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At first, I thought it was because of crossing-over, but when I thought more about it, that didn't seem reasonable. Why don't cells just do meiosis like this? (I know that we don't understand all the why's and how's in biology yet, but if anybody has an explanation for this I would appreciate it)

  1. Diploid cell has 46 unique chromosomes - 23 homologous pairs
  2. Cell crosses over the homologous pairs WITHOUT duplicating them first
  3. Metaphase and Anaphase produce two haploid daughter cells, each with a unique combination of genes within a chromosome - 23 total

Instead, this is what they do (or at least, how I understand them to do)

  1. Diploid cell has 46 unique chromosomes - 23 homologous pairs
  2. Cell duplicates all chromosomes - 92 in total - 46 unique chromosomes - 23 homologous pairs
  3. Cell crosses over the homologous pairs
  4. Metaphase and Anaphase produce two haploid daughter cells, each with 46 chromosomes - but only 23 are unique - the others are duplicates of that 23 (with the exception of the variance caused by the crossing over - what I mean is, all 46 were all initially from the same parent instead of the 23/23 found in a diploid cell)
  5. The two daughter cells undergo Meiosis II in order to produce four total daughter cells - Each has 23 chromosomes - All 23 are unique

So why does the cell take this more difficult path in order to achieve the same thing? Am I missing something out here? I would really appreciate some help. Thanks!

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here is what I think why cells evolved to do meiosis like as it is now and not like you mentioned.

First I will try to tell you briefly (not in detail) how crossovers are made and how they ensure the segregation of chromosomes. During meiosis the cells are programmed to make DNA double strand breaks (DSBs). These DSBs are repaired by a pathway called Homologous Recombination (HR), where the breaks can be repaired into cross overs (CO) or non cross overs (NCO) using a template DNA (which could be DNA from sister chromosome or from homologous chromosome). For the homologous chromosomes to properly segregate during Meiosis I, at-least there should be 1 CO per homolog pair. But in most studied organisms (including humans, mice), the number of DSBs largely outnumbers the CO number. For instance in mice there are 20 homolog chromosome pairs, so they need 20 COs for proper homologous chromosome segregation; but to produce 20 COs the cells are making 200-300 DSBs. And the remaining are repaired as NCO with homologous chromosome or as both CO and NCO with sister chromosomes.

So coming back to your possibility, if the cell was trying to do CO without DNA replication (without sister chromosome), it is risking its genome for less options for repair. More molecularly, sister chromosome is the ideal template to properly repair a DSB than homologous chromosome!

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