Meiosis II

Meiosis II is the second of the two successive divisions of meiosis, and it follows meiosis I directly, without any intervening round of DNA replication -- the two haploid cells produced by meiosis I proceed straight into meiosis II carrying chromosomes that are already duplicated (each still consisting of two sister chromatids) from the single round of DNA replication that occurred long before, back in the S phase that preceded meiosis I.

In its overall course, meiosis II closely resembles an ordinary mitotic division, and is described using the same four stages. In prophase II, the chromosomes, which may have partly decondensed during the brief interval between the two divisions, recondense, and a new spindle apparatus begins to form in each of the two haploid cells. In metaphase II, the chromosomes -- now single, unpaired chromosomes rather than the bivalents of meiosis I, since homologous pairing was already resolved in meiosis I -- line up individually at the equatorial plate of each cell, exactly as in a mitotic metaphase. In anaphase II, the centromere joining the two sister chromatids of each chromosome finally splits, and the separated chromatids, now individual daughter chromosomes, move toward the two opposite poles of each cell. In telophase II, the two new sets of chromosomes at each pole decondense, and a nuclear envelope re-forms around each, after which cytokinesis divides each of the two cells into two, completing the process.

Because meiosis II acts independently within each of the two haploid cells produced by meiosis I, the net result of the two divisions taken together is that one original diploid parent cell gives rise to four haploid daughter cells in all. In male animals, all four of these products typically develop into functional gametes (sperm); in most female animals, by contrast, unequal cytoplasmic division at one or both meiotic divisions produces only one large, functional egg cell together with two or three much smaller, non-functional cells called polar bodies, which eventually degenerate. …