1. MEIOSIS
Meiosis

2. Review of Mitosis
Mitosis is the division of a parent cell into two genetically identical daughter cells. Mitosis occurs during the M Phase of the Cell Cycle and is essential for growth and repair.

3. The Four Stages of Mitosis
PROPHASE
METAPHASE
ANAPHASE
TELOPHASE
In mitosis, a cell that begins with 46
chromosomes divides into two daughter cells each with 46 chromosomes.

4. Overview of Meiosis
The function of meiosis is to produce our sex cells that are used for reproduction.
Sex cells are also called GAMETES. In humans, our gametes are egg and sperm.

5. Fertilization of Gametes
When a sperm cell penetrates an egg cell, this is called FERTILIZATION and the result is a zygote, a fertilized egg which will then grow and divide and develop into an embryo.

6. Overview cont.
All other cells in your body are somatic cells and use the cell cycle which includes mitosis to replicate.

7. Why do we need a separate process for sex cells?
Eggs and sperm cells must have half the number of chromosomes as the parent cell, so that when they are put together (during sexual reproduction egg+sperm) they will have the same number of chromosomes as the parent.
Egg and sperm cells are NOT genetically identical to the parent cell. (If they were, you would be identical to your siblings!)

8. AMOEBA SISTERS CHROMOSOME NUMBERS
Everyone has 2 sets of chromosomes:
A set from Mom and a set from Dad, these two sets are homologous chromosomes.
AMOEBA SISTERS CHROMOSOME NUMBERS
The term sister chromatids refers to the identical copy of the chromosomes that is temporarily attached by a centromere after replication. Sister chromatids have identical DNA, homologous chromosomes do not as they came from different parents.

9. Diploid Cells
A cell containing both sets of homologous chromosomes is said to be diploid, which means “two sets.”
This number is usually represented by the symbol 2N.
Example: Humans have 46 chromosomes or 2 sets of N = 46
EXAMPLES: Skin Cells, Lung Cells, Heart Cells, Blood Cells, a Zygote

10. Haploid Cells
A cell containing only 1 set of chromosomes is said to be haploid, which means “one set.”
This number is usually represented by the symbol N.
Example: Humans 2N = 46 then N =23
EXAMPLES: Sperm Cell, Egg Cell

11. Important Information about Meiosis
To produce gametes, specific cells (called germ cells) must undergo the cell cycle and replicate their DNA during the S phase.
While in the M Phase (Cell Division) instead of Mitosis, these cells use Meiosis I and II where they will divide twice before entering into G0.
The result is four (instead of two) daughter cells that are all haploid (N).

12. The Stages of Meiosis
Meiosis is divided into two rounds: Meiosis I and Meiosis II. It is simply 2 rounds of phases very similar to mitosis.

13. MEIOSIS I Prophase I Interphase I
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. CROSSING OVER occurs in Prophase I
Interphase I
Cells undergo a round of DNA replication, forming duplicate chromosomes.

14. MEIOSIS I Metaphase I Anaphase I
Spindle fibers pull the homologous chromosomes apart and away toward opposite ends of the cell
Metaphase I
Homologous Chromosomes align in the middle and spindle fibers attach to their centromeres

15. MEIOSIS I Telophase I and Cytokinesis Two new nuclear membranes form
The cell separates into two cells

16. Meiosis I

17. WHY TWO ROUNDS?
Meiosis I results in two diploid (2N) daughter cells with the same number of chromosomes as the parent cell. So if an egg and sperm were to join there would be too many chromosomes!!!
To solve this issue, the two daughter cells DO NOT continue on in the cell cycle with Interphase. The DNA is NOT replicated again. Instead, the two daughter cells progress right into meiosis II.

18. MEIOSIS II Metaphase II Prophase II
Chromosomes align in the middle and spindle fibers attach to their centromeres
Prophase II
Nuclear membrane disappears and chromosomes once again begin to pair up

19. MEIOSIS II Telophase II and Cytokinesis Anaphase II
Nuclei reform and cells split. Meiosis II results in four haploid (N) daughter cells.
Anaphase II
Spindle fibers pull the sister chromatids apart and away toward opposite ends of the cell

20. Meiosis II Prophase II Metaphase II Anaphase II Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Telophase II
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.

21. SUMMARY OF MEIOSIS
By the end of Meiosis, the original diploid parent cell has now become FOUR sex cells. These gametes are haploid cells because they have only a single set of chromosomes.
3D SUMMARY OF THE PHASES OF MEIOSIS:

22. How Meiosis Encourages Genetic Differences
Crossing Over and Independent Assortment are two processes that happen during Meiosis I and II that ensure daughter cells are unique and different from the parent cell.

23. Crossing Over
Crossing-over happens during PROPHASE 1 when homologous chromosomes exchange portions of their chromatids, resulting in new and unique combinations of genes in the gametes.
CROSSING OVER CLIP

24. Independent Assortment
Independent Assortment ensures that chromosomes are randomly assorted. Chromosomes align randomly in the middle during Meiosis I and II, and they separate independently. This gives countless possibilities of gene combinations.

25. MITOSIS VS MEIOSIS Type of Reproduction Asexual Sexual
Number of times cell divides 1 2
Number of daughter cells made at completion of division(s) 4
Cells where this type of division occurs
Body Cells (somatic)
Gametes
Daughter cells are haploid/diploid
Diploid
Haploid
Daughter cells are genetically similar or different from parent
Identical
Different
Daughter cells are genetically similar or different from each other
If there were 6 chromosomes in the parent cell, how many would be in the daughter cells? 6 3
Type of Reproduction
Asexual
Sexual
MEIOSIS SQUARE DANCE

26. 46 Chromosomes 46 Chromosomes 46 Chromosomes 92 Chromosomes
MITOSIS
46 Chromosomes
46 Chromosomes
92 Chromosomes
AMOEBA SISTERS MEIOSIS
23 Chromosomes
23 Chromosomes
MEIOSIS
23 Chromosomes
46 Chromosomes
92 Chromosomes
23 Chromosomes
46 Chromosomes