Chromosomal duplication

[blastula]

Can someone explain this question...

If chromosomal duplication before tetrad formation occurred twice during spermatogenesis, while the other steps of meiosis proceeded normally, which of the following would result from a single spermatocyte?

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

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[bluemonkey]

The correct answer choice is B, 4 diploid sperm.

Synthesis occurs as the cell goes from a spermatogonium to primary spermatocyte. If you have two rounds of synthesis, then all of the chromosomes in the primary spermatocyte would 4 sister chromatids. After tetrad (or rather octet in this case) formation and meiosis I, each of the secondary spermatocytes will have 23 chromosomes, all of which still have 4 sister chromatids. Finally, after meiosis 2, each of the sperm cells will have 23 chromosomes, each of which has two sister chromatids. This is of course assuming that two sister chromatids go to each daughter cell during meiosis 2. Otherwise you would have a situation where some chromosomes were N, some 2N, some 3N.

I hope this helps...

 

[psanger]

I follow all of bluemonkeys reasoning... each of the 4 sperm ends up containing 23 chromosomes (each chromosome consisting of 2 nearly identical or identical sister chromatids). I have two problems with calling that situation "diploid":

1. The two sister chromatids are still attached at the centromere--so technically I think that's a single chromosome, not 2 chromosomes (or 23 chromosomes, not 46 chromosomes). If you think they aren't still attached, please tell me why and how.

2. Also, typically when we talk about diploid cells, we mean a cell that has two copies of genetic material -- one from the mother and one from the father. In this case, both copies are from a single individual (possibly with a small amount of other genetic material from crossing over).

In this case, the sperm ends up with twice the amount of genetic material as normal, BUT I don't think that is sufficient to call it "diploid". Thoughts?

 

[SBR249]

Here are my thoughts:

1). When meiosis has just finished, I would agree that the two sister chromatids are still attached. However, after the nuclear membranes are reformed, the chromosomes return to a non-condensed state. It is possible that sister chromatid adhesion at the centromered only occurs when chromosomes are in the condensed state during meiosis/mitosis. But I'm not a molecular biologist.

2). Having two sets of genetic material from the same individual would not preclude a designation as a diploid. Ploidy strictly refers to the number of sets of genetic material an organism has and makes no assumptions about their origin. By the same token, there are many organisms that can reproduce asexually yet not all of them give rise only to haploid offspring. Another example would be polyploid plants. In that case, the plant would have only two parents yet still be considered ployploid. So at least two sets must have come from the same parent.

 

[neil100]

The correct answer choice is B, 4 diploid sperm.

Synthesis occurs as the cell goes from a spermatogonium to primary spermatocyte. If you have two rounds of synthesis, then all of the chromosomes in the primary spermatocyte would 4 sister chromatids. After tetrad (or rather octet in this case) formation and meiosis I, each of the secondary spermatocytes will have 23 chromosomes, all of which still have 4 sister chromatids. Finally, after meiosis 2, each of the sperm cells will have 23 chromosomes, each of which has two sister chromatids. This is of course assuming that two sister chromatids go to each daughter cell during meiosis 2. Otherwise you would have a situation where some chromosomes were N, some 2N, some 3N.

I hope this helps...

proper explanation.

 

[Corvette]

I follow all of bluemonkeys reasoning... each of the 4 sperm ends up containing 23 chromosomes (each chromosome consisting of 2 nearly identical or identical sister chromatids). I have two problems with calling that situation "diploid":

1. The two sister chromatids are still attached at the centromere--so technically I think that's a single chromosome, not 2 chromosomes (or 23 chromosomes, not 46 chromosomes). If you think they aren't still attached, please tell me why and how.

2. Also, typically when we talk about diploid cells, we mean a cell that has two copies of genetic material -- one from the mother and one from the father. In this case, both copies are from a single individual (possibly with a small amount of other genetic material from crossing over).

In this case, the sperm ends up with twice the amount of genetic material as normal, BUT I don't think that is sufficient to call it "diploid". Thoughts?

Exactly my thoughts. I just took that test today and answered "four haploid cells" as well because I thought (and still do) that the four sperm would have N chromosomes with 1x genetic material, instead of the normal N chromosomes with 1/2x genetic material.

The sperms will just have chromosomes with sister chromatids, but not two separate chromosomes. Thus, twice the genetic material as normal, but same AMOUNT of chromosomes. Where is my line of reasoning wrong? I'd really appreciate some light on this as it's been bugging me ever since I took the test.

 

[EkramVahsedi86]

Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.[8]

Let's speculate that the duplicated chromosomes aren't counted doubly, then the Trisomy 21 embryo doesn't have 3 copies of chromosome 21, just 2 copies but one of the copies has more "material" than normal but is still one chromosome, which isn't correct.

I hope this backwards explanation is helpful.

 

[efle]

Sorry to necro like this, but I just encountered this question in a practice exam, and consulted some biology professors at my university.

The sperm would indeed be haploid, not diploid. The question writers made the mistake of saying that 1 chromosome = haploid, 2 chromosomes = diploid. In reality what you have is double the normal number of genes, but only genes from one parent (eg. each sperm has either 2 mother or 2 father instead of 1 mother or 1 father). But, ploidy is defined by the number of different contributors (one parent vs 2 parents), not the physical number of chromosomes.

By the test writer's logic, a somatic cell would be diploid immediately after division (1 copy of mom 1 of dad) but become tetraploid during the S phase (2 of mom 2 of dad) which obviously isn't how the nomenclature works.

Diagram: l = mom, ) = dad

Meiosis 1: l ) becomes ll )( known as a tetrad, divides into children cells ll and )( and these children cells are called HAPLOID despite containing two of the chromosome

Meiosis 2: each child cell divides into l and l ; ) and ( all four are HAPLOID

In the question:

Meiosis 1: l and ) becomes ll and )( and an EXTRA copying creates ll and ll and )( and )( ; separates into children cells with ll ll and )( )( they are HAPLOID despite containing four of the chromosome

Meiosis 2: each child divides into ll and ll ; )( and )( all four are HAPLOID despite containing two of the chromosome

Hope this helps and future test takers.