1. What is meiosis? The process of dividing the number of chromosomes in cells used for reproduction

2. Why is meiosis needed? Each organism gets ½ their DNA from each parent.  23+ 23 =46 Without meiosis:  46+46 = 92  92+92 = 184

3. Divisions of meiosis Meiosis I  One diploid cell becomes two haploid cells. Meiosis II  Two haploid cells become four haploid cells.

4. Stages of Meiosis I Prophase I  Starts with one diploid cell  Spindle fibers form  Nuclear membrane dissolves  Homologous chromosomes condense and pair  Crossing-over occurs Diagram of crossing-over

5. Stages of Meiosis I Diagram of prophase I

6. Stages of Meiosis I Metaphase I  Homologous chromosomes line up along the equator of the cell  Spindle fiber attach to chromosomes Diagram of Metaphase I

7. Stages of Meiosis I Anaphase I  Homologous chromosomes are pulled to opposite ends of the cell Diagram of Anaphase I

8. Stages of Meiosis I Telophase I  Nuclear membrane forms  Spindle fibers dissolve  Two haploid cells are formed Diagram of Telophase I

9. Stages of Meiosis II Prophase II  Nuclear membrane dissolves  Spindle fibers form  Chromosomes condense Diagram of prophase II

10. Stages of Meiosis II Metaphase II  Chromosomes line up across the center of the cell  Spindle fibers attach to chromosomes Diagram of Metaphase II

11. Stages of Meiosis II Anaphase II  Centromeres break, releasing chromatids  Chromatids move to opposite poles of the cell Diagram of Anaphase II

12. Stages of Meiosis II Telophase II  Nuclear envelope reforms  Spindle fibers dissolve  Ends with four haploid cells

13. Gender differences in meiosis Males  One cell becomes four sperm cells Females  One cell becomes one egg cell and 3 polar bodies

14. Gender differences in meiosis Why?  Early embryos need the nutrients in the egg to survive. Most of the cytoplasm from the one cell is transferred to the final egg.  Sperm cells do not provide nutrients to the embryo, so the cytoplasm is mostly discarded.