Meiosis II and the Haploid Product

After meiosis I, you already have two cells, and each is haploid: one chromosome from every homologous pair, no duplicate set. But each of those chromosomes is still doubled, made of two sister chromatids joined at a centromere. Meiosis II exists to pull those sister chromatids apart so each final cell gets just one chromatid per chromosome. Without it, gametes would carry chromosomes that are twice as big as they should be.

Meiosis II looks almost exactly like mitosis, run on a haploid cell. In prophase II the chromosomes condense again, in metaphase II they line up single-file at the equator, and in anaphase II the centromere finally splits, so sister chromatids separate and get pulled to opposite poles by spindle fibers. Telophase II and cytokinesis follow, giving each of the two meiosis-I cells two daughter cells. Start with one diploid cell, divide once (meiosis I) to get two haploid cells, divide again (meiosis II) to get four haploid gametes total, and because crossing over and independent assortment happened back in meiosis I, those four are genetically distinct from each other.

The key reason meiosis II skips replication: DNA copies itself only once, during S phase, before meiosis I even starts. That single replication is what creates the sister chromatids in the first place. Meiosis II just finishes the job of separating chromatids that are already there. Lined up against mitosis, both processes separate sister chromatids in their final division, but mitosis starts and ends diploid while meiosis starts diploid and ends haploid, because meiosis I cut the chromosome number in half first.