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There is something about the numbers of chromosomes that doesn't make sense to me:

Let's take this illustration: enter image description here

So a gamete has 23 chromosomes, which are haploid (have only one chromatid), is that correct? Before meiosis it replicates, so now we have 23*2, i.e 2n as in the first left picture where it is written "Parent cell (2n)". Then I don't understand how is it formed in a tetrad. Where do the additional two come from. Also in the start we have 2n (after replication) and in the end after two divisions we have n for each gamete of the total 4, so its total of 4n, what do I miss ?

Otherwise the first left picture is before the replication of the chromosomes, but then why are there 2n in a gamete cell, isn't it just 1n as in the end of the process ? Isn't the parent cell also a haploid with just 1n chromosomes with just one chromatid (because if it is before replication, it looks like a diploid cell).

acvill
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Ilya.K.
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  • see also: https://biology.stackexchange.com/questions/55832/what-is-the-purpose-of-two-cell-divisions-in-meiosis – Maximilian Press Apr 27 '22 at 19:34
  • I would recommend looking more closely at the image. I would suggest that it is in fact 1n. Those are two different chromosomes, not copies of the same chromosome, in the gametes. – Maximilian Press Apr 27 '22 at 19:36
  • Why do you think the parent cell is haploid? – Bryan Krause Apr 27 '22 at 19:50
  • @BryanKrause, that how it happens in humans, isn't it ? Meiosis is a cell division process, which happens from one cell to four or am I wrong ? I should be haploid isn't it ? Sex cells are haploids (gametes) in humans by definition. – Ilya.K. Apr 27 '22 at 19:53
  • @Ilya.K. Which cells in your diagram are gametes, and where do those gametes come from? – Bryan Krause Apr 27 '22 at 19:54
  • @BryanKrause, well it isn't said in the diagram itself, but meiosis is relevant only in gametes. Could it be that the parent cell isn't a gamete and the daughter cells are ? The daughter cells are gametes for sure, the parent cell logically assuming must be too. – Ilya.K. Apr 27 '22 at 20:23
  • Getting closer :) Where do gametes come from, and what is the purpose of meiosis? – Bryan Krause Apr 27 '22 at 20:24
  • @BryanKrause that's probably the part where my knowledge is a little bit unclear. Where do they come from : I'm not sure, like they it's a differentiation thing, each cell in the body has the whole genome set (but it differentiate by the set of actual proteins generated by the appropriate rna instructions) , which means I think that each cell in the body from one hand has the whole 23 chromosomes, but in gametes as much as I understand the chromosomes are complected each from only one chromatid, and no two, because they are haploid cells and not diploid. I've probably mistake anywhere here. – Ilya.K. Apr 27 '22 at 20:30
  • @BryanKrause, the purpose is cell replication, like part of the life cycle of the cells. – Ilya.K. Apr 27 '22 at 20:31
  • @BryanKrause, are you trying to tell me that to create gametes, we need the diploid cell, which the parent cell is ? I think our lecturer didn't told us that part. Like who is the parent cell ? Cuz in the second image I've just posted, it is obviously said that from one pair of chromosomes (a pair means it is diploid and has one chromatid from male and one from female) in the parent cell we get 4 haploid gamete cells. It's only makes sense if the parent cell is diploid. But who is it if not a gamete (and it can't be gamete) ? Like it can't be skin cell or some other tissue cell, can it ? – Ilya.K. Apr 27 '22 at 20:32
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    Yeah, I think you've just about got it. After fertilization, a typical human cell has 46 chromosomes, 2 copies of 23 chromosomes, with caveat that one of the pairs is the sex chromosomes which can be XX or XY. We call that a diploid, 2n cell, and there are 2 copies of each because one copy came from an egg gamete and the other copy came from a sperm gamete. Those 2n cells divide and divide to make all the cells of the body via mitosis. To make gametes, though, you need to make 1n cells; meiosis is how that happens. – Bryan Krause Apr 27 '22 at 20:38
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    The cells that give rise to gametes through meiosis are called germ cells; that's pretty much their job, simplifying things a bit. As far as "Like it can't be skin cell or some other tissue cell, can it ?" - as the wikipedia article describes, it depends - for animals, you're right, they're particular cells. For plants, they come from otherwise ordinary cells, a bit like making gametes from skin cells, yes, just in the right place at the right time. – Bryan Krause Apr 27 '22 at 20:40
  • @BryanKrause, thanks, that's a good point been clarified. – Ilya.K. Apr 28 '22 at 12:47

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