Unit 3- Genetics Meiosis

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

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

Recall where DNA is within the cell

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

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

this cell has 2 pairs of homologous chromosomes

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.

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

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

Your cell should look like this now

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

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

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

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

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

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

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

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

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: https://www.youtube.com/watch?v=uYgbWm4H_gU Use the Meiosis flipbook template if you need to

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

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

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

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

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. 

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)