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Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction. Chapter 7. Meiosis. Key terms Ploidy Diploid Haploid Polyploidy Recombination Crossing over. What is Meiosis?. Meiosis is a form of cell division that halves the number of Chromosomes. Produces specialized reproductive cells called gametes or spores .

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Meiosis and Sexual Reproduction

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  1. Meiosis and Sexual Reproduction Chapter 7

  2. Meiosis • Key terms • Ploidy • Diploid • Haploid • Polyploidy • Recombination • Crossing over

  3. What is Meiosis? • Meiosis is a form of cell division that halves the number of Chromosomes. • Produces specialized reproductive cells called gametes or spores. • Meiosis involves two divisions of the nucleus: Meiosis I and Meiosis II.

  4. Meiosis in a nutshell…

  5. Phases of Meiosis I • Prophase I • Chromosomes become visible (condense). • Nuclear Envelope breaks down. • Crossing over occurs. • Portions of homologous chromosomes break off and exchange with the other chromosome

  6. Phases of Meiosis I • Metaphase I • Pairs of homologous chromosomes move to the equator of the cell. • Homologous chromosomes remain together along what is called the metaphase plate.

  7. Phases of Meiosis I • Anaphase I • Homologous chromosomes move to opposite poles of the cell • Move along spindle fibers. • The Chromatids do not separate at the centromere…each chromosome is still comprised of two Chromatids.

  8. Phases of Meiosis I • Telophase I and Cytokinesis • Chromosomes gather at the poles of the cell, • The cytoplasm divides to create two daughter cells

  9. Phases of Meiosis II • Prophase II • A new spindle forms around the chromosomes

  10. Phases of Meiosis II • Metaphase II • Chromosomes line up at equator of the cell and are attached at their Centromeres to spindle fibers.

  11. Phases of Meiosis II • Anaphase II • The Centromeres divide, and the Chromatids (now called chromosomes) move to opposite poles of the cell.

  12. Phases of Meiosis II • Telophase II • Nuclear envelope forms around each set of chromosomes • Cell undergoes Cytokinesis • Results in four haploid cells.

  13. Meiosis and Genetic Variation • Meiosis allows for the fast generation of new genetic combinations • Three key mechanisms make this possible • 1. independent assortment • 2. crossing over • 3. random fertilization

  14. Independent Assortment • Most organisms have more than one chromosome. • Most organisms have more than one copy of each chromosome from each parent (ploidy) • Which copy of each chromosome is a matter of chance due to meiosis. • This is called independent assortment

  15. Crossing Over and Random Fertilization • The DNA exchange during crossing over adds even more genetic variation • Due to this process, combined with independent assortment, the genetic combinations that can occur among gametes is practically unlimited. • Since the joining of two gametes (an egg and sperm cell) is random, the number of possible outcomes is the total number of chromosomes squared. • Humans have 23 chromosomes. • 64 trillion possible outcomes.

  16. Meiosis and Gamete Formation

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