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BioFlix Meiosis

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Humans produce gametes [gA-meats] -- eggs and sperm -- through the process of meiosis [my-O-sis]. Here we'll follow the production of male gametes by focusing on this cell as it goes through meiosis. Let's begin in the nucleus, where genetic information is stored in chromosomes. Most of a person's cells are diploid [DIP-loyd], with two sets of chromosomes. One set is from their mother, shown here in red, and the other set is from their father, shown in blue. Each maternal chromosome has a corresponding paternal chromosome. These matched pairs are called homologous [huh-MOL-uh-gus] chromosomes. During interphase, chromosomes are duplicated. Each chromosome now consists of two identical copies called sister chromatids. Zooming in, we see that each sister chromatid is made up of DNA wound around histone proteins. Each strand coils up into a tight helical fiber. As meiosis begins, a spindle forms and duplicated centrosomes start to migrate toward opposite poles of the cell. Back in the nucleus, the chromosomes are condensing. In meiosis, homologous chromosomes stick together in pairs. The close association of homologous chromosomes allows segments of non-sister chromatids to trade places. This recombination of maternal and paternal genetic material is a key feature of meiosis. After the spindle forms and the nuclear envelope breaks down, microtubules from opposite poles attach to each chromosome of the homologous pair, resulting in a tug-of-war. At metaphase I, the chromosome pairs are positioned in the middle of the cell. The next stage begins when homologous chromosomes separate from each other and move toward opposite poles. Each chromosome still consists of two sister chromatids. This cell began meiosis with 46 chromosomes, but each daughter cell now has only 23 chromosomes. In meiosis II, microtubules from opposite poles attach to the chromosomes, which then move to the center of the cell. Next, the sister chromatids separate, becoming full-fledged chromosomes that move to opposite poles. Nuclear envelopes re-form, and each daughter cell divides into two cells. We started with a single diploid cell, and now that meiosis is complete, we have four haploid cells, cells with a single set of chromosomes. These haploid cells mature into gametes that can contribute to the next generation. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings