1. Meiosis

2. Meiosis
Is a special type of cell division that occurs in certain diploid cells to create gametes (egg/sperm) that are haploid
This is necessary for sexual reproduction to occur

3. Haploid Cells & Diploid Cells
A haploid cell is a cell that contains one complete set of chromosomes.
Gametes are haploid cells that are produced by meiosis.

4. Diploid Cells
A diploid cell is a cell that contains two sets of chromosomes.
One set of chromosomes is donated from each parent.
The diploid number of a cell is the number of chromosomes in the cell.
This number is commonly abbreviated as 2n, where “n” stands for the number of chromosomes.
For humans this equation would be 2n=46.

5. Diploid vs Haploid Body cells are diploid (2n)
chromosome number = 46
Gametes (sex cells - egg/sperm) are haploid (n)
chromosome number = 23
Haploid reproductive cells that unite during sexual reproduction to form a diploid zygote. Male gametes are sperm and female gametes are eggs.
egg + sperm  zygote
n + n  2n
 46

6. Meiosis

7. Homologous Chromosomes
Identical in size and shape and in the way the genes are arranged
One came from the mother; one came from the father
Also referred to as a tetrad

8. Meiosis I Prophase I nuclear membrane and nucleolus disappears
centrioles separate; spindle fibers form
synapsis occurs - homologous chromosomes pair up
crossing over (gene exchange) may occur

9. Prophase I

10. Crossing Over

11. Meiosis I, cont’d Metaphase I
homologous pairs line up along equator of cell

12. Metaphase I

13. Meiosis I, cont’d
Anaphase I
homologous pairs separate (segregation)

14. Anaphase I

15. Meiosis I, cont’d Telophase I
cytoplasm divides forming 2 daughter cells, each with the haploid number of chromosomes, however, the chromosomes are still in their replicated form (2 sister chromatids) and therefore another division is necessary to separate chromatids

16. Telophase I

17. Meiosis II Prophase II Metaphase II Anaphase II
nuclear membrane and nucleolus disappears
centrioles separate; spindle fibers form
Metaphase II
chromosomes line up at equator
Anaphase II
sister chromatids are separated to opposite poles

18. Meiosis II

19. Meiosis II, cont’d Telophase II
both daughter cells divide, forming 4 haploid cells
nucleolus and nuclear membrane reform
chromatin is visible

20. Meiosis Animation

21. Meiosis I Animation
                                                

22. Meiosis II Animation
                                                

23. Meiosis Animation