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# Meiosis watch

1. meiosis is when the chromosomes turn from 2n to n. The first stage of meiosis is when the DNA replicates therefore would it be right to say the chromosomes change from 2n to 4n. Then in the 1st meiosis division, the 4n splits to 2 2n cells. Then in the second meiosis division, the 2 2n cells split into 4 n cells.

I think what I am saying above is correct but in the textbook, it starts with 2n, and then when it replicates, it stays at 2n. Then divides to 2*2n and then 4*n

If there is such thing as 4n, what is it called (n=haploid, 2n=diploid, 4n=?)
2. In meiosis you start with 2n so in humans that's 2*23 chromosomes and 2*23 chromatids before DNA replication takes place. Once DNA replication takes place there are still same number of chromosomes (2*23) but double the number of chromatids so 2 chromatids per chromosome so (2*2*23 chromatids). The homologous chromatids dissociate after meiosis I and produce 23 chromosomes per nucleus and each chromosome will have 2 chromatids (23 chromosomes and 2*23 chromatids per nucleus). The sister chromatids dissociate after meiosis II and produce 23 chromosomes per nucleus and 1 chromatid per chromosome. (23 chromosomes and 23 chromatids per nucleus).
N will refer to the number of chromosomes but not the DNA content!

There are few plants with 3n (triploid) and 4n (tetraploid)

I hope this helps
3. (Original post by Sigma5116)
In meiosis you start with 2n so in humans that's 2*23 chromosomes and 2*23 chromatids before DNA replication takes place. Once DNA replication takes place there are still same number of chromosomes (2*23) but double the number of chromatids so 2 chromatids per chromosome so (2*2*23 chromatids). The homologous chromatids dissociate after meiosis I and produce 23 chromosomes per nucleus and each chromosome will have 2 chromatids (23 chromosomes and 2*23 chromatids per nucleus). The sister chromatids dissociate after meiosis II and produce 23 chromosomes per nucleus and 1 chromatid per chromosome. (23 chromosomes and 23 chromatids per nucleus).
N will refer to the number of chromosomes but not the DNA content!

There are few plants with 3n (triploid) and 4n (tetraploid)

I hope this helps
So the DNA replication forms double chromatids, not chromosomes. But then, after the first meiosis division, the chromosomes turn from 2 chromatids to 1 chromatid so then how can the second division take place.

Does this mean DNA replication happens 2 times
4. (Original post by nasira372)
So the DNA replication forms double chromatids, not chromosomes. But then, after the first meiosis division, the chromosomes turn from 2 chromatids to 1 chromatid so then how can the second division take place.

Does this mean DNA replication happens 2 times
No after meiosis I, the homologous chromosomes (bivalent) dissociates with 2 chromatids per chromosomes intact. In meiosis II the chromatids dissassembles from the single chromosome.

I hope this image will help.

5. (Original post by Sigma5116)
In meiosis you start with 2n so in humans that's 2*23 chromosomes and 2*23 chromatids before DNA replication takes place. Once DNA replication takes place there are still same number of chromosomes (2*23) but double the number of chromatids so 2 chromatids per chromosome so (2*2*23 chromatids). The homologous chromatids dissociate after meiosis I and produce 23 chromosomes per nucleus and each chromosome will have 2 chromatids (23 chromosomes and 2*23 chromatids per nucleus). The sister chromatids dissociate after meiosis II and produce 23 chromosomes per nucleus and 1 chromatid per chromosome. (23 chromosomes and 23 chromatids per nucleus).
N will refer to the number of chromosomes but not the DNA content!

There are few plants with 3n (triploid) and 4n (tetraploid)

I hope this helps
Would it still be alright to say when: In the interphase stage of meiosis:

That we start with 46 chromosomes that are 23 pairs with each chromosomes from maternal and paternal looking like this: l l

Then after replication the 46 chromosomes become 92 and now THE QUESTION can i say that: From before with 1 chromosome from the father lets say, and now 2 chromsomes or what we call chromatids a chromatid and sister chromatid, what I call a DOUBLE ARMED CHROMOSOME.

So now the chromosomes become double armed like this:

l-l l-l the - is the centromere

I think you are the best person to ask as you are a genetics person and I really want assuring about my understanding of the chromosomes etc

Thanks
6. (Original post by NinjaRikki)
Would it still be alright to say when: In the interphase stage of meiosis:

That we start with 46 chromosomes that are 23 pairs with each chromosomes from maternal and paternal looking like this: l l

Then after replication the 46 chromosomes become 92 and now THE QUESTION can i say that: From before with 1 chromosome from the father lets say, and now 2 chromsomes or what we call chromatids a chromatid and sister chromatid, what I call a DOUBLE ARMED CHROMOSOME.

So now the chromosomes become double armed like this:

l-l l-l the - is the centromere

I think you are the best person to ask as you are a genetics person and I really want assuring about my understanding of the chromosomes etc

Thanks
First of all in interphase, 23 pairs of chromosomes replicate, but are still intact by the centromere so they are still one chromosome but with 2 chromatids each. So it's still 46 chromosomes with 2 chromatids each (they are sister chromatids).
So |-| is still a single chromasome but with 2 chromatids, so |-|, |-| will be 2 homologous chromosomes with 2 chromatids each.
It's probabably better to say a chromosome with 2 chromatids instead of double armed chromosome.
I hope this helps
7. (Original post by Sigma5116)
No after meiosis I, the homologous chromosomes (bivalent) dissociates with 2 chromatids per chromosomes intact. In meiosis II the chromatids dissassembles from the single chromosome.

I hope this image will help.

So let me get this straight, you start with 2n, then after replication, you still have 2n but all chromosomes have sister chromatids. In meiosis 1, the chromatids seperate and each chromatid becomes a chromosome leaving you with 2 cells, each with 2n.

Am I right so far....

The confusion kicks in on the second meiosis division. Do the chromosomes replicate again and split like meiosis 1.

Please highlight the bits I have got wrong. I really appreciate this mate
8. (Original post by nasira372)

So let me get this straight, you start with 2n, then after replication, you still have 2n but all chromosomes have sister chromatids. In meiosis 1, the chromatids seperate and each chromatid becomes a chromosome leaving you with 2 cells, each with 2n.

Am I right so far....

The confusion kicks in on the second meiosis division. Do the chromosomes replicate again and split like meiosis 1.

Please highlight the bits I have got wrong. I really appreciate this mate
In terms of still having 2n that's correct but in meiosis I homologous chromosomes are segregated and in meiosis II sister-chromatids are separated.

I hope this helps.

"Describe using well labelled diagrams the two ways in which variation occurs in the process of meiosis."
10. (Original post by FrozenBeak)

"Describe using well labelled diagrams the two ways in which variation occurs in the process of meiosis."
One will be that effectively the segregation of homologous chromosomes are random process so it's like picking 23 different socks out of 23 pairs as the homologous chromosomes are not genetically identical.
Another will be that non-sister chromatids can go through crossing over if you know about them?

I hope this helps
11. (Original post by nasira372)
meiosis is when the chromosomes turn from 2n to n. The first stage of meiosis is when the DNA replicates therefore would it be right to say the chromosomes change from 2n to 4n. Then in the 1st meiosis division, the 4n splits to 2 2n cells. Then in the second meiosis division, the 2 2n cells split into 4 n cells.

I think what I am saying above is correct but in the textbook, it starts with 2n, and then when it replicates, it stays at 2n. Then divides to 2*2n and then 4*n

If there is such thing as 4n, what is it called (n=haploid, 2n=diploid, 4n=?)
Haploid is half and diploid is full so you can't have 4n because that would be double the information needed right?
12. (Original post by HEJ92)
Haploid is half and diploid is full so you can't have 4n because that would be double the information needed right?
Diploid is not necessarily full. some organisms like salmon and trout or some plants are able to be 4n.
13. (Original post by Sigma5116)
Diploid is not necessarily full. some organisms like salmon and trout or some plants are able to be 4n.
I did not know this, but then again, I am only learning about people (Nursing)
14. After mieosis there are 23 chromatids in a single gamete, how then does a parent cell have 46 chromosomes? please help

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