1. Section 11-4: Meiosis
Start with 4 chromosomes
The word "meiosis" comes from the Greek meioun, meaning  "to make small," since it results in a  reduction in chromosome number in the gamete cell.
End with 2 chromosomes

2. A. Why is Meiosis Important?
Consider these Questions …
1. How many chromosomes would a human sperm or an egg contain if either one resulted from the process of mitosis?
a. 46 chromosomes
2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo?
b = 92; a developing embryo would not survive if it contained 92 chromosomes.
3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
 c. Sperm and egg should each have 23 chromosomes.
*Meiosis  how you make sperm and egg cells with only 23 chromosomes*

3. B. Chromosome Number 1.  Number varies by organism
a. Ex: turkey  82, fruit fly  6, human 46
2. Organisms that reproduce sexually have pairs of similar chromosomes
a. Called  HOMOLOGOUS CHROMOSOMES
b. Similar in size, structure, and carry genes for the  same traits (but may be different forms of the gene)
c. Each member of a pair comes from  each parent
d. Ex: humans have  23 pairs of homologous chromosomes, or  46 total
i.  23 in sperm,  23 in egg, pair up

4. B. Chromosome Number a. Diploid b. Haploid 3. Diploid vs. Haploid
i.  Double set of chromosomes
ii.  “2n”
iii. Humans – 23 sets/pairs,  2n = 46
b. Haploid
i..  Single set of chromosomes
ii.  “n”
iii. Humans – one chromo. from each set,  n = 23

6. B. Chromosome Number 4. Chromosome numbers & cell types
a. Somatic cells
i. “Regular” body cells
ii.  Always diploid
iii. Humans  46 chromo.
b. Gametes
i. “sex cells”
ii. Usually egg & sperm (or pollen/spores)
iii.  Always haploid
iv. Humans  23 chromo.

7. C. Phases of Meiosis Meiosis I 1. Interphase I
a. Cells undergo a round of  DNA replication, forming duplicate chromosomes

8. 2. Prophase I
a. Each chromosome pairs with its corresponding homologous chromosome to form a  tetrad
i. tetrad  contains 4 chromatids
b. Homologous chromosomes exchange portions of their chromatids
ii.  Called crossing over
tetrad
crossing-over

9. Crossing-Over

10. 3. Metaphase I a.  Spindle fibers attach to the chromosomes
b. Tetrads line up in the  middle of the cell

11. 4. Anaphase I
a. Fibers pull homologous chromosomes toward  opposite ends of the cell
b. Centromere  stays intact

12. 5. Telophase I and Cytokinesis
a.  Nuclear membranes form
b. Cell separates into  2 cells

13. Meiosis II
1. Prophase II
a. Meiosis I results in  2 haploid (N) daughter cells each with  half the number of homologous chromosomes as the original cell

14. 2. Metaphase II
a. Chromosomes line up in the  center of the cell

15. 3. Anaphase II
a. Paired  sister chromatids separate and move towards  opposite ends of the cell
b. Centromere  is broken

16. 4. Telophase II and Cytokinesis
a. Meiosis II results in  4 haploid (N) daughter cells
b. Each cell contains  half the number of chromosomes as the original cell

17. Meiosis I
Tetrad = 4 chromatids
Homologous chromosome

18. Meiosis II Prophase II Metaphase II Anaphase II Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.

19. D. Gamete Formation
Unequal division of cytoplasm
2. Females  1 egg formed (larger) and 2 polar bodies (smaller) a.  only the egg is used in reproduction
1. Males  4 sperm formed

20. E. Comparing Mitosis and Meiosis
1. Known as  cell division
2. Consists of  1 division
3. Forms  2 genetically identical daughter cells
4. Daughter cells have  same amount of chromosomes as parent cell (humans = 46)
5. Occurs only in  body cells
somatic cells
6. Purpose =  make more body cells for growth and repair
Meiosis
1. Known as  cell reduction division
2. Consists of  2 divisions
3. Forms  4 genetically different daughter cells
4. Daughter cells have  half the amount of chromosomes as the parent cell (humans = 23)
5. Occurs only in  sex cells
gametes
6. Purpose =  form egg and sperm cells for fertilization

21. Meiosis Animations