19.3: Meiosis II
Meiosis II entails cell division and segregation of the sister chromatids, resulting in the production of four unique haploid gametes. The steps for meiosis II are similar to mitosis, except that meiosis II occurs in haploid cells, whereas mitosis occurs in diploid cells.
The timing and cell division patterns of meiosis differ between males and females. In male meiosis, the centrosomes are part of the formation of the meiotic spindle. However, in oocytes, including that of humans, Drosophila, and mice, the meiotic spindle forms without centrosomes. Further, in mammalian oocytes, cyclin A2 plays a vital role in the stabilization of the meiotic spindle and the separation of the sister chromatids in anaphase II. In mouse oocytes, inhibition of cyclin A2 results in disordered spindle formation and failure of segregation of sister chromatids.
Telophase II is followed by cytokinesis, which completes one meiosis cycle with the production of four haploid daughter cells. In male meiosis, all four daughter cells have equal amounts of cytoplasm; however, in female gametes, asymmetric distribution of the cytoplasm occurs in meiosis I, which leads to the production of an egg cell and a haploid polar body. The egg cell undergoes meiosis II only after fertilization and produces the haploid mature ovum and the secondary polar body. The mature ovum then fuses with the sperm and forms the diploid zygote.