1. Lesson 5.5: Meiosis

2. Purpose
Meiosis is a process of cellular division that is used to make gametes (eggs and sperm)
Egg cells from the ovaries
Sperm cells from the testes

3. Chromosome number Diploid Haploid All cells in your body but sex cells
Have twice the number of chromosomes as haploid cells
Only sex cells, including egg and sperm cells
Have half the number of chromosomes as normal cells.

4. Prophase I Chromatin condenses (coil) into chromosomes
Centrosomes form the mitotic spindle
Homologous chromosomes pair up (1 maternal/1 paternal)
Nuclear envelope breaks down

5. Prophase I – Crossing Over
Crossing over may occur = Homologous chromosomes exchange genetic material.
(Notice the colors have switched places)

6. Metaphase I
Spindles are attached to the Homologous chromosomes that are lined up on the equatorial plate
Chromosomes assort independently (different colors can go to different sides of the plate)

7. Anaphase I Homologous chromosomes separate and move to opposite poles
Each chromosome now consists of sister chromatids
They may not be identical due to crossing over
Sister Chromatids

8. Telophase I Chromosomes decondense or uncoil Nuclear membranes reform
Cytokinesis occurs
DNA replication does not occur (it did before Prophase I)

9. Prophase II Chromatin again condenses into chromosomes
Centrosomes form the mitotic spindle
The cells are haploid
Remember there were 4 total chromosomes (2 pairs of 2) in prophase I
Now there is 1 of each pair in prophase II
Nuclear envelope breaks down

10. Metaphase II
Spindles are attached to the chromosomes that are lined up on the equatorial plate

11. Anaphase II
Centromeres divide and each chromatid moves to opposite poles of the cell

12. Telophase II Chromosomes uncoil Nuclear membranes reform.
Cytokinesis occurs
Result are 4 haploid gametes (either egg or sperm cells)
Each of four daughter cells are genetically unique due to independent assortment and crossing over.