1. Meiosis
EQ: How is meiosis different from mitosis?

2. WHAT IS MEIOSIS?

3. I. Overview
Organisms can reproduce asexually (DNA from only one parent, identical) or sexually (DNA from two parents)
Asexual(mitosis) Sexual (meiosis)

4. Body cells of every species have a characteristic number of chromosomes. Human have 46 chromosomes (23 pairs) in each body cell  

5. A cell that contains two complete sets of chromosomes (2n) are diploid
A cell that contains two complete sets of chromosomes (2n) are diploid. In humans, body cells are diploid (2n)  

6. Human sex cells (gametes) (egg/sperm) are haploid and contain 23 individual chromosomes (half the number of a diploid body cell).

7. Why is it important that gametes be haploid?    

9. Meiosis is a reproductive process that produces 4 cells in which the number of chromosomes in each cell is reduced by half (haploid).

10. Homologous Pair
Chromosomes Pairing   Body cells contain 46 chromosomes These 46 cells are arranged in 23 pairs Each of these pairs is called a homologous pair. 
Each pair contains chromosomes that are exactly alike in size, with one exception.  

11. Phases of Meiosis   Before meiosis begin the cell is in interphase (growth and development phase). Just like in mitosis the DNA is replicated during this phase, the cell is then ready to begin meiosis.   Meiosis consists of two stages:
Meiosis I and Meiosis II

13. Nucleus disappears Spindle fibers forms
Homologous chromosomes align with one another to form a homologous pair (Tetrads)
Crossing over Occurs
centrioles
spindle fiber
TETRAD 13

14. Homologs line up at equator or metaphase plate
Tetrads line up on the equator.
Homologs line up at equator or metaphase plate OR 14

15. Homologous chromosomes separate and move towards opposite poles.
Sister chromatids remain attached at their centromeres. 15

16. Cytoplasm begins to pinch in.
Nuclear membrane forms around each set of chromosomes, creating two new nuclei.
Cytoplasm begins to pinch in.
Cytokinesis occurs and two haploid daughter cells are formed. 16

17. cytokinesis 17

18. Sister Chromatids Separate
Meiosis II 18

19. No Interphase II or very short No DNA Replication
Remember: Meiosis II is similar to mitosis 19

20. Same as Prophase in mitosis
Nucleus disappears
Chromosomes condense
Spindle fibers form 20

21. Sister chromatids line up on the equator. Same as Metaphase in mitosis21

22. SISTER CHROMATIDS separate22

23. haploid Sperm cell fertilizes haploid egg to form diploid zygote
Nuclear membrane reforms, spindle fibers disappear
CYTOKINESIS occurs.
FOUR HAPLOID daughter cells are produced.
Cells are called GAMETES (eggs and sperm)
haploid Sperm cell fertilizes haploid egg to form diploid zygote 23

24. 24

25. 25

26. Meiosis Provides Genetic Variation Two Ways:
  1. Independent Assortment of chromosomes    

27. Meiosis Provides Genetic Variation
Crossing Over meiosis allows for the shuffling of chromosomes.

28. Crossing Over - the exchange of genetic material between homologous pairs of chromosomes   Remember that genetic variation is important to species survival, especially in a changing environment  

29. Mitosis vs. Meiosis

31. Differences Between Mitosis and Meiosis
 1. Number of daughter cells produced? 
Mitosis = 2 Meiosis = 4
2. How many chromosomes in each daughter cell?
Mitosis = 46 (diploid) Meiosis = 23 (haploid)
How do the daughter cells compare to the parent cell?
Mitosis = genetically identical Meiosis = NOT gen. identical
How many times does the cell divide?
Mitosis = once Meiosis = twice  

32. Meiosis Video