1. Meiosis

2. MEiosis
Look at the first two letters of the word meiosis. It spells the word me.
Meiosis is how ME got here!
“Me” got here from sexual reproduction.
Two gametes (one egg and one sperm) came together to make an organism (me!)
Meiosis is the cell division that makes sex cells or gametes.

3. Meiosis Cells divide into sex cells (gametes)
Gametes are haploid, or n (1/2 the correct number of chromosomes for organism)
Male sex cell
Sperm
Made in testes

4. Meiosis Female sex cell Egg Made in ovaries
Gametes contain ½ the number of original chromosomes.

5. Human Chromosome Numbers
Human body cells have 46 chromosomes.
Human sex cells formed from meiosis have 23 chromosomes.
Human sex cells or gametes have ½ the number of chromosomes found in the body cells of an organism.

6. Prophase I Chromosome pairs (homologous) pair up Spindle fibers form
Crossing over, the exchange of genetic information between chromosomes occurs. (increases variation)
Pictture:

7. http://www. wereyouwondering

8. Metaphase I
Paired chromosomes line up side by side across the center of the cell
Pictture:

9. Anaphase I Paired chromosomes move apart
Pulled to opposite ends of the cell
Pictture:

10. Telophase I Center pinches cell into two cells
Each new cell contains half of the genetic information.
Pictture:

11. Let’s Review!

12. Prophase II New cells divide again
Spindle fibers form and attach to chromosomes in each cell
Pictture:

13. Metaphase II
Chromosomes line up single file across center of the cell.
Pictture:

14. Anaphase II Sister chromatids separate and move to opposite sides
Double stranded chromosomes are separated into single stranded chromosomes
Pictture:

15. Telophase II Nuclei reform Spindle fibers break down
Cytoplasm divides (cytokinesis)
Pictture:

16. Let’s Review

17. Meiosis produces 4 new cells from one.
Meiosis produces 4 new cells from one.
The new cells each contain ½ number of chromosomes.
Notice that no two sex cells are alike!

18. Meiosis forms 4 male gametes (sperm)
Meiosis forms 1 egg and 3 smaller cells in female.

19. Why Meiosis? Results in sex cells (gametes) for sexual reproduction
Cells made by meiosis contain ½ the number of chromosomes for the organism (23 for humans)
Sex cells or gametes are haploid or have an “n” number of chromosomes

20. Why Meiosis?
The beginning of the word haploid sounds like the beginning of the word half.
Source of variation in organisms (as well as sexual reproduction and mutations)
Half the number of chromosomes is important so that fertilization can take place

21. Meiosis What is the second letter in the word meiosis?
It is the letter e.
All of the cells that are formed from meiosis have an e in them!
Sex cells
Gametes
Egg
Sperm
Meiosis happens in ovaries and testes

22. Let’s Look at Meiosis 2n n n n n n n

23. Why is meiosis important?
Meiosis must reduce the number of chromosomes by half for fertilization
In fertilization the egg and sperm come together to make one cell called a zygote.
Fertilization creates a single-celled zygote with the correct number of chromosomes for the organism (46 for humans).
Zygote then grows by mitosis, making body cells.

24. Fertilization Fertilization egg (n) + sperm (n)
Growth of cells by mitosis
Zygote
(2n)

25. Let’s Review Father makes sperm by meiosis
Mother makes eggs by meiosis
Sperm and eggs have ½ the number of chromosomes for an organism
Sperm (n) haploid
23 chromosomes
Growth by mitosis
Zygote (2n) diploid
46 chromosomes
Formed by meiosis
Fertilization
Egg (n) haploid
23 chromosomes

26. Meiosis I

27. Meiosis II