1. Meiosis

2. Meiosis
Defined: Process where the chromosome number is reduced by half
Cells that undergo meiosis will start diploid, but…
end haploid.

3. Meiosis
Defined: Process where the chromosome number is reduced by half
Gametes created (egg & sperm)
End Result: Four non-identical Haploid Cells 2
Sperm cell 1 3 4
Egg cell (ovum)

4. Interphase Same process G1: Growth S: Chromatin duplicated G2: Growth
6 strands of DNA to start
Interphase
Chromatin strands duplicated
Same process
G1: Growth
S: Chromatin duplicated
G2: Growth

5. Prophase I Similar to mitosis, except… Chromosomes form
6 strands of DNA to start
Similar to mitosis, except…
Chromosomes form
Dad 1
Mom 3
Mom 1
Dad 2
Dad 3
Mom 2
Nucleus dissolves

6. Prophase I Similar to mitosis, except… Two Events:
6 strands of DNA to start
Similar to mitosis, except…
Two Events:
1) Synapsis: Pairing of homologous chromosomes
Dad’s #1 chromosome pairs with Mom’s #1, etc…
The four chromatids equals a TETRAD
2) Crossing Over: Exchange of genes
Dad 1
tetrad
Mom 3
Mom 1
Dad 2
Dad 3
Mom 2
tetrad
tetrad

7. Prophase I Similar to mitosis, except… Two Events:
6 strands of DNA to start
Prophase I
Similar to mitosis, except…
Two Events:
1) Synapsis: Pairing of homologous chromosomes
Dad’s #1 chromosome pairs with Mom’s #1, etc…
The four chromatids equals a TETRAD
2) Crossing Over: Exchange of genes
Lets zoom into one tetrad
Dad #1
Mom #1
tetrad
Dad 1
Mom 1
Dad 3
Mom 3
tetrad
Mom 2
Dad 2
Lets zoom back out
tetrad

8. Crossing over Importance:
Prophase I
6 strands of DNA to start
Crossing over Importance:
Creates genetic diversity. Each sperm and egg cell are genetically unique.
Similar to 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
tetrad
Mom 2
Dad 2
tetrad
Mom 2
Dad 2
tetrad

9. Metaphase I Homologous chromosomes (tetrads) align along cell equator
6 strands of DNA to start
Homologous chromosomes (tetrads) align along cell equator

10. Anaphase I Tetrads separated
6 strands of DNA to start
Tetrads separated
Homologous chromosomes are pulled to opposite ends

11. Telophase I Cytokinesis divides the cells
6 strands of DNA to start
Telophase I
Cytokinesis divides the cells
Each cell contains 1 of the homologous chromosomes
Humans: 23 full chromosomes each
Sometimes the nucleus will regrow…
sometimes it doesn’t

12. Prophase II Same as mitosis
6 strands of DNA to start
Prophase II
Same as mitosis
Note: No new DNA was created (no 2nd interphase)
Nucleus dissolves;
Spindle fibers form

13. Metaphase II Remaining chromosomes are aligned at cell equator
6 strands of DNA to start
Remaining chromosomes are aligned at cell equator

14. Anaphase II Sister chromatids are pulled to opposite ends
6 strands of DNA to start
Sister chromatids are pulled to opposite ends

15. Telophase II Cytokinesis divides the cells
6 strands of DNA to start
Cytokinesis divides the cells
Nucleus reforms on the four new cells
Chromatids uncoil
End Result: Four genetically unique haploid cells
3 strands of DNA
3 strands of DNA
3 strands of DNA
3 strands of DNA

16. Mitosis vs. Meiosis

17. Spermatogonium (diploid)
Spermatogenesis
Meiosis 1
Defined: Production of sperm cells
Location: male testes
Makes final changes to sperm cell
Four non-identical haploid sperm cells at a time
Meiosis 2
Spermatogenesis
Mature sperm cells

18. Spermatogenesis Video Clip

19. Oogenesis Defined: Production of the egg cell (ovum) In female ovaries
Oocyte (diploid)
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
Mature ovum

20. Oogenesis Video Clip

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

22. Quiz
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?