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Chapter 12 Meiosis

The chromatids that meet to form a chiasma are

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The chromatids that meet to form a chiasma are homologous but not sisters. Morgan proposed that a physical exchange of paternal and maternal chromosomes occurs at chiasmata. According to Morgan's hypothesis, paternal and maternal chromatids break and rejoin at each chiasma, producing chromatids that have both paternal and maternal segments. This process of chromosome exchange is called crossing over. In metaphase I, the kinetochore microtubules move the pairs of homologous chromosomes to a region called the metaphase plate, in the middle of the cell. o Each tetrad moves to the metaphase plate independently of the other tetrads, and the alignment of maternal an d paternal homologs from each chromosome is random. During anaphase I, the homologous chromosomes in each tetrad separate and being moving to opposite sides of the cell. Meiosis I concludes with telophase I, when the homologs finish moving to opposite sides of the cell. When meiosis I is complete, cytokinesis occurs and two haploid daughter cells form. End result of meiosis is that one chromosome of each homologous pair is distributed to a different daughter cell. The daughter cells of meiosis are haploid. Summary of the steps of meiosis I: o Early Prophase I- Replicated chromosomes condense, the spindle apparatus forms, and the nuclear envelope disappears. Synapsis of homologs forms pairs of homologous chromosomes, or tetrads. Kinetochore microtubules attach to the kinetochores at the centromeres of chromosomes. o Late Prophase I- Crossing over results in a mixing of chromosome segments from maternal and paternal chromosomes. o Metaphase I- Pairs of homologous chromosomes (tetrads) move to the metaphase plate and line up. o Anaphase I- Homologs separate and begin moving to opposite sides of the cell. o Telophase I- Homologs finish moving to opposite sides of the cell. In some species, a nuclear envelope re-forms around each set of chromosomes.
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Chromosome movement takes place via motor proteins that are attached to the kinetochore and walk along kinetochore microtubules. When meiosis I is complete, the cell divides. The Phases of Meiosis II During prophase II, a spindle apparatus forms in both daughter cells. Kinetochore microtubules attach to each side of every chromosome- one kinetochore microtubule to each sister chromatid- and begin moving the chromosomes toward the middles of each cell. In metaphase II, the chromosomes are lined up at the metaphase plate. The sister chromatids of each chromosome separate during anaphase II and move to different daughter cells during telophase II. Once separated, each chromatid is considered an independent chromosomes. Meiosis II results in four haploid cells, each with one chromosome of each type. Summary of Steps of Meiosis II: o Prophase II- The spindle apparatus forms. If a nuclear envelope formed at the end of meiosis I, it breaks apart.
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The chromatids that meet to form a chiasma are homologous...

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