1. Meiosis is an important aspect of sexual reproduction

2. Gametes & Fertilization
Gametes – specialized cells necessary for reproduction
Males – sperm, Females – egg
The zygote goes on to develop into an embryo, and on into a complete individual.
When fertilization takes place:
(Sperm, 23 chromosomes) + (Egg, 23 chromosomes) = (Zygote, 46 chromosomes)
Normal body cells have a diploid (2n) chromosome number, meaning chromosomes occur in pairs.
In humans, the male and female each contribute half (haploid (n)), 23 chromosomes.

3. Sexual Reproduction 2 parents; offspring not genetically identical
The process of meiosis shuffles genetic information (DNA) and results in variation in genes. Therefore producing a variation in offspring

4. Meiosis
Note: occurs in reproductive organs and only produce gametes with half the chromosome compared to body cells
46 chromosomes,
23 pairs
23 chromosomes
Interphase
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1 &
Cytokinesis
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2 &
Cytokinesis
Meiosis 1
Meiosis 2

5. Centrioles
Spindle fibers
Prophase I
Corresponding homologous chromosomes from each parent pair up to form homologous pairs
When sister chromatids exchange DNA its called crossing over.
Crossing over creates a number of genetic variations in gametes
Homologous Pairs (Humans have 23 pairs making 46 total chromosomes)

6. Importance of crossing over
Siblings look alike but not exactly the same….
Crossing over increases genetic diversity

7. Metaphase I
The centrioles send out spindle fibers to line up homologous pairs in the middle of cell along the metaphase plate
Centrioles
Spindle fibers
Homologous Pairs

8. Anaphase I
The centrioles use the spindle fibers to separate the homologous pairs
Each homologous chromosome is pulled to the opposite pole of the cell
Centrioles
Spindle fibers
Homologous Chromosomes

9. Independent Assortment
A shuffling of the 23 homologous pairs, in which there are two possibilities for how chromosomes will sort themselves into daughter cells.
“independent assortment” leads to variations in species as there are more than 8 million combinations possible for the 23 pairs in any egg or sperm cell.
resulting in 70 trillion different zygotes possibilities

10. Telophase I & Cytokinesis
Telophase I – the cell creates a temporary nucleus around the two homologous chromosome sets
Cytokinesis – the cell divides into two cells
Centrioles
Homologous Chromosomes
Nuclear Membrane

11. Meiosis I - Homologous Pair Separate
Prophase I - Homologous chromosomes pair up, non-sister chromatids exchange genetic material.
(crossing over)
Metaphase I - Homologous chromosomes pair up at the equator. (one chromosome from each parent)
Anaphase I - Homologous chromosomes separate and are pulled to opposite poles by the spindle fibres.
Telophase I - One chromosome from each homologous pair is at each pole of the cell.
Interkinesis - results in 2 daughter cells with ½ # of chromosomes. This stage between cell divisions the cell will grow and make proteins as in interphase of mitosis. Unlike interphase in mitosis, there is no replication of DNA during this stage.

12. Prophase II
Each of the Meiosis II stages are running in 2 cells at the same time.
Similar to Prophase of Mitosis
Centrioles attach spindle fibers to the chromosomes
Centrioles
Spindle fibers
Chromosomes

13. Prophase II
Each of the Meiosis II stages are running in 2 cells at the same time.
Similar to Prophase of Mitosis
Centrioles attach spindle fibers to the chromosomes
Centrioles
Spindle fibers
Chromosomes

14. Metaphase II
- The X-shaped chromosomes form a single line across the middle of the cell

15. Telophase II & Cytokinesis
Telophase II – the cells creates a permanent nucleus around the two haploid chromosome sets
Cytokinesis – the cells divides into four haploid daughter cells
Chromatids
Nuclear Membrane

16. Meiosis II - Separates sister chromatids
Meiosis II is like mitosis because in both processes the chromatids of each chromosome are pulled to opposite poles.
Each daughter cell inherits one chromatid from each chromosome resulting in four haploid cells each with half the N of chromosomes.
Prophase II - There is one chromosome of the homologous pair in each cell.
Metaphase II - The X-shaped chromosomes form a single line across the middle of the cell.
Anaphase II - Sister chromatids move to opposite poles of the cell. Once they separate, each sister chromatid is considered to be a chromosome. (nondisjunction could occur here)
Telophase II - Spindle fibres begin to disappear, and a nuclear membrane forms around each set of chromosomes.
Cytokinesis - In cytokinesis, the two daughter cells are separated.

17. Meiosis
23 chromosome
46 chromosomes
23 chromosomes

18. Male and Female Gametes
In males, all 4 cells resulting from meiosis develop into sperm
In females, 1 cell gets most of the cytoplasm and becomes the egg while other 3 die, due to unequal cytoplasm and organelles

19. Mitosis vs. Meiosis

20. Differences in Mitosis and Meiosis
Review of
Mitosis vs.
Meiosis
Comparison animation
Differences in Mitosis and Meiosis
Mitosis
# daughter chromosomes = # parent chromosomes
Two identical daughter cells
Occurs in all cells except reproductive cells
Produces diploid cells
Diploid cells = 2n
Somatic Body Cells (skin)
Meiosis
# daughter chromosomes = half the # parent chromosomes
Four non-identical daughter cells
Occurs only in reproductive cells
Produces haploid cells
Haploid cells = n
Gamete Sex Cells (sperm or egg)