1. Meiosis

2. Remember these key points…
How many sets of genes are in each organism?
2 sets - one from the mom, one from the dad
A cell with 2 sets of genes is said to be DIPLOID

3. If the fertilized egg has 2 copies of the genes, how many copies are in 1 sperm cell or one egg cell?
One copy of the genes in the egg cell. One copy of the genes in the sperm cell.
These cells are called HAPLOID
They have only one set of genes

4. How does an organism create haploid cells?
Glad you asked!
This happens through a process called MEIOSIS
Meiosis - the process of cell division in which the number of chromosomes per cell is cut in half

6. The Phases of Meiosis
Before Meiosis I begins the chromosomes are replicated
In Prophase I the nuclear envelope breaks down and HOMOLOGOUS chromosomes (chromosomes that look the same) pair up to form TETRADS.

7. The Phases of Meiosis Prophase I continued –
Portions of the tetrads get exchanged in a process called CROSSING OVER. This makes new combinations of alleles so the new cells will be genetically different from the original cell.

8. The Phases of Meiosis Metaphase I –
Microtubules grow from the centrioles and connect to the centromeres
Tetrads line up along the center of the cell

9. The Phases of Meiosis Anaphase I –
The chromosome pairs are split apart (the sister chromatids are still together)
Each cell now has the haploid number of replicated chromosomes (2N)

10. The Phases of Meiosis Telophase I –
The chromosomes decondense in some species
Nuclear membrane reforms

11. The Phases of Meiosis Cytokinesis I – Two new cells are formed
Each cell has only one replicated copy of each gene and each is genetically different from the mother cell

12. The Phases of Meiosis Prophase II – Nuclear membrane dissolves
Centrioles replicate

13. The Phases of Meiosis Metaphase II –
Sister chromatids line up in the center of the cell
Spindle fibers attach to the centromeres

14. The Phases of Meiosis
Anaphase II –
Sister chromatids are pulled apart

15. The Phases of Meiosis Telophase II and cytokinesis II –
Four new haploid daughter cells are formed

16. Meiosis
Why do it?
creates cells that are haploid (N) which allows reproduction to create a baby with the correct number of genes
Creates cells that are genetically unique
which gives the baby a new set of traits, that might make it better able to survive.

17. The four cells produced by Meiosis II are called GAMETES
In a male, the gametes produced are called sperm
In a female, only one of the four cells created becomes the egg. The other three cells are called polar bodies.

18. Let’s think…
What are some ways meiosis is different from mitosis? (Other than their names!)

20. Comparing Mitosis and Meiosis
Place each of the following words or phrases in one section of your Venn Diagram:
Makes 2 cells Makes 4 cells
Genetically different cells Genetically identical cells
Makes haploid cells Makes diploid cells
A process of cell division Produces gametes
Produces normal body cells
Cells produced are used for growth
Cells produced are used for reproduction