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Report Thread starter 6 years ago
This is a pretty embarrassing question to ask considering I'm a medical student and it's something a GCSE student should know.

Now, I know that diploid = 2n which is 46 chromosomes and haploid = n which is 23 chromosomes.
Diploid cells are all somatic cells and haploid cells are only gametes.
So I've stated the main differences between haploid and diploid cells myself, why am I getting confused?

Because if you look at the image below
Name:  oogenesis.jpg
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for the first 2 cells right at the top, the germ cell and oogonium, it shows 2 separate chromatids and it is labelled as diploid. I'm perfectly okay with that. However, the next one down, the primary oocyte, it now has double that number and it is still labelled as diploid. Why?
It's not just this, look down further at meiosis II and the secondary oocyte which has 2 chromosomes is called haploid. I don't understand that either. But if you look further down to the egg and polar body that has only 1 chromosome, that is labelled haploid as well. I that the egg and polar body should be called haploid but what about the secondary oocyte with 2 chromosomes that I just said? Why is that called haploid?

And this isn't the only thing that is saying this. Other sources are saying this as well.

Can someone please explain to me what is going on? Have I got the meaning of haploid and diploid wrong? What's happening?
Hype en Ecosse
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Report 6 years ago
First of all (and I don't mean to patronise you when I say this!), but special notes need to be given to the fact that that diagram describes oogenesis and the process is obviously different for spermogenesis!

Haploid and diploid refer to the number of chromosomes that a cell possesses, rather than the number of chromatids or amount of DNA present. I think you might want to refresh these definitions! You're right that as we transfer from an oogonium to a primary oocyte that we double the amount of DNA present in our cell, but we do not double the number of chromosomes, hence why it's still diploid.

So at the stage of the oogonium, we have two unduplicated chromosomes. Then at the stage of the primary oocyte, we have two duplicated chromosomes (each made up of two chromatids). Then our cells undergo meiosis I, and we end up with our secondary oocyte, and it gives off a polar body.

Now in our secondary oocyte, we only have one chromosome (although it's still made up of two chromatids). It only has one set of DNA (from either mum or dad) rather than both sets, hence why it's haploid. Then we undergo meiosis II that pulls the two identical sister chromatids apart, leaving us with one daughter chromosome, which still contains only one set of DNA. Do you get what I mean?

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