1. Meiosis

2. Meiosis – A Source of Distinction
Why do you share some but not all characteristics of each parent?
What are the rules of this sharing game?
At one level, the answers lie in meiosis.

3. Meiosis

4. What controls each characteristic or “trait” in an organism?
The instructions for each trait are located in genes, which are segments of DNA on a chromosome.
Gene= a segment of DNA that controls the production of one particular protein (1 gene = instructions for 1 protein)
How many genes are on each of your 43 chromosomes? HUNDREDS

5. Chromosomes and Chromosome Number
Reminder: Human body cells have 46 chromosomes. When your cells need to repair or replace or grow, they duplicate cells with 46 chromosomes; this is MITOSIS.
But to first create life, cells need to undergo MEIOSIS.
Each parent contributes 23 chromosomes- resulting in 23 pairs that match up- homologous chromosomes.

6. Homologous chromosomes have the same length and the same centromere position, and they carry genes that control the same inherited traits.

7. Organisms produce gametes, which are sex cells that have ½ the number of chromosomes. This number will vary from species to species.
These cells are called haploid cells (n).
Fertilization is the process where one haploid gamete (n) combines with another haploid cell (n). The cell will now have 2n chromosomes and is called a diploid cell.

8. Why do we need meiosis?
Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.
Genetic diversity is important for the evolution of populations and species.

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

10. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

11. The Stages of Meiosis:
aka: Reduction Division- reduces the chromosome number by half
Gametes (sex cells) are produced

12. Meiosis I : Separates Homologous Chromosomes
Interphase
Carry out normal cell function (metabolism)
Each of the chromosomes replicate
The result is two genetically identical sister chromatids which remain attached at their centromeres

13. Prophase I This is a crucial phase for meiosis.
During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.
Extremely IMPORTANT!!! It is during this phase that crossing over can occur.
Crossing Over is the exchange of segments during synapsis- pieces of each chromosome switch places.

14. Metaphase I Spindle fibers attach to centromeres
Homologous pairs of chromosomes line up together at the equator

15. Anaphase I
Homologous chromosomes separate and move to either side of the cell (NOT as sister chromatids- the ENTIRE chromosome!)
The cell goes from diploid (2n) to haploid (n)

16. Telophase I This is the end of the first meiotic cell division.
Homologous chromosome reach the cell’s opposite poles. Each pole contains only one of the original pair of chromosomes.
The sister chromatids are still attached but might not be identical to each other any longer due to crossing over.
Chromosomes relax in the 2 new nuclei. Cytokinesis occurs but the cell does NOT enter interphase.

17. Figure 13.7 The stages of meiotic cell division: Meiosis I

18. Meiosis II : Separates sister chromatids
Proceeds similar to mitosis
THERE IS NO INTERPHASE II !

19. Prophase II
Spindle forms and chromosomes condense

20. Metaphase II
Chromosomes line up at the center

21. Anaphase II
Centromeres split and sister chromatids NOW move to opposite poles

22. Telophase II and Cytokinesis
Nuclei form around the 4 chromosomes
After completion of cytokinesis there are four daughter cells
All are haploid (n)

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

24. Mitosis vs. Meiosis

25. 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

26. What is the end result of meiosis?
4 genetically different haploid cells
What type of cell division do cells use in asexual reproduction?
Mitosis – end result is 2 identical haploid gametes

27. What is the importance of meiosis in terms of sexual reproduction?
Meiosis provides genetic variation. The 4 resulting haploid cells are all genetically different because of crossing over and random assortment. When sperm and egg join to form a new organism, there is genetic variability.

28. One 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.

29. Compare mitosis and meiosis
Mitosis – 2 genetically identical diploid body cells
Meiosis – 4 genetically different haploid sex cells

30. Mitosis vs. Meiosis

31. On page 276- answer questions 2,4, and 5 in COMPLETE SENTENCES or label drawings where appropriate.
*NO CREDIT for incomplete answers or unclear, unlabeled drawings.