1. Unit 5.3 Meiosis

2. What we need to know: Genes are located on
chromosomes in the cell’s nucleus
Each organism inherits a single copy of each gene from each of its parents
When an organism produces its own gametes (sperm/egg) the 2 sets of genes must be separated so each gamete ends up with only 1 set.

3. Chromosomes Consider a fruit fly with 8 chromosomes
4 chromosomes from mom
4 chromosomes from dad
These 2 sets of chromosomes are homologous because each of the 4 chromosomes from mom match up with each of the 4 from dad

5. Ploidy Ploidy is the number of sets of chromosomes in a cell
A cell that contains both sets of homologous chromosomes is called diploid (2 sets, 2N)
2 complete sets of chromosomes and 2 complete sets of genes

6. Ploidy
Gametes (sperm/egg), on the other hand, contain only a single set of chromosomes
Thus, they have only a single set of genes
These cells are referred to as haploid (1 set, N)

7. Meiosis
Meiosis is a process of reduction division where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a 2N cell (diploid)
2 divisions:
Meiosis I
Meiosis II

8. Meiosis I: Interphase I
Cells go through a round of replication , forming duplicate chromosomes

9. Meiosis I: Prophase I
Chromosomes have replicated then centrioles migrate to poles
Each chromosome from mom pairs with it’s corresponding chromosome from dad
This structure is called a tetrad. Tetrads contain 4 chromatids.
Crossing-over occurs. This is an exchange in alleles between homologous chromosomes & produces new allele combinations

10. Crossing over - during meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair.
These points of contact can allow the exchange of genetic information between chromosomes.

11. Crossing over
Crossing over occurs during prophase I and means that each daughter cell formed in meiosis has chromatids with a different combination of alleles.

12. Meiosis I: Metaphase I
Spindle fibers attach to each chromosome in a tetrad
The tetrads line up at the metaphase plate

13. Meiosis I: Anaphase I
Spindle fibers pull the homologous chromosomes toward the opposite end of the cell

14. Meiosis I: Telophase and Cytokinesis
Nuclear membranes form around the now separated chromosomes
The cytoplasm squeezes in a pinches off into 2 cells (cleavage)
The 2 cells have 4 chromatids as in mitosis BUT since there was crossing-over, each cell only has half of the parent cell’s information, this means these new cells are haploid

15. Meiosis II: Prophase II
There’s NO chromosome replication before prophase II.
(no interphase for Meiosis II)
Centrioles migrate to opposite poles.

16. Meiosis II: Metaphase II
Chromosomes line up in the center of the cell at the metaphase plate (like in mitosis). Spindle fibers attach to the centromeres of each chromatid.

17. Meiosis II: Anaphase II
The spindle fibers pull the sister chromatids apart at the centromeres. Chromatids move toward opposite end of the cell.

18. Meiosis II: Telophase II and Cytokinesis
New nuclear membranes form around each of the groupings of chromatids.
The cytoplasm draws in a pinches off forming 2 new cells from each of the 2 original cells.
The final result of Meiosis is 4 haploid (N=4) daughter cells with 2 chromosomes each

19. Gamete Formation In males the haploid gametes are called sperm.
In some plants, pollen grains contain haploid sperm cells

20. Gamete Formation
In females, normally only 1 of the 4 haploid cells becomes an egg (has most of the cytoplasm). The other 3 cells are called polar bodies and do not take part in reproduction

21. *In both mitosis and meiosis the cell divides.
Mitosis vs Meiosis
1 round of cell division
Results in 2 genetically identical daughter cells
Diploid= 2N
Somatic (body)Cells
2 rounds of cell division
Results in 4 different daughter cells
Haploid= N
Involved in gamete formation
*In both mitosis and meiosis the cell divides.

22. http://www. youtube. com/watch

23. Additional videos
Meiosis vs. Mitosis Overview Square Dance!

24. Inheritance Phenotype: