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What phase can I use to correctly identify the nucleotide sequence

Discussion in 'MCAT Study Question Q&A' started by inaccensa, Aug 6, 2011.

  1. inaccensa

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    An EK q is basically asking whether the study of prophase I or metaphase I can accurately identify the nucleotide sequence of a cell undergoing meiosis? I thought it was neither, since the homologus chromosomes dont separate until anaphase I. Arent they held at Chiasmata after undergoing genetic recomb? During metaphase, they line up at the met plate. They separate at anapahse, which i think renders easy visualization.
     
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  3. hamsterman

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    What I think they're concentrating on here is the fact that in Prophase 1, genetic recombination occurs through crossing over. Therefore, you wouldn't be able to know for certain until after Prophase 1.
     
  4. inaccensa

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    Yeah thats what I thought. But in any event, if I do want to look at the DNA after meiosis, I will look at Anaphase I, correct?
     
  5. paul411

    paul411 ANES
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    If you're looking for the original nucleotide sequence of the cell undergoing meiosis, you'd want to do it before recombination occurs.

    Easy visualization of chromosomes won't help us determine nucleotide sequence (order of individual base pairs) any better.

    Can you give more info about where you found the question? (EK 1001?)
     
  6. MT Headed

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    It's EK Bio, chapter 2, quiz 3, question 43.

    "A scientist monitors the nucleotide sequence of the third chromosome as a cell undergoes normal meiosis. What is the earliest point in meiosis at which the scientist can deduce with certainty the nucleotide sequence of the third chromosome of each gamete?"

    The point is that at the end of prophase I all crossing over has occurred and the four possible gametes of chromosome 3 (and every other chromosome) have been determined. No, the homologous pairs and sister chromatids aren't physically separated yet. No you don't know which chromosome combos will appear in each gamete. But that's not the question.

    Towards the end of prophase I, after crossing over, the scientist could deduce with certainty the nucleotide sequence of the third chromosome of each gamete.

    As long as the question gets you thinking about the topic and you understand the answer, consider it time well spent. Nobody is expected to get 100% on the practice quizzes, and half the time the true learning is in the answer explanations. It's just the nature of the game.
     
  7. inaccensa

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    hmm, I understand that at the end of prophase I, we have all the possible recombinants, but you cant visualize them accurately. I know they are practice q's,but if I follow their explanations blindly, I'm not sure I will fare too well. The question is poorly worded.
     
  8. MT Headed

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    Nowhere in the question does it ask at what stage the recombinants can be visualized accurately. It only asks when is the earliest that the nucleotide sequences can be deduced with certainty.

    I got the question wrong, too. I thought it was anaphase II or telophase II, because that is when the chromatids are completely separated.

    I still remember this question because it taught me to read the questions carefully, and that is a lesson far more valuable than knowing meiotic details.
     
  9. paul411

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    :thumbup:
     

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