1. Unit 9 lesson 2: Meiosis

2. Common words in today’s notes
gametes (sex cells)
sperm
egg
haploid
chromosomes

3. Meiosis overview
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.

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

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

6. 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.

7. Overview of Meiosis

8. Meiosis overview (2)

9. 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!

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

11. 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

12. Why Meiosis?
Produces cells with HALF the number of chromosomes so that fertilization can take place
The beginning of the word haploid sounds like the beginning of the word HALF!
Source of variation in organisms
All sperm and egg are different
Chromosomes can “exchange” DNA in a process called crossing over
=Increases variety!
Image:

13. 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

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

15. 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.

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

17. Father makes sperm by meiosis Mother makes eggs by meiosis
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

18. *Notice meiosis starts with 1 cell with 4 chromosomes

19. *meiosis ends with 4 cells with 2 chromosomes in each