The reason for two divisions is to reduce the chromosome number by half while still allowing genetic mixing. Body cells are diploid (2n) – they have two sets of chromosomes. Gametes (sperm and egg) must be haploid (n) so that when sperm + egg fuse, the zygote is diploid again (2n). If there was only one division, you would get diploid gametes and chromosome number would double every generation – not good! So: Meiosis I separates homologous chromosomes ? two haploid cells (each chromosome still has two chromatids). Meiosis II separates sister chromatids ? four haploid cells with one chromatid each. Also, during meiosis I (especially prophase I) crossing over happens and in metaphase I independent assortment occurs – both increase genetic diversity. That’s why meiosis needs two rounds. This page explains all phases step-by-step: phases of meiosis. Hope it’s clearer now!
