The biggest difference between the separation of chromosomes in meiosis and meiosis II is:
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Meiosis (specifically Meiosis I): Homologous chromosomes (pairs of similar chromosomes, one from each parent) are separated. This reduces the chromosome number by half, so each new cell gets one chromosome from each pair.
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Meiosis II: The sister chromatids (identical copies of a chromosome) are separated. This is similar to what happens in mitosis, and the chromosome number stays the same in each cell produced.
In summary, Meiosis I separates pairs of chromosomes, while Meiosis II separates sister chromatids.