1. The Other Cell Division: Making Sex Cells

2. Meiosis – A Source of Distinction
Ever wonder why you don’t look exactly like either your mother or father? Or why you and your siblings are not identical?
It’s all in MEIOSIS!

3. 2 Major Roles of Meiosis
Make GAMETES (egg and sperm); diploid (2n) cells make haploid (n) cells
Genetic Variation

4. GAMETES
Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).
This change (diploid  haploid) is critical if two gametes combine to make a new individual
In meiosis, one diploid cells produces four haploid cells.

5. Genetic Variation
Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives.
Increases genetic diversity (accomplished through independent assortment and crossing-over).
Genetic diversity is important for the evolution of populations and species.

6. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

7. Homologous Chromosomes
Homologous Chromosomes are:
The same size
The same shape
Have the same genes
Different forms of gene

8. Meiosis I : Separates Homologous Chromosomes
Interphase
Each of the chromosomes replicate
The result is two genetically identical sister chromatids which remain attached at their centromeres

9. Prophase I
Each pair of sister chromatids move to their homologous pair and join together (synapsis) in a group of four called a tetrad.
This is when crossing over can occur.
Crossing Over is the exchange of segments during synapsis.

12. Metaphase I
The chromosomes line up at the equator (metaphase 1 plate) attached by their centromeres to spindle fibers from centrioles.
Still in homologous pairs

13. Anaphase I
The spindle guides the movement of the chromosomes toward the poles
Sister chromatids remain attached
Move as a unit towards the same pole
The homologous chromosomes are separated to opposite poles

14. Telophase I This is the end of Meiosis I.
The cytoplasm divides, forming two new daughter cells.
Each of the newly formed cells has half the number of the parent cell’s chromosomes (23 unique chromosomes)
but each chromosome is already replicated (sister chromatids) and the daughter cells are ready for Meiosis II

15. Cytokinesis Occurs simultaneously with Telophase I
Forms 2 daughter cells
Plant cells – cell plate
Animal cells – cleavage furrows
NO FURTHER REPLICATION OF GENETIC MATERIAL (S Phase) PRIOR TO THE SECOND DIVISION OF MEIOSIS

17. Meiosis II : Separates sister chromatids
There is no Interphase.
Results in 4 haploid daughter cells (gametes)

18. Prophase II
Each of the daughter cells forms a spindle, and the sister chromatids move toward the equator

19. Metaphase II
The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

20. Anaphase II The sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles
Now individual chromosomes, we no longer call them chromatids!

21. Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell and cytokinesis occurs
After completion of cytokinesis there are four daughter cells
All are haploid (n)
This is the whole point of meiosis!

22. Figure 13.7 The stages of meiotic cell division: Meiosis II

23. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 223 =
That’s a lot of diversity by this mechanism alone.

25. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

27. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and distinguishing) division of meiosis