1. Objectives 11.4 Meiosis -Contrast the number of chromosomes in body cells and in gametes. -Summarize the events of meiosis. -Contrast meiosis and mitosis. -Describe how alleles from different genes can be inherited together.

2. Lesson Overview Lesson OverviewMeiosis Diploid Cells ( Body Cells ) A human body cell has 46 chromosomes. A cell that has a full set of chromosomes is said to be DIPLOID. *23 comes from Mom and 23 come from Dad.* These two sets of chromosomes are homologous, meaning that each of the 23 chromosomes from Dad has a matching chromosome from Mom.

3. Lesson Overview Lesson OverviewMeiosis Haploid Cells ( Gametes ) Some cells contain only a single set of chromosomes. Such cells are HAPLOID, meaning “HALF”. The gametes of sexually reproducing organisms are haploid.

4. Meiosis KM 4 Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues.

5. Meiosis KM 5 In humans … 23 chromosomes donated by each parent (total = 46 or 23 pairs). Gametes (sperm/ova): –Contain 22 autosomes and 1 sex chromosome. –Are haploid (haploid number “n” = 23 in humans). Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid. –Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Only gametes are produced by meiosis.

6. Meiosis KM 6 Chromosomes Karyotype: –ordered display of an individual’s chromosomes. –Collection of chromosomes from mitotic cells. –Staining can reveal visible band patterns, gross anomalies.

7. Meiosis KM 7 Karyotyping

8. Meiosis KM 8

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11. Meiosis KM 11 Animation

12. Lesson Overview Lesson OverviewMeiosis Phases of Meiosis Meiosis is a process in which the number of chromosomes per cell is cut in half. Meiosis usually involves two distinct divisions: 1. Meiosis I ~ Prophase I (crossing over) Metaphase I Anaphase I Telophase I 2. Meiosis II ~ Prophase II Metaphase II Anaphase II Telophase II By the end of meiosis II, the diploid cell becomes four haploid cells.

13. Lesson Overview Lesson OverviewMeiosis Prophase I As homologous chromosomes pair up and undergo a process called crossing-over, where portions of the chromosomes are swapped.

14. Lesson Overview Lesson OverviewMeiosis Metaphase I and Anaphase I During metaphase I of meiosis, chromosomes line up at the center of the cell. During anaphase I, spindle fibers pull each chromosome toward opposite ends of the cell.

15. Lesson Overview Lesson OverviewMeiosis Telophase I and Cytokinesis During telophase I, a nuclear membrane reforms around each cluster of chromosomes. Cytokinesis follows telophase I, forming two new cells.

16. Lesson Overview Lesson OverviewMeiosis Meiosis II The two cells produced by meiosis I now enter a second division. Unlike the first division, the cell does not replicate its DNA before entering meiosis II.

17. Lesson Overview Lesson OverviewMeiosis Prophase II As the cells enter prophase II, their chromosomes become visible again.

18. Lesson Overview Lesson OverviewMeiosis Metaphase II During metaphase II, chromosomes line up in the center of each cell.

19. Lesson Overview Lesson OverviewMeiosis Anaphase II As the cell enters anaphase II, the paired chromatids separate.

20. Lesson Overview Lesson OverviewMeiosis Telophase II and Cytokinesis II These four daughter cells now contain the haploid (half) number of chromosomes.

21. Lesson Overview Lesson OverviewMeiosis Gametes to Zygotes The haploid cells produced by meiosis II are gametes. In male animals, these gametes are called sperm. In female animals, the gamete is called an egg.

22. Lesson Overview Lesson OverviewMeiosis Gametes to Zygotes Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote.