Metaphase I is similar to the metaphase of mitosis except that the paired chromosomes are aligned at the equatorial plate with one member on either side. The homologous chromosomes are still held together by chiasmata. At late metaphase, chiasmata are cleaved and the chromosomes separate. Once the nuclear envelope has broken down, the spindle micro-tubules begin to interact with the chromosomes through the multilayered protein structure, the kinetochore, which is usually positioned near the centromere (see Fig. 3.13). The chromosomes undergo movement to ultimately align their centromeres along the equator of the spindle.
Anaphase I and telophase I are similar to the same phases in mitosis except that the centromeres do not split. The  sister chromatids, held together by cohesin complexes and by the centromere, remain together. A maternal or paternal member of each homologous pair, now containing exchanged segments, moves to each pole. Segregation or random assortment occurs because the maternal and paternal chromosomes of each pair are randomly aligned on one side or the other of the metaphase plate, thus contributing to genetic diversity. At the completion of meiosis I, the cytoplasm divides. Each resulting daughter cell (a secondary spermatocyte or oocyte) is haploid in chromosome number (1n) and contains one member of each homologous chromosome pair. The cell is still diploid in DNA content (2d).
After meiosis I, the cells quickly enter meiosis II without passing through an S phase. Meiosis II is an equatorial division and resembles mitosis. During this phase, the proteinase enzyme separase cleaves the cohesion complexes between the sister chromatids. Cleavage of the cohesin complexes in the region of the centromere releases the bond between both centromeres. This cleavage allows the sister chromatids to separate at anaphase II and move to opposite poles of the cell. During meiosis II, the cells pass through prophase II, metaphase II, anaphase II, and telophase II. These stages are essentially the same as those in mitosis except that they involve a haploid set of chromosomes (1n) and produce daughter cells that have only haploid DNA content (1d). Unlike the cells produced by mitosis, which are genetically identical to the parent cell, the cells produced by meiosis are genetically unique.
In humans, as in all other multicellular organisms, the rates of cell proliferation and cell death determine the net cell production. An abnormality in any of these rates can cause disorders of cell accumulation (e.g., hyperplasia, cancer, autoimmune diseases) or disorders of cell loss (atrophy, degenerative diseases, AIDS, ischemic injury). Therefore, the balance (homeostasis) between cell production and cell death must be carefully maintained (Fig. 3.17).
Cell death may occur as a result of acute cell injury or an internally encoded suicide program.
Cell death may result from accidental cell injury or mechanisms that cause cells to self-destruct. The major two different mechanisms of cell death are necrosis and apoptosis.
 Necrosis, or accidental cell death, is a pathologic process. It occurs when cells are exposed to an unfavorable physical or chemical environment (e.g., hypothermia, hypoxia, radiation, low pH, cell trauma) that causes acute cellular injury and damage to the plasma membrane. Under physiologic conditions, damage to the plasma membrane may also be initiated by viruses, or proteins called perforins. Rapid cell swelling and lysis are two characteristic features of this process.