Meiosis-haploid... and then????

[TheresaW.]

First thread ever! So in A&P we learned about cell division in meiosis and mitosis last week. I understand that mitosis results in 2 daughter cells which are identical to the parent cell and also has diploid number of chromosomes. I understand that meiosis results in 4 daughter cells with haploid amount of chromosomes. Here's what I cannot wrap my brain around: In meiosis, after anaphase I, there will be 2 daughter haploid cells and then telophaseI, prophase II, metaphase II, then anaphase II... resulting in 4 daughter HAPLOID cells... why haploid? When the cells divided again during meiosis II, how do they remain haploid and not less than that??? I've been trying to figure this out for hours... any help would be greatly appreciated... THANKS!!

[PacoUSA, BSN, RN]

If I recall correctly, haploid cells by definition only have one set of chromosomes. You can't have less than 1 chromosome (absent some genetic defect I guess). This is why sperm and eggs are haploid cells, combining to produce a human (which is made up of diploid cells (two sets of chromosomes).

[Stcroix, ASN, PhD, RN]

ok, in Meiosis 1 the PAIRS of homologous chromosomes line up. When the cell divides, the daughter cells only have one copy of each chromosome, and by definition this is haploid. In meiosis II the 2 chromatids of each remaining chromosome split apart. Now you have 4 daughter cells with one chromatid of each chromosome. Later, the haploid gamete will join with the opposite sex gamete, making a diploid cell with only one chromotid from each. the chromatids form copies, and you are back to the normal diploid cell with both copies of the chromatids. I hope that made sense.

[TheresaW.]

Thank you very much!! I am understanding all the divisions and the results from them... I'm still confused. I don't usually get stumped like this!! I'm going to ask my instructor first thing in the morning and then come home and figure it out! Thanks a lot for the help! :)

[TheSquire, DNP, APRN, NP]

Short answer: Chromatids do not a chromosome make.

Long answer: Eukaryotic chromosomes are counted by the number of centromeres present in the cell. After S phase, sister chromatids are joined at a common centromere until pulled apart. At Metaphase I, pairs of chromosomes line up (to allow for crossing-over) and then separate, pulled at their respective centromeres. The number of chromosomes per cell goes from 2n to n.

Metaphase II works just like Metaphase in Mitosis, in that all the chromosomes line up at the center without a buddy and the two sister chromatids (ideally) get pulled apart into singleton chromosomes. Because this is the same process as Mitosis, there is no change in the number of chromosomes involved, which stays at n.