aamc9 #144

[charju]

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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

answer is B. I believe however, it should be C. correct me if im wrong with the following deductions.

If there is one extra replication during S phase. then I went from 2N 2C -> 2N 8C.

the homologous pairs should still pair up during metaphase I, and then the two daugther cells would be 1N 4C.
meiosis II would leave us with 1N 2C.

i would have two chromosomes yes, but of the same one.. not two different chromosomes.

 

[salim271]

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

answer is B. I believe however, it should be C. correct me if im wrong with the following deductions.

If there is one extra replication during S phase. then I went from 2N 2C -> 2N 8C.

the homologous pairs should still pair up during metaphase I, and then the two daugther cells would be 1N 4C.
meiosis II would leave us with 1N 2C.

i would have two chromosomes yes, but of the same one.. not two different chromosomes.

haploid means 23 chromosomes. Diploid means 46. By your own admission there are 2 chromosomes, it doesnt matter if they are the same one. Its still diploid.

 

[charju]

by that logic then the products of normal cells after meiosis I would still be considered diploids...since they are 1N 2C.

when i'm pretty sure textbooks says they are haploids by that point.

 

[Phlame217]

No... meiosis 1 produces diploid cells, meiosis 2 is what furthers it to a haploid state.

 

[charju]

really... i know wikipedia is not the most reliable source but
"Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional division."

again, i just quickly googled and they all indicate that after meiosis I, we will be at a haploid state.

just pulled out some books like EK and they all state after meiosis I is haploid.

 

[Phlame217]

Alright lets go through this bit by bit to be clear:

Chromosomal material is doubled during the cell cycle so lets use a 1 chromosome organism as an example.

M = Maternal
P = Paternal

Organism = 2N aka MP aka diploid

Say this organism undergoes meiosis

Meiosis 1 by metaphase would duplicate the chromosomal material, but the chromosomes still joined by 1 centromere. Therefore the organism is still 2N but 2M2P (you count chromosomes by centromeres as my genetics professor hammered in).

Crossing over occurs in prophase I but with this respect, we will still use the MP terminology.

The cell proceeds to anaphase and telophase which results in two cells which are technically Haploid since they now only have one chromosome (counted by the centromere) but technically have 2 copies of that chromosome (thinking diploid in my head so therefore either 2M, 2P, or MP based on random selection at metaphase)

The cell then undergoes meiosis 2 resulting in 4 cells with only one copy of each chromosome. (either M or P)

Now simply realize that throughout this whole process that if the genetic material is quadrupled at the start instead of doubled like normal, your going to end up with double the genetic material at the end and hence the cell would technically be diploid since it would have to copies of each chromosome.

TL;DR

Normal:
2N (MP) = 2N (2M2P) = 1N (2M or 2P or MP) = 1N (M or P)

This scenario:
2N (MP) = 2N (4M4P) = 1N (4M or 4P or 2M2P) = 2N (2M or 2P or MP)

Just some outside information; once the material is separated and meiosis is completed the centromeres dissolve therefore you count chromosomes by the copies present, no longer by centromeres.

 

[charju]

i see. I guess the centromere counting part was what I was missing.

so to wrap it up. If two sister chromatids no longer stick to the same centromere, you can count them as two independent chromosomes?

 

[Phlame217]

i see. I guess the centromere counting part was what I was missing.

so to wrap it up. If two sister chromatids no longer stick to the same centromere, you can count them as two independent chromosomes?

Correct