Meiosis I
Metaphase I: Homologue pairs line up at the metaphase plate. Anaphase I: Homologues separate to opposite ends of the cell. Sister chromatids stay together. Telophase I: Newly forming cells are haploid, n = 2. Each chromosome still has two sister chromatids, but the chromatids of each chromosome are no longer identical to each other. When the homologous pairs line up at the metaphase plate, the orientation of each pair is random. For instance, the pink version of the big chromosome and the purple version of the little chromosome happen to be positioned towards the same pole and go into the same cell. But the orientation could have equally well been flipped, so that both purple chromosomes went into the cell together. This allows for the formation of gametes with different sets of homologues Diagram showing the relationship between chromosome configuration at meiosis I and homologue segregation to gametes. The diagram depicts a simplified case in which an organism only has 2n = 4 chromosomes. In this case, four different types of gametes may be produced, depending on whether the maternal homologues are positioned on the same side or on opposite sides of the metaphase plate. Diagram showing the relationship between chromosome configuration at meiosis I and homologue segregation to gametes. The diagram depicts a simplified case in which an organism only has 2n = 4 chromosomes. In this case, four different types of gametes may be produced, depending on whether the maternal homologues are positioned on the same side or on opposite sides of the metaphase plate. In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don't come apart. Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes decondense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II^{2,3} Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells. Meiosis II Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis for haploid cells." The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid—have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids. In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes. Phases of meiosis II Prophase II: Starting cells are the haploid cells made in meiosis I. Chromosomes condense. Metaphase II: Chromosomes line up at the metaphase plate. Anaphase II: Sister chromatids separate to opposite ends of the cell. Telophase II: Newly forming gametes are haploid, and each chromosome now has just one chromatid. Phases of meiosis II Prophase II: Starting cells are the haploid cells made in meiosis I. Chromosomes condense. Metaphase II: Chromosomes line up at the metaphase plate. Anaphase II: Sister chromatids separate to opposite ends of the cell. Telophase II: Newly forming gametes are haploid, and each chromosome now has just one chromatid. During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes.The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate. In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell. In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells.