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)

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

Why is the egg so much bigger than sperm?

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

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

Chromosome Reduction Since a healthy zygote needs 46 chromosomes the egg and sperm cells each need 23 chromosomes 23 + 23 =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)

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

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

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

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

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

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

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 23 + 23 =46 Meiosis involves two rounds of cell division so the number of chromosomes gets reduced by half to create haploid cells

What happens when meiosis goes wrong?

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

Non-disjunction and age

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)