bio question: An egg cell contain..

[Hope2007]

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Hi everyone, I just want to check if I am right about this biology question which seems pretty easy but..
I think: An Egg cell (N) contains 23 chromosomes and 46 chromatids. (each chromosome has 2 chromatids which are joined at centromere). However, the answer said: an egg has 23 chromosomes, 23 chromatids.
Am I wrong? (I took gen biol 5 yrs ago).

 

[au5233]

I thought it would be 23 chromosomes and 46 chromatids as well. I remember there was a similar destroyer question which I disagreed with. My reasoning:

I believe an egg cells arrests in prophase of meosis II, until the point of fertilization AT WHICH POINT it is no longer an egg, it is a zygote and we get our second polar body. So to me, if an egg is considered everything up to and including prophase II, then it should have 23 chromosomes and 46 chromatids.
Anyone else want to debate this?

 

[REH]

I believe the answer is actually correct. Let's walk through it...You have a primary oocyte (diploid, 46 CRX, 4N) undergo meiosis I to form two secondary oocytes (haploid, 23 CRX, 2N). One of these will be lost as a polar body, resulting in a single secondary oocyte. The secondary oocytes undergo meiosis II to form one ovum (haploid, 1N). If fertilization of the ovum occurs, we get a 2N zygote which restores our diploid chromosome number. I'm not sure of this is what you're having troubles with...

If it's chromosomes/chromatids that's troubling you, it might be helpful to think about the structure of a chromosome in metaphase II. We see an X, which represents a single chromosome. This X contains two sister chromatids which are bound together at the central portion of the chromosome, the centromere. In anaphase II, the centromere splits and the sister chromatids migrate towards opposite poles of the cell. At this point, we refer to the sister chromatids as chomosomses. This produces a haploid ova with one sister chromatid for every diploid chromosome.

Hope this helps,

REH.

 

[au5233]

If it's chromosomes/chromatids that's troubling you, it might be helpful to think about the structure of a chromosome in metaphase II. We see an X, which represents a single chromosome. This X contains two sister chromatids which are bound together at the central portion of the chromosome, the centromere. In anaphase II, the centromere splits and the sister chromatids migrate towards opposite poles of the cell. At this point, we refer to the sister chromatids as chomosomses. This produces a haploid ova with one sister chromatid for every diploid chromosome.

...but this doesn't happen until AFTER fertilization...the egg is arrested in Prophase II until fertilization. So how can an egg have 23 chromatids if it didn't finish meiosis II? And then as soon as its fertilized it finishes meiosis II but at the moment of fertilization its a zygote. So I dont think an egg ever truly has 23 chromatids.

 

[REH]

You're right! I didn't think this through. Thanks for pointing this out.

...but this doesn't happen until AFTER fertilization...the egg is arrested in Prophase II until fertilization. So how can an egg have 23 chromatids if it didn't finish meiosis II? And then as soon as its fertilized it finishes meiosis II but at the moment of fertilization its a zygote. So I dont think an egg ever truly has 23 chromatids.[/QUOTE]

 

[au5233]

No problem. Good teamwork to solve this problem...what I like to see 🙂 Again, I think there was a problem in destroyer similar to this which I did not agree with because of the whole fertilization fact...just to make u guys aware. Maybe Dr. Romano can shed some light on it.

 

[Donjuan]

You're right! I didn't think this through. Thanks for pointing this out.

...but this doesn't happen until AFTER fertilization...the egg is arrested in Prophase II until fertilization. So how can an egg have 23 chromatids if it didn't finish meiosis II? And then as soon as its fertilized it finishes meiosis II but at the moment of fertilization its a zygote. So I dont think an egg ever truly has 23 chromatids.

Correct me if I'm wrong, but I read in examkrackers that the egg is arrested in Metaphase II, and once fertilization occurs, the zygote completes anaphase II. Feel free to offer you two cents guys. Happy studying

- Donjuan

 

[au5233]

...it could be metaphase II..but that still wouldn't affect the chromosome/chromatid number of the egg b/c at metaphase they are all still joined.

 

[Hope2007]

Thank you all for the information!!
At the end of miotic division I, secondary oocyte (23xx, 46chromatids) and a polar body are produced. At this stage, each chromosome is still composed of 2 sister chromatids. These chromatids do not separate until devision II of meiosis (anaphase II). After fertilization, secondary oocyte will futher divided. As the result an Egg (ova, 23xx, 23chromatids) and 3 polar bodies are produced.

 

[chessxwizard]

You got it. 👍

 

[jimmy1fernan]

Immediately after a mitosis, where a cell has divided in two, but not yet duplicated its DNA, there are still 23 chromosome pairs (46 chromosomes). However, each chromosome only has one chromatid. Thus there are 46 chromatids (2xN)
Alternatively, a haploid cell with two chromatids per chromosome also has 46 chromatids. However, this doesn't occur naturally in humans.