1. Meiosis – a form of cell division that halves the number of chromosomes when forming reproductive cells, such as gametes (egg & sperm cells)

2. Stages of Meiosis: *Before meiosis begins, DNA in
the original cell is replicated.
In meiosis, the cell undergoes two
cell divisions: Meiosis I and Meiosis II.
The cell then goes through two cell
divisions: Meiosis I and Meiosis II.

3. Meiosis I: Meiosis I: Homologous chromosomes
separate; This stage consists
of prophase I, metaphase I,
anaphase I, and telophase I.

4. Prophase I: chromosomes condense; nuclear envelope breaks down;
homologous chromosomes pair up forming a tetrad;
crossing over can occur here

5. Metaphase I homologous chromosomes line up in the middle of the cell
independent assortment occurs

6. Anaphase I
homologous pairs separate

7. Telophase I nuclear membrane reforms; the cytoplasm divides
two cells are produced each containing 1 set of the homologous pairs

8. Meiosis II: Meiosis II: The 2 chromatids (referred to
as sister chromatids) separate;
This stage consists of prophase
II, metaphase II, anaphase II,
and telophase II.

9. Prophase II
a new
spindle forms
around the
chromosomes

10. Metaphase II sister chromatids line up along the equator
independent assortment occurs

11. Anaphase II
centromeres divide
sister chromatids separate

12. Telophase II nuclear envelope forms around each set of chromosomes
the cytoplasm divides
four haploid cells are produced

13. 2n
Meiosis: -reduces the number of chromosomes by half to form gamete cells
centromere n n

14. Three Mechanisms that Contribute to Genetic Variation:
1. independent assortment – the random distribution of homologous chromosomes during meiosis
2. crossing over – when portions of a chromatid on one homologous chromosome is broken and exchanged with the corresponding chromatid portion of the other homologous chromosome
3. random fertilization – the random joining of 2 gametes

15. Importance of Genetic Variation:
It allows organisms to change so they can better adapt to the changing environment !

16. Gametogenesis – the formation of gametes
1. spermatogenesis – the process by which sperm are produced in male animals; occurs in the testes
2. oogenesis – the process by which eggs are produced in female animals; occurs in the ovaries

17. Spermatogenesis vs. Oogenesis