AAMC #9 Bio Question

[UMICHPremed]

So the question states if two cycles of DNA replication in a germ cell occurred before tetrad formation what would be the product after normal meiotic division?

A. One tetraploid sperm
B. Four diploid sperm
C. Four haploid sperm
D. Eight haploid sperm

The solution given was B. Four diploid sperm. However, I choose C. This was my reasoning:

Say we have a (2n) cell with chromosomes X and Y: Xx and Yy

After one round of replication we have: XXxx and YYyy

Then a second round of replication: XXXXxxxx and YYYYyyyy

So now the first meitotic division separating homologous chromosomes to yield 2 cells: XXXX with YYYY or xxxx w/ yyyy or xxxx w/ YYYY or XXXX w/ yyyy
whatever the combination but I'll just pick the first two and continue.

So now the second mitotic division separating sister chromatids to yield 4 cells: cell 1(XXYY) Cell 2(XXYY) Cell 3(xxyy) Cell 4(xxyy)

So even though there are two chromatids of the same chromosome per cell, there are not 2 homologs per cell. Would this not mean that the cell is haploid but just contains 2 copies of each chromatid? From cell bio to genetics, I have always been taught that diploid refers to homologs and not copies of chromatids. Or is that wrong?

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[UMICHPremed]

help

 

[Fifty 3rds]

I haven't had genetics in 3 years, so take this with a grain of salt.

I read the question and answered it correctly before looking at your response and I thought of it like this.

If there were only one normal round of replication and meiosis occurred, you would be left with haploid sperm.

If two cycles of replication occur BEFORE tetrad formation and then meoisis, you will have double the DNA per sperm. This would make it diploid.

There would be 2 "amounts" of DNA per sperm. However you want to phrase it, there should be double the amount of DNA normally found in a sperm (diploid)

Very simplistic understanding but I truly believe that is what they are asking.

Also, I will quote an often repeated saying by many who have been successful on the MCAT, "Try to get into the minds of the test-writers."

AKA- what are they trying to accomplish with this question.

I think they are testing basic knowledge of n vs. 2n vs. 4n and mitosis, meiosis. (and what the result of tetrad formation is)


I hope this helps.

 

[illegallysmooth]

You're overthinking the problem. MCAT genetics problems generally pretty simple.

You have a 2N cell.

One replication = 4N.

Second replication = 8N.

Now meiosis.

1 8N cell --> 2 4N cells --> 4 2N cells.

The key here is not to forget that meiosis is made up of two divisions.

 

[UMICHPremed]

Thanks for the responses guys.

I guess what I am saying is that in all the textbooks they define diploid as having two copies of chromosome of different alleles and not as having two copies of the same chromosome.

For example in all the mitosis diagrams, even after a round of replication, the cell isn't labeled as a 4N cell. Instead, it is a 2N cell and off to the side they note 2x each chromatid. meh

 

[illegallysmooth]

Thanks for the responses guys.

I guess what I am saying is that in all the textbooks they define diploid as having two copies of chromosome of different alleles and not as having two copies of the same chromosome.

For example in all the mitosis diagrams, even after a round of replication, the cell isn't labeled as a 4N cell. Instead, it is a 2N cell and off to the side they note 2x each chromatid. meh

Stop confusing yourself with this nonsense. Diploid means 2N. Haploid means 1N.


Technically, sister chromatids are not two separate chromosomes. This doesn't matter for the question, and probably would never matter for any MCAT question.

All they want you to know for these types of questions is: haploid is 1N, diploid is 2, meoisis is two rounds of division that produces cells with half the N number of the parent cell, mitosis is essentially a cloning of a cell.

 

[UMICHPremed]

Alright thanks!