1. Meiosis

2. Meiosis
Defined: Process where the chromosome number is reduced by half
Gametes created (egg & sperm)
End Result: Four Haploid Cells

3. Interphase
Same process
G1: Growth
S: Chromatin duplicated
G2: Growth

4. Synthesis (S stage)
Chromatin strands duplicated

5. Prophase 1 Similar as mitosis, except… Two Events:
Mom 1
Dad 1
Similar as mitosis, except…
Two Events:
1) Synapsis: Pairing of homologous chromosomes
Dad’s #1 chromosome pairs with Mom’s #1, etc…
The four paired chromatids equals a TETRAD
2) Crossing Over: Exchange of genes

6. Early Prophase Chromosomes form Nucleus dissolves Dad #3 Dad #2 Dad #1
Mom #3
Mom #1
Mom #2
Nucleus dissolves

7. Synapsis Spindle fibers pull homologous chromosomes together tetrad
Dad #3
Spindle fibers pull homologous chromosomes together
Dad #2
Dad #1
Mom #3
Mom #1
Mom #2
tetrad

8. Homologous chromosomes (tetrads) align along cell equator
Metaphase 1
Homologous chromosomes (tetrads) align along cell equator
Dad #3
Mom #3
Mom #2
Dad #2
Mom #1
Dad #1

9. Homologous chromosomes are pulled to opposite ends
Anaphase 1
Tetrads separated
Homologous chromosomes are pulled to opposite ends
Mom #2
Dad #2
Mom #1
Dad #1
Dad #3
Mom #3

10. Telophase 1 Cytokinesis divides the cells
Each cell contains 1 of the homologous chromosomes
Humans: 23 full chromosomes each
Mom #2
Dad #2
Mom #1
Dad #1
Dad #3
Mom #3

11. No Interphase 2 The Point of Meiosis: If Interphase 2 were to happen…
Reduce the amount of chromosomes by half
If Interphase 2 were to happen…
DNA duplicated during the S stage
Dad #2
Dad #2
Mom #2
Mom #2
Mom #1
Mom #1
Dad #1
Dad #1
Mom #3
Mom #3
Dad #3
Dad #3

12. Telophase 1 Cytokinesis divides the cells
Each cell contains 1 of the homologous chromosomes
Humans: 23 full chromosomes each
Mom #2
Dad #2
Mom #1
Dad #1
Dad #3
Mom #3

13. Note: No new DNA was created (no 2nd interphase)
Prophase 2
Same as mitosis
Note: No new DNA was created (no 2nd interphase)
Mom #2
Dad #2
Mom #1
Dad #1
Dad #3
Mom #3

14. Remaining chromosomes are aligned at cell equator
Metaphase 2
Remaining chromosomes are aligned at cell equator
Mom #2
Dad #2
Mom #1
Dad #1
Dad #3
Mom #3

15. Sister chromatids are pulled to opposite ends
Anaphase 2
Sister chromatids are pulled to opposite ends

16. Telophase 2 Cytokinesis divides the cells
Nucleus reforms on the four new cells
Chromatids uncoil
End Result: Four haploid cells

17. Mitosis vs. Meiosis

18. Spermatogenesis Defined: Production of sperm cells
spermatogonium
Spermatogenesis
Meiosis 1
Defined: Production of sperm cells
Location: male testes
Makes final changes to sperm cell
Four haploid sperm cells at a time
Meiosis 2
Spermatogenesis

19. Spermatogenesis Video Clip

20. Oogenesis Defined: Production of the egg cell (ovum)
oocyte
Oogenesis
Meiosis 1
Defined: Production of the egg cell (ovum)
In female ovaries
During meiosis I: Cytoplasm distributed unequally
After meiosis II:
1 cell matures
3 “Polar Bodies” dissolve
Meiosis 2
Oogenesis

21. Oogenesis Video Clip

22. Mitosis Meiosis Division of gametes Division of somatic cells
Ex: Sperm and Egg cells
End Result: 4 Haploid cells
Importance: Reduces the gamete chromosome number by half
Division of somatic cells
Ex: Blood cells, skin cells, nerve cells…
End Result: 2 Diploid Cells
Importance: Replaces aging cells
Importance: Form of asexual reproduction

23. Recap
1) Name three ways that meiosis and mitosis differ. 2) Pick a stage of meiosis…any stage. Describe what happens in it. 3) What type of cells does meiosis create? 4) What are homologous chromosomes? 5) When do homologous chromosomes move to opposite ends of the cell? 6) When do chromatids move to opposite ends of the cell? 7) Why is no DNA created in meiosis 2? 8) How does meiosis differ in males vs. females?