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Why does meiosis involve two rounds of cell division instead of stopping after meiosis I, where each daughter cell would have one chromosome randomly selected from each pair of homologous chromosomes? Theoretically, this could be achieved without DNA replication prior to meiosis I? Why do we need four daughter cells rather than two? Where in meiosis, effectively, one chromosome from each homologous pair goes into two of the four gametes, due to the chromosomes in meiosis I being composed of two sister chromatids. I am clearly oversimplifying the process and overlooking some important components.


In an attempt to answer my own question, I asked myself: whether the two daughter cells from meiosis II (I am referring to one pair that is produced from each parent cell), were identical. I realised I had forgotten to consider recombination. While it may be true that sister chromatids are identical copies of a chromosome created during DNA replication, crossing over during meiosis I can create genetic diversity between sister chromatids. Crossing over occurs between non-sister chromatids of homologous chromosomes. As a result, even though sister chromatids are separated during meiosis II, the daughter cells produced may not be genetically identical due to crossing over that occurred during meiosis I. The result I assume would look something like this. However, I realised recombination would still be possible, albeit with two unique gametes, rather than four.

If at any point I say anything incorrect feel free to point that out.

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  • $\begingroup$ I saw a potential answer here, however, the answer is arranged in a way that is unclear and difficult to read for me. $\endgroup$ Aug 7, 2023 at 4:31

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If I understand correctly, your idea is to say, it would also be possible to start with unduplicated chromosomes (no S-Phase), perform crossing over to increase the genetic variety and then to distribute the single chromosomes into different cells - and now you want to know what's the difference to regular Meiosis and "why" it happens that way? As you hopefully know, "why" questions are not really scientific. We cannot really know why something happened. We can only speculate that this could be a reason or provide an advantage for evolution. One explanation why it would be advantageous to have S-Phase before and use Meiosis 2 to reduce the chromosome number is (as explained in the question you cited) the following:

  • Crossovers (CO) play a huge and important role for generating gametes and is the basis of genetic diversity.
  • COs are generated by double-strand breaks (DSBs).
  • Most DSBs are not converted into COs and need repair through different mechanisms.
  • The sister chromatid helps as a template for that.
  • My speculation is that without a second sister chromatid during crossovers, the cell would simply die due to DNA damage.
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