Question regarding meiosis and ploidity

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Grah
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Question regarding meiosis and ploidity

Post by Grah » Wed Feb 24, 2010 10:06 pm

Hello everyone!

I am confused with the ploidity of cells after meiosis I. How can they be haploid if there are 46 chromosomes present (homologous pairs)?!

I would really appreciate it if someone could help me with this.

plasmodesmata11
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Post by plasmodesmata11 » Thu Feb 25, 2010 1:56 am

they're haploid after meiosis 2. The DNA replicates itself the first time. It's similar to mitosis save for the crossing over.
mathematically... it's 46X2= 92 (for the replication)
then divide by 2= 46 (telophase I)
then divide by 2 for telophase II
AND YOU GET HAPLOID CELLS!
hope that helps

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mamoru
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Re: Question regarding meiosis and ploidity

Post by mamoru » Thu Feb 25, 2010 5:11 am

Grah wrote:Hello everyone!
I am confused with the ploidity of cells after meiosis I. How can they be haploid if there are 46 chromosomes present (homologous pairs)?!

After meiosis I, what is in the daughter cells is not ploidy because at this point the homologous pairs have been segregated. Think of it like this:

1.) Going into meiosis I: 23 homologous pairs (46 chromosomes) replicate, so you have two copies of each chromosome per pair. The copies are attached to each other, so looking under the microscope, you would see what appear to be 2 X shapes per pair (half of the X shape represents 1 copy: > and < would then be individual chromosomes). So, you would actually see 46 such X shapes looking under the microscope (23 pairs), because each individual shape is a chromosome and it's copy).
NB: by saying "X", I'm referring to the generic shape of the chromosomes we see under the microscope during cell division, not specifically the X chromosome.

2.) During Anaphase of meiosis I, the homologous pairs segregate into the 2 new daughter cells. Under the microscope, it would look like 23 X shapes going to one side and 23 going to the other. Telophase separates the daughter cells. Now, in a sense, each cell is haploid, because every chromosome it carries is half of a homologous pair. However, I guess it would be possible to think of it as ploidy, because there are two copies of each of those halves.

Then, during meiosis 2, the copies separate (as replicated DNA does in normal mitosis), and the result is 4 haploid cells.

Here, perhaps seeing it as an example using A and B for a homologous pair would work:

Start: AB
Replication and Prophase and Metaphase of Meiosis I: AABB
Anaphase and Telophase of Meiosis I: AA BB
Anaphase and Telophase of Meiosis II: A A B B

Make sense?
"Empathise with stupidity, and you're halfway to thinking like an idiot." - Iain M. Banks

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