1. Cell Reproduction (Meiosis)

2. How Do Cells Reproduce?
Most of the time cells simply divide in half to make new cells that are identical, this is called mitosis.
This is known as asexual reproduction (no partner needed).

3. How do Cells Reproduce?
Other times cells join together to create new combinations of DNA, this is called meiosis.
This is known as sexual reproduction (partner needed).

4. Asexual vs. Sexual Reproduction
Asexual reproduction occurs quickly, and makes identical copies of cells.
Sexual reproduction occurs much slower, but allows for more diversity.

5. Meiosis = Sexual Reproduction
Meiosis is important for two reasons:
A) Meiosis reduces the number of chromosomes in a cell by ½.
B) Meiosis creates opportunities for genes to be exchanged (cross over).

6. Meiosis = Sexual Reproduction
Meiosis is sometimes called “Reduction Division” because it reduces the normal chromosome number in half.
DIPLOID CELL
>
HAPLOID CELL
46 Chromosomes
23 Chromosomes

7. Mitosis = Asexual Reproduction
This is unlike in Mitosis where the chromosome number stays the same.
DIPLOID CELL
>
DIPLOID CELL
46 Chromosomes
46 Chromosomes

8. The Meiosis Process
Meiosis begins in Interphase where the cell is going through G1, S, and G2.
INTERPHASE

9. The Meiosis Process
When directed by certain chemicals the cell begins meiosis, instead of mitosis.
Meiosis is divided into two stages:
A) Meiosis I
B) Meiosis II

11. Meiosis I – Prophase I
The nuclear envelope of the cell begins to break up and starts to disappear.
Chromosomes become visible.

12. Meiosis I – Prophase I
Homologous chromosomes pair up (similar chromosomes from mom & dad).
This is called a tetrad.
Mom’s Chromosome
Dad’s Chromosome

13. Meiosis I – Prophase I
When tetrads get too close segments of genes can be exchanged (cross over.)
This allows for more combinations of DNA to occur and even more diversity.
Mom’s Chromosome
Dad’s Chromosome

15. Meiosis I – Metaphase I
The tetrads (homologous chromosomes) move to the center (equator) of the cell.

16. Meiosis I – Anaphase I
Tetrads (homologous chromosomes) get pulled apart and moved to opposite sides.

17. Meiosis I – Telophase I
The chromosomes gather at opposites sides and the cytoplasm divides to make two new cells.

18. Meiosis I – Cytokinesis I
Two cells are made.
Chromosomes stay wound up.

19. So far…
The sister chromatids have NOT been pulled apart at their centromeres.
Interphase does NOT occur again.
The chromosomes are NOT copied.

20. Meiosis II – Prophase II
This occurs in both of the cells made during Meiosis I.
Chromosomes are still visible (wound up).
Spindle fibers surround the chromosomes.

21. Meiosis II – Metaphase II
This occurs in both of the cells made during Meiosis I.
The sister chromatids move to the center (equator) of the cell.

22. Meiosis II – Anaphase II
This occurs in both of the cells made during Meiosis I.
Sister chromatids get pulled apart at their centromeres and move to opposite sides.

23. Meiosis II – Telophase II
This occurs in both of the cells made during Meiosis I.
The chromosomes gather at opposites sides and the cytoplasm divides.

24. Meiosis II – Cytokinesis II
This occurs in both of the cells made during Meiosis I.
Two more cells are made (4 total).
The chromosomes unwind and Interphase begins again.

27. Meiosis Animation

28. Meiosis Summary We start with 1 diploid cell and make 4 haploid cells.
A cell goes through 2 separate cell division processes (Meiosis I and II).

30. Meiosis Summary Parent cells begin with 46 chromosomes.
In S phase the 46 chromosomes make identical copies to become sister chromatids (92 chromosomes).

31. 46 chromosomes
92 chromosomes

32. Meiosis Summary
During Meiosis I homologous chromosomes are separated into 2 cells that now have 46 chromosomes.
92 chromosomes
46 chromosomes
46 chromosomes

33. Meiosis Summary Both cells from Meiosis I, go to Meiosis II.
Now sister chromatids are separated into cells that each have 23 chromosomes.
46 chromosomes
46 chromosomes 23 23 23 23

34. Meiosis Summary
A grand total of 4 cells each containing 23 chromosomes are formed!

36. Diversity in Meiosis
When tetrads are formed during Meiosis I they can cross over segments of DNA.

37. More Diversity in Meiosis
The tetrads (homologous chromosomes) randomly line up at the equator and can separate independently.
This is called independent assortment.

39. Even More Diversity in Meiosis
A zygote is made by a random sperm joining with a random egg.

40. Diversity! Combined all three processes: 1) crossing over,
2) independent assortment,
3) random fertilization,
…create an almost infinite number of possible outcomes.

41. Meiosis in Males
The process of Meiosis in males is known as spermatogenesis.
It occurs in the testes.
It creates 4 functional sperm.

43. Meiosis in Females
The process of Meiosis in females is known as oogenesis.
It occurs in the ovaries.
It creates 1 functional egg and 3 polar bodies.

45. Mitosis vs. Meiosis

46. Mitosis vs. Meiosis In sexual and asexual reproducing organisms.
Occurs in body cells.
Cells divide 1 time and make 2 cells.
No crossing over
Cells are diploid
Only in sexual reproducing organisms.
Occurs in sex cells.
Cells divide 2 times and make 4 cells.
Yes, crossing over
Cells are haploid

47. Mitosis vs. Meiosis No crossing over. Cells are diploid (46)
All new cells made are identical.
Yes crossing over.
Cells are haploid (23)
Not all new cells made are identical (DIVERSITY!).

48. Any Questions?
“Like a ten-speed bike, most of us have gears we do not use.”
--Charles Schulz
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--Eric Butterworth