1. Homework: Homework Packet #10 (read article)
If a healthy zygote needs 46 chromosomes to develop, how many chromosomes would egg and sperm cells need?
Homework: Homework Packet #10 (read article)

2. Asexual vs. Sexual Reproduction
Read and annotate the text
Answer the reading questions
You can answer them as you read or after you read the entire text

3. Why is the egg so much bigger than sperm?

4. Purpose of Meiosis
To create sex cells (also called gametes) with HALF the number of chromosomes of typical human cells
Meiosis in females creates eggs in the ovaries
Meiosis in males creates sperm in the testes

5. Unique Aspects of Meiosis
Chromosome reduction- the number of chromosomes get cut in half
Crossing over- Mom and Dad’s DNA can get mixed together on chromosomes
Independent Assortment- Chromosomes divide into sex cells randomly
Two rounds of division- 1 cell divides twice to create 4 cells

6. Chromosome Reduction
Since a healthy zygote needs 46 chromosomes the egg and sperm cells each need 23 chromosomes
=46
But…every cell in our bodies contains 46 chromosomes
Therefore, in meiosis, the number of chromosomes has to be cut in half
We refer to these cells as ‘haploid’ cells (23 chromosomes)

7. Homologous pairs
During prophase 1 of meiosis, the chromosomes from your mom and dad match up
You have two copies of each chromosome and each chromosome carries DNA for the same proteins
(ex: Chromosome 13 carries the trait for development of the retina in your eye)
Each pair of chromosomes is called a homologous pair

8. Crossing Over
Crossing Over – During prophase 1, homologous pairs swap segments of DNA’

9. Crossing Over
Since segments of your mom and dad’s DNA switch, you end up with greater genetic variety in the chromosomes
Each sex cell you make will be unique because crossing over happens differently each time

10. Independent Assortment
As the chromosomes from your mom and dad line up before division, they line up in a random order
This occurs differently each time, resulting in unique gametes

11. Modeling Meiosis
1. Answer the meiosis review questions using your notes from today]
2. Go to a lab station, read through the directions carefully
3. Draw the pictures of meiosis 1 and meiosis 2 from the board
4. Go through the activity by reading carefully and answering any questions.
BEFORE YOU GLUE- I need to check you work

12. Homework: HP #10- read, annotate, answer reading questions
Explain why the number of chromosomes must be reduced by half during the process of meiosis.
Homework: HP #10- read, annotate, answer reading questions

13. Chromosome Reduction- Review
Meiosis is the process of making egg and sperm cells (gametes, sex cells)
These two cells will combine to form a zygote
A healthy zygote needs 46 chromosomes
Each sex cell must carry 23 chromosomes instead of 46 in order to create a healthy zygote
=46
Meiosis involves two rounds of cell division so the number of chromosomes gets reduced by half to create haploid cells

14. What happens when meiosis goes wrong?

15. Non-disjunction
Non-disjunction is the failure of the chromosomes to split properly during meiosis resulting in sex cells with too few or too many chromosomes

16. Non-disjunction and age

17. How can doctors know if the baby has the right number of chromosomes?
Doctors can use a karyotype
A karyotype is a picture of all the chromosome pairs (one from mom and one from dad)