1. Problem:
Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?

2. Answer:
Meiosis

3. Meiosis Cell division to produce gametes What are gametes?
Egg and sperm!

4. How does meiosis solve the problem of too many chromosomes?
The number of chromosomes is cut in half
46 to 23
Your chromosomes are in pairs: one from mom and one from dad. One from each pair goes into each gamete

5. In humans: Mitosis Cell division to produce 2 identical daughter cells
Each cell has 46 chromosomes: 2 sets (one from mom, one from dad)
Diploid (2n): 2 sets of chromosomes
Meiosis
Cell division to produce genetically different gametes
Each gamete has 23 chromosomes: 1 set (about half from mom, half from dad)
Haploid (1n): 1 set of chromosomes

6. Diploid and haploid Our body cells (somatic cells) are diploid
Our sex cells are haploid

7. Meiosis Sex cells divide to produce gametes (sperm or egg).
Gametes have half the # of chromosomes.

8. Fertilization n=23 egg 2n=46 zygote
The fusion of a sperm and egg to form a zygote.
A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

9. Meiosis has two cell divisions
Meiosis I: Cuts chromosome number in half.
Meiosis II: sister chromatids separate

10. Meiosis I
Two unique things happen that create genetic variation – the reason for diversity within a species.

11. Unique thing #1. Crossing Over
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Now the chromatids are not alike—genetic variation

12. First, homologous chromosomes come together
sister chromatids from mom
sister chromatids from dad
Link together

13. Crossing Over - variation
nonsister chromatids
Variation: now all 4
chromatids are different

14. What does crossing over produce?
4 genetically different chromatids!

15. Unique thing #2. Independent assortment
The random assortment of genes you got from your mom and your dad into gametes.
Kind of like shuffling a deck of cards

16. Happens during Metaphase I
metaphase plate OR
metaphase plate

17. Meiosis II
Similar to mitosis
sister chromatids separate

18. So what do we have at the end of meiosis?
4 genetically different haploid gametes

19. http://highered. mcgraw-hill