1. Unit 3- Genetics
Meiosis

2. Fertilization Which is diploid ? Which is haploid ? Egg Sperm
____ chromosomes
Sperm
_____ chromosomes X
Zygote
46 chromosomes
Which is diploid ?
Which is haploid ?

3. Sex cells How can a diploid cell become haploid cells?
23 chromosomes
Egg
23 chromosomes
How can a diploid cell become haploid cells?
Answer: by the process of meiosis
Through meiosis, one diploid parent cell produces four haploid daughter cells;
- Each daughter cells has half the number of chromosomes found in the parent cell

4. Recall where DNA is within the cell

5. Interphase DNA replicates/duplicates (S phase)
In the form of chromatin
Not easily visible
Similar to interphase in Mitosis

6. Before any cell division, DNA must be duplicated
We’re going to use a simpler cell in which 2n=8 (or n= 4)
This cell have 4 unduplicated chromosomes right before meiosis occurs

7. this cell has 2 pairs of homologous chromosomes

9. 2 parts of meiosis Meiosis 1: reduction division
Meiosis 2: Gametes are produced
Unlike mitosis, meiosis does NOT occur in somatic cells of our body but in reproductive organs.

10. Prophase 1 Tetrad: the homologous pair made of 4 chromatids.
Chiasmata: regions where crossing-over occurs.

11. Prophase 1
Chromosomes condense and become visible; each with 2 identical sister chromatids.
Nuclear membrane dissolves; centrioles move to opposite cell poles; spindle fibers form
Homologous chromosomes pair up and form a tetrad
Crossing-over occurring between homologous chromosomes allows exchange of genetic information

12. Your cell should look like this now

13. Draw this cell. This is NOT metaphase 1
Homologous chromosomes line up in PAIRS in the middle of the cell
Again: Homologous chromosomes do not line up in a single file but rather in pairs in metaphase 1

14. Anaphase 1
Homologous chromosomes (NOT individual sister chromatids) separate and move to opposite end of cell.

15. Telophase 1 Homologues reach opposite sides of the cell.
Remember each chromosome is still made up of 2 sister chromatids because the chromatids were not seperated but rather the whole replicated chromosome.
Fix Two haploid cells (half of the original amount in each cell that is, 2 each)
Original cell (n= 4 chromosomes)
Result: 2 daughter cells with half the amount of chromosomes as the original parent cell. Haploid!
Hence the reduction division.
Homologues reach opposite sides of the cell.
A nuclear member reforms forming 2 new nuclei
Chromosomes relax

17. Meiosis 2: similar to mitosis Prophase 2
Chromatin coil again
Spindle fibers attach to chromosomes
THERE IS NO DUPLICATION OF CHROMOSOME AT THIS STEP
No Crossing over either

18. Metaphase 2
Chromosomes line up RANDOMLY in the middle of each cell

19. Anaphase 2
Centromeres split pulling sister chromatids (NOT homologous pairs anymore) to opposite end of each cell

20. Four nuclei form around chromosomes Spindle fibers dissolve
Telophase 2
Four nuclei form around chromosomes
Spindle fibers dissolve
Cells divide

21. Final result of meiosis
Initial parent cell
2n=4
4 haploid daughter cells
n = 2

23. Task: Create a Meiosis Flipbook
Use Playdoh to recreate the detailed events of meiosis.
Take picture of each phase of meiosis and compile them into a flip book
Sample at:
Use the Meiosis flipbook template if you need to

24. What you need 3 different colored playdoh
One color for paternal chromosome
One color for maternal chromosome
One color for centromere
Paper or surface of whiteboard
Pen/marker to draw other details such as cell membrane, spindle fibers, centrioles etc.
Handout
Group of 3

25. Gametogenesis= the formation of gametes
involves
Spermatogenesis in male
and oogenesis in female

26. No DNA duplication occur here
What is the original # of chromosome in the cell of this species? 4
At the end of meiosis I, how many chromosome are there? 2; # chromosomes has been reduced by half
At the end of meiosis II, the # of chromosomes is still haploid but the # of cells is twice as many. Why is that? Will learn in later slides

28. Chromosome Crossing over
Only one chiasma is illustrated but many occur per pair
(Plural: chiasmata)

30. 22 = 4 possible combinations to forms a gamete
When they are lined up, there is a chance that one daughter cell will get the paternal chromosome and an equal chance that it will get the maternal one. This 50/50 split can occur with any homologous chromosome pair. 

31. Some new words Gamete: a sex cell; includes sperm cells and egg cells
Zygote: a cell produced by the fusion of two gametes
Fertilization: the formation of a zygote by fusing two gametes
Haploid: a cell containing half the usual number of chromosomes (n)
Diploid: a cell containing two copies of each chromosome (2n)