AAMC 9 Meiosis

[tmh]

This was posted in the main forum by another user but didn't get much attention. I figured it'd be best here.

In the bio section of AAMC 9, a question asks:
If two cycles of DNA replication in a spermatocyte occurred before tetrad formation what would be the product after normal meiotic division?

The answer is 4 diploid sperm. This is what I answered, but I'm wondering now why it wouldn't be haploid. The sperm wouldn't have extra homologous chromosomes, they would just have copies. Isn't that still haploid, like when a cell copies chromosomes before mitosis, but is still diploid?

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[user003]

In the bio section of AAMC 9, a question asks:
If two cycles of DNA replication in a spermatocyte occurred before tetrad formation what would be the product after normal meiotic division?

The fact that the chromosomes replicated twice indicates that the final result is 4 diploid sperm cells instead of the normal 4 haploid cells.

It doesn't matter what the extra chromosomes are or where they one from, in human the haploid (N) chromosome number is 23, diploid (2N) number is 46 chromosomes, triploid (3N) is 69 chromosomes, etc.

Edit: when a cell is undergoing meiosis I, the chromosomes are called tetrads, since there are 2 chromosomes made up of 4 chromatids

I hope that clears things up

 

[tmh]

The fact that the chromosomes replicated twice indicates that the final result is 4 diploid sperm cells instead of the normal 4 haploid cells.

It doesn't matter what the extra chromosomes are or where they one from, in human the haploid (N) chromosome number is 23, diploid (2N) number is 46 chromosomes, triploid (3N) is 69 chromosomes, etc.

Edit: when a cell is undergoing meiosis I, the chromosomes are called tetrads, since there are 2 chromosomes made up of 4 chromatids

I hope that clears things up

But isn't it possible to have haploid cells with 2x as much DNA and still be haploid?

 

[user003]

But isn't it possible to have haploid cells with 2x as much DNA and still be haploid?

I don't believe so. Haploid is defined as having one copy of each autosome along with one sex chromosome (either X or Y). Near haploid cells are defined as having a range of less than 23 but no more than 35 chromosomes. If a cell has more than 35 chromosomes (regardless of how many chromosomes are duplicates) it is said to be near diploid. These are definitions set by the International System for Chromosome Nomenclature (ISCN).

I think the confusion stems from the assumption that exact copies of chromosomes are not harmful, they are. If a sperm is diploid and it fertilized a haploid egg, the fetus would be triploid (69,XXX or 69,XXY or 69,XYY). Keep in mind all these options are viable with life so the fetus would survive, but it will have mental & physical defects.

 

[3m31250n]

But isn't it possible to have haploid cells with 2x as much DNA and still be haploid?

in a human being, it is possible for a 'diploid' cell to have 2 times the amount of DNA as another diploid cell, because as long as sister chromatids are still joined at the centromere, they are still considered "one chromosome"

However, I don't think that this occurs naturally in humans for HAPLOID cells, simply because gametes (sperm and eggs) don't go through an S phase...



I do see where you're coming from, but if you define a "chromosome" to be a physically contiguous mass of DNA (that includes the centromere that holds sister chromatids together during S phase), you will see that there is no error on the part of the test makers. Besides, the chromosomes in the final four sperm cells aren't technically identical, since crossing over occurrs prior to their formation in meiosis 1, which further disparages the analogy between these special diploid sperm cells and the chromosomes of a somatic cell during the S phase.

Let me know what you think

EDIT: on second thought, they don't describe HOW the cell undergoes a second S phase... so assuming two consecutive S phases result in four sister chromatids with three centromeres, (synapsis would produce "octads"), this would lead to 4 sperm cells with n (haploid) number of chromosomes each, since the chromosomes would still be joined at the centromeres and thus would only count as one chromosome.


I think that this is a technicality and as long as you understand what is going on and don't overestimate the test writers, you will select the correct answer, which is the most important part.