1. Meiosis

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

3. 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 genetically different daughter cells
Makes diploid daughter cells (2N)
Makes haploid daughter cells (1N)

4. What is an “n”? Haploid (n) is the number of chromosomes in a gamete.
Diploid (2n) – number of chromosomes in a
somatic cell. You have one copy from
the mother, and one copy from the
Father.

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

6. 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

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

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

9. Occurs when homologous chromosomes exchange segments of genes
PROPHASE I
CROSSING OVER
Occurs when homologous chromosomes exchange segments of genes

10. 
METAPHASE I
Homologous pairs of chromosomes line up along metaphase plate

11. 
ANAPHASE I
Homologous pairs pulled apart to opposite poles

12. 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

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

14. 2 haploid daughter cells then go through MEIOSIS II
2 haploid daughter cells then go through MEIOSIS II
In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!

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

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

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

18. 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!

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

20. Is This Meiosis I or Meiosis II?
Why?

21. Meiosis I
Meiosis II
Check – Point: At what point do the cells become haploid?

22. Mitosis v. Meiosis

23. 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

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

25. Meiosis and Sexual Reproduction Guarantee Variation and Diversity!

26. 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)