1. Meiosis Sex Cell Formation

2. Sexual Reproduction
Involves the production of sex cells (gametes) and the fusion of their nuclei (fertilization), producing a fertilized egg cell (zygote)
Gametes
Fertilization

3. Each body (somatic) cell contains a diploid (2n) number of chromosomes
Chromosomes are present in homologous pairs.
Homologous chromosomes: contain genes for the same traits, and look alike.
Gametes have a monoploid (1n) number of chromosomes

4. Chromosome Structure

5. Gametogenesis Formation of Gametes (sex cells)
Occurs in gonads (ovaries and testes)
Involves MEIOTIC cell division
The end result of Meiosis is 4 daughter cells each containing half the original chromosome number

6. Meiosis = Reduction Division
The Chromosome # is reduced by ½
Humans have:
46 chromosomes in our body cells
23 in our gametes (sperm and egg)

7. First Meiotic Division
1. Each single stranded chromosome is replicated during the non dividing resting period. This results in double stranded homologous chromosomes being formed. 2. Chromosomes shorten and thicken with the spindle apparatus being formed. 3. Homologous chromosomes pair up side by side in a double line at the equator. (center of the cell) These homologous chromosome pairs are called tetrads. It is here that synapsis and crossing over occurs.

8. 4.During synapsis the chromatids may twist around each other and exchange segments. This is called crossing over, which increases the variation in gametes, and therefore offspring.
5. Homologous chromosomes (in tetrad formation) then line up on the equator (Metaphase I)
6. Disjunction – homologous chromosomes then separate and move towards opposite poles (Anaphase I)
7. Cytoplasm divides (cytokinesis)
8. The 2 new cells are now monoploid (1n)

9. The first Meiotic Cell Division
Replication – results in double stranded chromosomes.
Synapsis – an intimate pairing of homologous chromosomes, resulting in a tetrad.
Tetrad- Two pairs of homologous chromosomes

10. The Second Meiotic Cell Division
1. Double stranded chromosomes line up on equator (Metaphase II)
Centromeres replicate, to allow chromatids to separate
3. Single stranded chromosomes are pulled to opposite ends of the cell by spindle fibers (Anaphase II)
4. Cytokinesis: cytoplasm divides again.
5. The result of meiosis is the formation of 4 monoploid cells that are all DIFFERENT. Distribution of homologous chromosomes between the resulting nuclei is random, resulting in variation.

11. Mitosis and Meiosis Compared
Mitosis is for
growth
repair and replacement of cells
Asexual reproduction
Results in two IDENTICAL (2n) cells
Meiosis is strictly for sex cells.
production of gametes
Results in 4 DIFFERENT cells with
½ the number of chromosomes (1n)
n=haploid or monoploid

12. Spermatogenesis The Male gonads (testes) produce sperm
4 monoploid cells mature into motile sperm

13. Oogenesis Occurs in ovaries The ovum is much larger than the sperm
Results in one monoploid non-motile egg (ovum)
3 non-functional polar bodies are produced that disintegrate and are reabsorbed

14. Male and Female Meiosis Compared

15. Meiosis diagram 1

16. Meiosis diagram 2

17. Meiosis diagram 3

18. Meiosis diagram 4

19. Mitosis and Meiosis compared