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Meiosis: Formation of gametes

Meiosis: Formation of gametes

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Meiosis: Formation of gametes

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  1. Meiosis: Formation of gametes Utilizing BioCoach Activity “Meiosis”

  2. Meiosis occurs only in sexually reproducing organisms. In animals, where the somatic (body) cells are diploid, the products of meiosis are the gametes. Some plants, algae and fungi have more complex cycles involving both mitosis and meiosis. Introduction to Meiosis

  3. A two-step cell-division process in sexually reproducing eukaryotes. Results in cells (typically gametes) with one-half the chromosome number of the original parental cell. One diploid cell (2N) results in 4 haploid cells (N). Introduction to Meiosis

  4. Introduction to Meiosis • Meiosis consists of two successive nuclear divisions: meiosis I meiosis II • Each division consists of : • Prophase • Metaphase • Anaphase • Telophase

  5. Premeiotic Interphase in Animal Cells • All chromosomes are duplicated • The two centriole are produced by the duplication of a single centriole

  6. MeiosisI: Prophase I • Chromosomes become visible as tetrads • A tetrad is a pair of homologous chromosomes (ex. both copies of chr #3) • Crossing-over occurs • Homologous chromosomes exchange portions of their chromatids • Produces new combinations of alleles

  7. MeiosisI: Prophase I • Nucleolus disappears • Meiotic spindle forms • Nuclear envelope disappears Prophase I consumes 90% of the time for meiosis

  8. Crossing Over During Meiosis crossing-over results in a physical exchange of equivalent segments of maternal and paternal homologous chromosomes

  9. Crossing Over During Meiosis

  10. Prophase I Meiotic prophase I

  11. Meiosis I: Metaphase I • The centrioles are at opposite poles of the cell • The pairs of chromosomes (tetrads) are arranged along the center of cell • Spindle fibers are attached to one chromosome of each pair

  12. Metaphase I Meiotic metaphase I

  13. Meiosis I: Anaphase I • Chromosomes of each tetrad separate and move to opposite poles Note: Sister chromatids remain attached and move together to the pole

  14. Anaphase I Meiotic anaphase I

  15. Meiosis I: Telophase I • Homologous chromosome pairs reach the poles of the cell • Nuclear envelopes form around them • Cytokinesis follows to produce two cells

  16. Telophase I Meiotic telophase I

  17. Product of Meiosis I Two cells with a haploid number of chromosomes is produced. Each cell has two copies of each chromatid. The two cells have sets of chromosomes that are different from each other and from the parent cell that entered meiosis. Animation

  18. Meiosis II There is no interphase between the two stages (chromosomes are not duplicated).

  19. Meiosis II: Prophase II • Centrioles duplicate • Nuclear envelope breaks down • Spindle apparatus forms

  20. Prophase II Meiotic prophase II

  21. Comparing Prophase Iand Prophase II

  22. Meiosis II: Metaphase II • Spindle apparatus attaches to each chromosome • Single chromosomes align along center of each cell

  23. Metaphase II Meiotic metaphase II

  24. Comparing Metaphase Iand Metaphase II

  25. Centromeres separate Sister chromatids — now individual chromosomes — move toward the opposite poles of the cell. Meiosis II: Anaphase II

  26. Anaphase II Meiotic anaphase II

  27. Comparing Anaphase Iand Anaphase II

  28. Meiosis II: Telophase II • Nuclear envelope forms around each set of chromosomes • Cytokinesis occurs • Four daughter cells are produced, each with a haploid set of chromosomes.

  29. Telophase II Meiotic telophase II

  30. Comparing Telophase Iand Telophase II

  31. Product of Meiosis II Four cells with a haploid number of chromosomes is produced. Each cell has a single copy of each chromatid. The four cells have sets of chromosomes that are genetically different from each other and from the parent cell that entered meiosis. Animation

  32. Mitosis and Meiosis Reviewed

  33. Compare meiosis to mitosis How are they alike? • In interphase prior to both, replication of DNA occurs • Both are methods of cell division • Both include Prophase, Metaphase, Anaphase and Telophase • Cytokinesis occurs in both

  34. Compare meiosis to mitosis How are they different? Meiosis Two cell divisions Sexual Reproduction --makes gametes Results in 4 haploid cells Mitosis One cell division Asexual reproduction --makes body cells Also for growth and regeneration Results in 2 diploid cells

  35. Compare meiosis to mitosis How are they different? Meiosis Two parts - eight phases DNA in daughter cells differs from parent cell Crossing over occurs Mitosis One part – four phases DNA in daughter cells is same as parent cell Crossing over doesn’t occur

  36. Compare anaphase in mitosis to anaphase I in meiosis Mitotic Anaphase Meiotic Anaphase I How are they different?