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AAMC 3 No.99

Discussion in 'MCAT Study Question Q&A' started by SaintJude, Apr 21, 2012.

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  1. SaintJude

    SaintJude

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    I don't get it. What's the "64-cell stage" referring to?

    [​IMG]
     
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  3. chiddler

    chiddler 5+ Year Member

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    morulation? it doesn't matter what it is referring to, but the realization is that cells remain diploid during development.
     
  4. SaintJude

    SaintJude

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    Wait, Kaplan says this regarding choice C. Is that really how the cell looks like after the S phase? This looks more like a cell with polyploidy to me...

     
  5. MrNeuro

    MrNeuro 5+ Year Member

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    as an embryo it's at whatever n and will no longer be 1/2 n (if it undergoes sexual reproduction)

    Ex

    Zygote is 2n when the animal is at the 64 cell stage during development every cell will still be 2n
     
  6. SaintJude

    SaintJude

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    I don't understand what you're saying.
     
  7. chiddler

    chiddler 5+ Year Member

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    you remember that S phase duplicates the chromosomes, right? the number of chromosomes goes from 46 to 92.

    however, haploid/diploid tells us how many sets of independent chromosomes the organism has. if it is haploid, it has only one type of chromosome 1. if it is diploid, it has two chromosome 1's.

    since we just made more copies of the chromosome, but did not introduce new sets chromosome 1's, the cell remains diploid.

    although the number of chromatids have doubled, and there are more chromosome 1's, they are connected to their sister chromatid so they are not independent chromosomes. they are not an indepedent set of chromosomes. it is not considered polyploidy for this reason.
     
  8. MrNeuro

    MrNeuro 5+ Year Member

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    the only edit id make is the number of chromosomes do NOT become 92. Rather the number of chromatids is 92 after dna replication (s phase). the number of chromosomes remains at a constant level

    its a common mistake found on the internet you DO NOT go 2n > 4n > 2n during mitosis you go 2n > 2n >2n (interphase, during mitosis, cytokinesis)
     
  9. SaintJude

    SaintJude

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    Right....DNA content amount is 4n, but it's still considered a diploid 2n during S phase. That's what you're saying, I think.
     
  10. MrNeuro

    MrNeuro 5+ Year Member

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    yep but "n" isn't the correct term they usually use c because n refers to the number of chromosomes in a haploid individual or gamete
     
  11. MedPR

    MedPR Banned

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    The 64 cell stage is just a point further in development than the 2 cell stage. Like chiddler said, you just have to realize that the number of chromosomes within an organism does not change at any point in development. If our cells are diploid, they are always diploid.
     
  12. MrNeuro

    MrNeuro 5+ Year Member

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    Unless they're in the gonads.
     
  13. MedPR

    MedPR Banned

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    Yes, but an embryo has the ploidy number of somatic cells, not germ cells.
     
  14. MDschoolorbust

    MDschoolorbust SDN Gold Donor Gold Donor 2+ Year Member

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    I just took this test and guessed correctly on this one. I chose it because the passage states something about sea urchin two celled embryos containing enough DNA information to produce their own organisms by themselves. So at a random number like 64 cells the DNA distribution should be the same? Keep in mind this is total BS and I just tried to justify my answer while guessing (not too bad of a strategy while guessing on the MCAT)
     

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