AQA Alevel Biology Question Answer is wrong?

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helpme135
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For the AQA Biology A level 2nd Edition Book page 502, a question says "a mutation causes three bases in the DNA of a gene to become duplicated. Explain how the effects of this mutation might differ if the duplicated bases are consecutive rather than in three separate locations on the DNA molecule."
The answer then says "When the duplicated bases are consecutive the frame shift is three bases long and so the subsequent codons are not affected. The polypeptide will have an additional amino acid but otherwise unchanged. If the bases are separate the frame shift will initially be one base long, becoming two bases long after the second duplication will be changed and the polypeptide will have many different amino acids. After the third duplicate base the codons will be unchanged."

When I saw the answer, I was like... why? If the three consecutive bases are added or duplicated, then there will be no frame shift as it is a multiple of three. Only one additional amino acid will be added but nothing else changed. And if three bases were duplicated at different separate spot....then there will be a frame shift. Am I right? Also doesn't duplication and addition have the same end result so is pretty much similar? Thank you
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Marsharko
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If three consecutive bases were added in, I agree with you that there would be no frameshift (just extra amino acid).


If three bases were added in, but not consecutively (in a row), then between the 1st and 2nd extra gene, there would be a shift of 1. Between the 2nd and 3rd gene, there would be a shift of 2. After the 3rd gene until the stop codon, there would be no shift. A diagram shows this best, but I've tried to type it out for you.

ATC GAT CGA TCG ATC GAT CGA

ATC GAA TCG ATC GAT CGA TCG

ATC GAA TCG GAT CGA TCG ATC

ATC GAA TCG GAT CCG ATC GAT CGA

The first row is the original gene sequence. My gene sequence is just a repeating ATCG combination.

The second row is original gene sequence + base duplication on 5th position
The third row is original gene sequence + base duplication on 5th position + base duplication on 9th position
The fourth row is original gene sequence + base duplication on 5th position + base duplication on 9th position + base duplication on 13th position

The underlined section shows how, after 3 bases being duplicated (or by chance, adding a base that happens to be the same as the one before it) randomly, the rest of the sequence is the SAME as the unmutated original. Look at everything after the 13th base, and compare it to the original.

Addition is NOT the same as duplication unless the addition just happens to be the adding genes that coincidently happen to be the same as the section before it.
Last edited by Marsharko; 2 months ago
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helpme135
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(Original post by Sharmarko)
If three consecutive bases were added in, I agree with you that there would be no frameshift (just extra amino acid).


If three bases were added in, but not consecutively (in a row), then between the 1st and 2nd extra gene, there would be a shift of 1. Between the 2nd and 3rd gene, there would be a shift of 2. After the 3rd gene until the stop codon, there would be no shift. A diagram shows this best, but I've tried to type it out for you.

ATC GAT CGA TCG ATC GAT CGA

ATC GAA TCG ATC GAT CGA TCG

ATC GAA TCG GAT CGA TCG ATC

ATC GAA TCG GAT CCG ATC GAT CGA

The first row is the original gene sequence. My gene sequence is just a repeating ATCG combination.

The second row is original gene sequence + base duplication on 5th position
The third row is original gene sequence + base duplication on 5th position + base duplication on 9th position
The fourth row is original gene sequence + base duplication on 5th position + base duplication on 9th position + base duplication on 13th position

The underlined section shows how, after 3 bases being duplicated (or by chance, adding a base that happens to be the same as the one before it) randomly, the rest of the sequence is the SAME as the unmutated original. Look at everything after the 13th base, and compare it to the original.

Addition is NOT the same as duplication unless the addition just happens to be the adding genes that coincidently happen to be the same as the section before it.
Thanks. What about this question then? "Suggest two reasons why the addition of a single base into a DNA sequence may not alter the amino acid sequence in the resultant polypeptide."

The answer says " Some codons will be changed to ones that code for the same amino acid (degenerate code). The frame shift might not alter some codons because the replacement bases are the same as the originals. (e.g. GCT TTT CGA - a single base frame shift to the right does not alter the TTT codons."

I don't quite understand this bit "The frame shift might not alter some codons because the replacement bases are the same as the originals. (e.g. GCT TTT CGA - a single base frame shift to the right does not alter the TTT codons." Could you explain?
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