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Transcription AQA Question (incorrect MS?)

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  1. Feb 24, 2012 #1
    1. The problem statement, all variables and given/known data
    http://www.xtremepapers.com/AQA/Human%20Biology/2004%20Jun/AQA-BYA2-W-QP-JUN04.pdf [Broken]

    Look at Q1bii)
    "Give the DNA base sequence that codes for threonine"

    2. Relevant equations
    3. The attempt at a solution
    So the DNA base sequence that CODES is on the sense strand
    mRNA is complementary to the anti-sense strand and identical to the sense strand (except T is replaced by U)

    So the codons on mRNA should be the same as the coding DNA strand
    hence the answer SHOULD BE ACC
    BUT the answer given is TGG
    why?
    thanks
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 24, 2012 #2

    Moonbear

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    You're right, the codon for threonine is ACC. TGG codes for tryptophan. Best explanation is an error in the key. Good job!
     
    Last edited by a moderator: May 5, 2017
  4. Feb 25, 2012 #3
    but this was a real A-level paper
    and there was no erratum notice
    the MS said "TGG"
    are you sure I am not missing anything?
     
  5. Feb 25, 2012 #4

    Moonbear

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    Was the answer key you got "official" or did someone else independent of the testing agency provide it.

    The only other possibility is they meant the antisense strand that's translated to the mRNA, not the codon. If that's what they meant, the question is written too ambiguously to answer correctly since they use the word "codes" and don't specify which DNA strand, nor give any indication they're talking about the strand being translated while leaving it to the student to understand the antisense is being translated into mRNA.
     
  6. Feb 25, 2012 #5
    It was the official key.
    http://en.wikipedia.org/wiki/Sense_(molecular_biology)#Antisense_DNA

    Read this and it makes sense (although it was DEFINITLEY ambiguous. i think the point is that the DNA base sequence that codes for any amino acid would be the bases on the antisense strand as this is the strand that is used for protein synthesis (the sense strand plays NO ROLE in protein synthesis)

    would you agree with this?
     
  7. Feb 25, 2012 #6

    Moonbear

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    Yes, that is the only way to interpret it, but you shouldn't have to work backward from an answer to understand an exam question. The sense strand is the one usually referred to as the coding strand, so asking what DNA sequence "codes" for threonine was completely ambiguous if they wanted the antisense (noncoding) strand complementary to the RNA. I don't know if there is a mechanism to contest questions on that exam, but if there is, the exam writers should be made aware of the ambiguity in the question.

    On the plus side, you've clearly demonstrated you understand the material enough to catch the ambiguity.
     
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