Meiosis

1. Meiosis The formation of gametes Gametes – Sex cells Sperm – Male Egg – Female Consists of 2 steps – Meiosis I and Meiosis II Prior to Meiosis I the DNA replicates There is no replication prior to Meiosis II

2. Prophase I Nucleus dissolves Chromatin condenses to form chromosomes Centrioles move to the poles Spindle forms Homologous chromosomes pair up  Form tetrads Crossing over occurs 4N

3. Crossing Over • Homologous Chromosomes - Chromosomes that contain the same alleles • During crossing over, homologous chromosomes exchange genetic information.

4. Metaphase I • Tetrads move to the midline • Spindle attaches to the centromere of each chromosome • In humans, there would be 46 pairs of chromosomes • 4N

5. Anaphase I • Spindle pulls chromosomes to the poles • 4N

6. Telophase I • Cytokinesis Occurs • In females cells do not split equally • In males cells split equally • 4N  2N

7. End of Telophase I • New cells are called daughter cells • Both male cells are viable, but only the large female cell is viable • New cells are genetically different from one another • 2N

8. Meiosis II • Unlike meiosis I, there is not DNA replication • Cells that undergo this process are 2N • At the conclusion of meiosis I: • the chromosomes remain in their current state • the nucleus does not reform

9. Prophase II • The centrioles move to the poles • Spindle begins to form • X 2 • 2N

10. Metaphase II • Chromosomes line up at the midline • Spindle attaches to chromosomes at the centromere • X 2 • 2N

11. Anaphase II • Centrioles pull chromatids to the poles • X 2 • 2N

12. Telophase II • Cytokinesis Occurs • Chromosomes return to chromatin • Nucleus reforms • X 2 • 2N  N

13. End of Telophase II New cells are called daughter cells Both male cells are viable, but only the large female cell is viable New cells are genetically different from one another X 2 N

14. Female Meiosis

15. Male Meiosis

16. Fertilization • When sperm and egg fuse they form a 2N zygote • Zygote = Fertilized egg