(A) In mitosis, diploid cells replicate chromosomes during S phase and segregate sister chromatids during M phase, so that diploid daughter cells are produced. (B) In meiosis, two chromosome-segregation phases, meiosis I and meiosis II, follow a single round of DNA replication during the premeiotic S phase. During meiosis I, homologous chromosomes (shown in red and blue) are segregated to opposite poles. Sister chromatids then segregate to opposite poles during meiosis II, which results in the formation of nonidentical haploid gametes. Note that the lengths of the cell-cycle stages are not drawn to scale.
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How is the same process responsible for genetic recombination and diversity also the cause of aneuploidy? Understanding the steps of meiosis is essential to learning how errors occur.
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