Why do we even have meiosis II?
I don’t get why meiosis needs two divisions. Mitosis makes two identical cells in one division. Meiosis I already separates homologues and makes haploid cells. So why do we need meiosis II at all? Wouldn’t one division be enough? Someone explain please, my brain hurts.
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"After meiosis I you have two haploid cells, but each chromosome still consists of two sister chromatids (so DNA content is still 2c even though chromosome number is n). If you stopped here, gametes would have double the normal amount of DNA.
Meiosis II exists exactly to separate those sister chromatids so each final gamete gets only one chromatid per chromosome (1c, 1n — correct haploid amount).
That’s why the whole process is two divisions but only one DNA replication. Meiosis I reduces chromosome number by half (separates homologues), Meiosis II separates identical copies (like mitosis).
Without meiosis II our sperm and eggs would have twice as much DNA as needed → after fertilization the zygote would have 4n instead of 2n and everything would be messed up.
Very good visual explanation and summary of both divisions here: phases of meiosis
So short answer: meiosis II is necessary to produce gametes with the correct single-copy DNA amount. "