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 are moved by the spindle to the equator of the cell; independent assortment occurs

6. Anaphase I
homologous pairs separate

7. Telophase I
individual chromosomes gather at the poles of the cell; 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
chromosomes line up at the equator and are attached at their centromeres to spindle fibers; independent assortment occurs

11. Anaphase II
centromeres divide; sister chromatids separate & move to opposite poles of the cell

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