1. Meiosis
Is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells (gametes).

2. Meiosis
Involves two divisions of the nucleus – meiosis I and Meiosis II
Starts with one diploid cell, and ends with four haploid cells.

3. Review of Terms Haploid (n)-- one set chromosomes
Diploid (2n)-- two sets chromosomes
(46 chromosomes)
Chromatid – Two exact copies of DNA (each single copy called a chromatid) makes up a chromosome
Centromere – point at which the two chromatids are attached
Homologous chromosomes – chromosomes similar in size, shape, and genetic content.

4. Meiosis

5. Meiosis I
Prophase I Metaphase I Anaphase I
Telophase I

6. Prophase I
Homologous Chromosomes synapse (pair together) Each pair is called
a tetrad because there are 4 chromatids.
Crossing – over occurs when
portions on one homologous chromosome are broken and exchanged with the corresponding portion on one of the chromatids of the other homologous chromosome.

7. Crossing - Over

8. Meiosis II
Prophase II Metaphase II Anaphase II Telophase II

9. Between Meiosis I & II
The chromosomes do NOT replicate between Meiosis I and Meiosis II.
Start of Meiosis I Start of Meiosis II
DNA copy made during Interphase

10. Prophase I & II A new spindle forms around the chromosomes
Spindles – are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division
Centrioles - two cylindrical cellular structures that form at the poles of a cell during meiosis.

11. Metaphase I & II Chromosomes line up in the center of the cell.
In metaphase I, chromosome tetrads line up, each with their sister chromatids attached.

12. Anaphase I & II Centromeres divide
Chromatids move to opposite poles of the cell
In anaphase I, the tetrad chromosomes with attached sister chromatide moves to
poles.
* Remember centromeres are what hold the two chromatids together to make a chromosome.

13. Telophase I Nuclear membrane forms around each set of chromosomes
Cytokinesis begins -division of cytoplasm
Left with two identical haploid daughter cells.

14. Telophase II Nuclear membrane forms around each set of chromosomes
Cytokinesis begins
Left with four haploid cells.

15. Male vs Female Meiosis
Spermatogenesis
Oogenesis

16. Spermatogenesis (Males)
Cytoplasm divided equally between all 4 cells during cytokinesis
Results in 4 haploid sperm cells

17. Oogenesis (Female)
Cytoplasm divided unequally between 4 cells during cytokinesis; 1 cell receives more cytoplasm than other cells
Results in 1 mature egg and 3 polar bodies

18. Sexual vs. Asexual Reproduction
2 Parents required
Offspring are similar to parents
(USES MEIOSIS)
Advantages
-genetic variation.
- Allows species to rapidly
adapt to changing
environment.
Only 1 parent required
Offspring identical
(NO MEIOSIS)
Ex: Binary Fission & Budding
Advantages
- Less energy required
- Faster reproduction time