, won't one extra not code for anything? If tRNA can't bind and grab an AA won't this just be ignored? Unless they are stating that this nucleotide was inserted somewhere within the chain, but then wouldn't that cause a frameshift and cause a mutation in all of the proteins not just the last one?

The answer states that Proteins are synthesized form the Amino Terminus to the Carboxyl Terminus which I understand and already knew, I just can't seem to figure out how we are supposed to know an extra base makes a change to the carboxyl terminus or why? Is there a nuance in this question i am missing. Maybe someone who has taken Genetics can explain?

EDIT:
Some previous SDN answers suggest that this answer is actually describing a frameshift, which is what I thought. If that is the case how does it not cause the stop codon to change?

 

[BB8730]

During translations, If the mRNA is transcribed 3 Bases at a time, won't one extra not code for anything? If tRNA can't bind and grab an AA won't this just be ignored? Unless they are stating that this nucleotide was inserted somewhere within the chain, but then wouldn't that cause a frameshift and cause a mutation in all of the proteins not just the last one?

The answer states that Proteins are synthesized form the Amino Terminus to the Carboxyl Terminus which I understand and already knew, I just can't seem to figure out how we are supposed to know an extra base makes a change to the carboxyl terminus or why? Is there a nuance in this question i am missing. Maybe someone who has taken Genetics can explain?

EDIT:
Some previous SDN answers suggest that this answer is actually describing a frameshift, which is what I thought. If that is the case how does it not cause the stop codon to change?

Dropping an extra nucleotide into the "open reading frame" (aka the region that is going to be translated) will cause a frameshift mutation. The question doesn't say where, so I would assume somewhere in the middle, or more relevantly, I would assume that it is unlikely that it was dropped into the code for the first few AA's. A frameshift mutation causes disruptions in the AA sequence starting from the point at which the extra nucleotide is inserted. As you pointed out, it would be more than just the last AA that would be changed, but we don't know where the nucleotide is dropped so we can't say exactly how much of the chain is changed. We can predict, though, that the most likely outcome is that the first amino acid (at the amino terminal) is unchanged while the last amino acid (carboxy terminal) is changed.

It's not uncommon for a frameshift mutation to mess up the stop codon as well, which is why it's a terrible mutation to have, but the question explicitly states that it does not remove the stop codon.

This whole question is just a messy way to test the concept that proteins are synthesized N -> C terminus and that dropping an extra nucleotide into the sequence will mess up things downstream, leaving the N terminus unchanged and probably changing the C terminus.

 

[impact2d]

Thanks! This cleared it up for me. I guess I was reading into it to much.

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