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EK Biology In-Class Exam Lecture 2 MC question

Discussion in 'MCAT Study Question Q&A' started by futuredoctor10, Dec 30, 2008.

  1. futuredoctor10

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    EK Biology: In-Class Exam Lecture 2, Question 32 (from passage II)

    In R looping the displaced DNA strand contains the complementary nucleotide sequence for:
    A) the entire DNA strand shown in figure 2 [correct]
    B) the entire RNA strand shown in figure 2
    C) the DNA introns only
    D) the RNA introns only

    I picked C. My reasoning: the R looping technique involves using a fully processed mRNA strand (contains only exons) and hybridizing with a double stranded DNA counterpart to form hybrids between DNA and RNA strands. "In this process, the RNA strand displaces one DNA strand and binds to the other DNA strand along complementary sequences."

    So, the DNA strand that hybridizes to the RNA is exonic DNA sequences (binding to the exonic/processed mRNA). Thus, the displaced DNA strand - the loops - must represent DNA introns.

    I thought the "loops" are the displaced DNA strands - if so then based on question 31, shouldn't the answer to 32 be C as well?

    Thanks!!!
     
  2. G1SG2

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    Before getting displaced and becoming a loop, the strand was bound to its complementary strand, and thus contains the complementary sequence for the entire strand, not just the introns.
     
  3. futuredoctor10

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    OK that makes sense, should've thought it through more slowly when answering this one!

    What confused me is that the "displaced strand" is not shown in the picture. I thought they were talking about the "loops" in the picture as the displaced strand... but that would be called technically the 'hybridized strand' or something else I think.

    thanks so much!!!
     
    #3 futuredoctor10, Dec 30, 2008
    Last edited: Dec 30, 2008

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