1. Meiosis

2. What is an “n”?  Haploid (n): Number of chromosomes in a gamete.
Diploid (2n):
Number of chromosomes in a somatic cell
One copy from mother, one copy from father.
Homologous pairs of chromosomes

3. Two Types of Diploid Cells In Sexual Organisms
Two Types of Diploid Cells In Sexual Organisms
Somatic
GERM
Body Cells
Diploid
Divide by mitosis to make exact copies
In every tissue except reproductive tissue
“Germinating” (reproductive) cells
Diploid
(Most) Divide by meiosis to make gametes (sex cells) (sperm & ova)
In testes & ovaries

4. Mitosis vs. Meiosis Asexual Reproduction Sexual Reproduction
Body cells (somatic)
Sex cells (gamete)
DNA replicates once
# cell divisions: 1
# cell divisions: 2
2 identical daughter cells
4 different daughter cells
Makes Diploids (2N)
Makes Haploids (1N)

5. Meiosis is part of SEXUAL Reproduction (requiring 2 parents)

6. In Meiosis: ONE diploid, germ cell divides to become FOUR
In Meiosis: ONE diploid, germ cell divides to become FOUR
(4) daughter cells
(*circle*) … diploid / haploid
(*circle*) … somatic / gamete

7. In order to go from diploid to haploid, Meiosis Involves Two Divisions
In order to go from diploid to haploid, Meiosis Involves Two Divisions
Meiosis I
Meiosis II
Splits homologous pairs of chromosomes
Splits sister chromatids apart

8. Time To Name The Phases PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II

9. TELOPHASE I & CYTOKINESIS
Make 2 Tables
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I & CYTOKINESIS
WHAT IT LOOKS LIKE
WHAT’S GOING ON
TABLE I

10. TELOPHASE II & CYTOKINESIS
Make 2 Tables
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II & CYTOKINESIS
WHAT IT LOOKS LIKE
WHAT’S GOING ON
TABLE II

11. PROPHASE I  Nuclear envelope disintegrates
Centrioles migrate, produce spindle fibers
Homologous chromosomes pair up
Homologous pairs CROSS OVER!

12. Exchange of segments of genes between homologous chromosomes
PROPHASE I
CROSSING OVER
Exchange of segments of genes between homologous chromosomes

13. 
METAPHASE I
Homologous pairs of chromosomes line up along metaphase plate

14. 
ANAPHASE I
Homologous pairs pulled apart to opposite poles

15. TELOPHASE I & CYTOKINESIS
TELOPHASE I & CYTOKINESIS
Chromosomes (no longer homologous pairs!) prepare for meiosis II
Cytoplasm begins to split into first 2 (of 4) daughter cells

16. TELOPHASE I & CYTOKINESIS
TELOPHASE I & CYTOKINESIS
NOTE:
At the end of meiosis I, daughter cells are already Haploid!
(because homologous pairs were separated)

17. 2 haploid daughter cells then go through MEIOSIS II
In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!

18. PROPHASE II  Centrioles migrate, produce spindle fibers
Just like in mitosis!

19. METAPHASE II  Chromosomes line up along metaphase plate
Just like in mitosis!

20. ANAPHASE II  Sister chromatids pulled apart to opposite poles
Just like in mitosis!

21. TELOPHASE II & CYTOKINESIS
TELOPHASE II & CYTOKINESIS
Nuclear envelopes reform around 4 daughter cells
Cytoplasm splits to separate the 4 daughter cells
Each of the 4 daughter cells is a new gamete with genetic information different from either parent!

22. Is This Meiosis I or Meiosis II?
Why?

23. Is This Meiosis I or Meiosis II?
Why?

24. Meiosis I
Meiosis II
Check Point: When do the cells become haploid?

25. Mitosis v. Meiosis

26. 200 to 400 million sperm per ejaculate!
But what happens to the other 3 eggs?
The largest ovum absorbs the others!
Only one wins per cycle, usually

27. Finally, Fertilization
Male and female gametes unite
Fusion of two haploid nuclei produces diploid nucleus
Egg + Sperm = Zygote (new life in one diploid cell)

28. Meiosis and Sexual Reproduction Guarantee Variation and Diversity!
In Three Ways
Independent Assortment
Homologous Chromosomes separate randomly
Crossing Over
Homologous pairs exchange gene segments
Random Fertilization
Which sperm meets which egg? (game of chance)