1. Meiosis
Formation of Gametes

2. Facts About Meiosis
Similar in many ways to mitosis, but division occurs twice
Original cell is diploid (2n), resulting cells are haploid (n)
Results in 4 non-identical cells, each with ½ the number of chromosomes
Start with 1 cell with 46 double-stranded chromosomes (2n), then divides to make 2 cells with 23 double stranded chromosomes (n), then divides a second time to make 4 cells with 23 single stranded chromosomes (n)

3. Facts About Meiosis
The cells that are produced are the gametes (or sex cells) – eggs (ova) in females and sperm in males
Ova are produced in the ovaries (this is called oogenesis)
Sperm are produced in the testes (spermatogenesis)
Called Reduction-Division
We are dividing cells twice
We are reducing the number of chromosomes in each cell

4. Facts About Meiosis

5. Why is Meiosis Important?
It is the fundamental basis of sexual reproduction
Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote

6. Prior to Meiosis Just like in mitosis, Interphase must occur first
This proceeds just like it does with mitosis: G1, S, G2
During the S phase, the homologous DNA chromosomes are replicated
The replicated copies are called sister chromatids, and are held together at the centromere

7. Meiosis I
Proceeds like mitosis – prophase, metaphase, anaphase telophase
In prophase I the nucleus breaks down
A critical component of prophase I is the event crossing over when homologous chromosomes pair together (tetrads) in the middle of the cell. They connect in an area called the chiasma

8. Crossing Over
This leads to an exchange of genes that results in new gene combinations (increased variation/diversity)
This is the main advantage of sexual reproduction

9. Meiosis I
In metaphase I, chromosomes line up in the middle (in 2 lines)
In anaphase I the homologous chromosomes move apart
In telophase I, 2 new nuclei form (1 cell becomes 2 cells, each with 23 paired chromosomes)

10. Meiosis I

11. Meiosis II Prophase II – both nuclei break down
Metaphase II – chromosomes line up in the middle (single file)
Anaphase II – sister chromatids are ripped apart
Telophase II – New nuclei form
*Cytokinesis occurs after meiosis II (2 cells become 4 cells, each with 23 single chromosomes)

12. Meiosis II

13. Overall Meiosis

14. Meiosis Animation

15. Mitosis Vs. Meiosis