1. Mitosis, Meiosis and Heredity: Meiosis
Unit 7

2. What we know: Genes are located on chromosomes in the cell’s nucleus
Each organism must inherit a single copy of every gene from both it’s 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

4. Ploidy
A cell that contains both sets of homologous chromosomes is called diploid (2 sets, 2N)
Drosophila 2N=8
2 complete sets of chromosomes and 2 complete sets of genes

5. 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 called haploid (1 set, N)
Drosophila N=4

6. 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.
2 divisions:
Meiosis I
Meiosis II

7. Meiosis I: Interphase I
Remember the steps G1, S, G2
Cells go through a round of replication , forming duplicate chromosomes

8. 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

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

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

11. Meiosis I: Telophase and Cytokinesis
Nuclear membranes form around the now separated chromosomes
The cytoplasm squeezes in a pinches off into 2 cells
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

12. Meiosis II: Prophase II
There’s NO chromosome replication before prophase II.
Centrioles migrate to opposite poles.

13. 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.

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

15. 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

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

17. 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

18. *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 Cells
2 rounds of cell division
Results in 4 different daughter cells
Hapliod= N
Involved in gamete formation
*In both mitosis and meiosis the cell divides.

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