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Modeling Meiosis

Explore the process of meiosis, the differences between meiosis and mitosis, and how chromosomes undergo homologous pairing, crossing over, and segregation. Use pop beads to model the stages and understand the purpose of meiosis in passing on our DNA.

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Modeling Meiosis

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  1. Modeling Meiosis How do we pass on our DNA?

  2. Meiosis is the process by which the body make sex cells (gametes) The process will remind you of Mitosis, but there are many differences… To show these differences we will model the process at our lab tables with pop beads First things first….. Use the large paper provided on which you’ll place your chromosomes The Purpose of Meiosis

  3. Modeling meiosis • Our sex cells have 23 chromosomes each. • It is hard to model meiosis using 23 chromosomes… and that’s a lot of chromosomes in a cell! • We are going to use only 1 pair with the knowledge that what happens to this pair will be happening to the other 22 pairs. Got it?

  4. Interphase: Step 1 of meiosis • During interphase the chromosomes in the cell double. This means they go from being single stranded (just a line) to double stranded (2 lines that cross over each other) • This cell has 1 chromosome pair –1 from mom and 1 from dad • Make 4 strands of beads: 2 of one color and 2 of another. • Cross them over one another • For each stage you model, also draw it on the diagram on your paper. COLOR CODE your chromosomes so you can follow their changes!

  5. Prophase I: Step 2 • One thing that happens in meiosis that doesn’t happen in mitosis is “homologous pairing.” These are chromosomes that have the same genes, but one is from mom and one from dad. • Show this by putting your homologous partner chromosomes side by side. • Once the homologous chromosomes meet, the chromosomes touch arms and actually swap sections! • End result: chromosomes will have a segment of different color (which represents “crossing over”). See the next slide. Don’t draw anything in the prophase box yet.

  6. “Crossing over” • Cross over one arm of the first color homologue with one armof the other homologue. • Swap two beads between the ends of these two arms only. • Draw a picture of what your chromosomes look like now inthe appropriate cell.

  7. Metaphase I: Step 3 in meiosis • During metaphase the homologous chromosomes line up together in the center of the cell. • Model this by setting the pair of homologues in the middle of your cell. • The chromosomes don’t line up in a specific way or pattern, it’s random. • Draw in the metaphase cell how your chromosomes are arranged.

  8. Anaphase I: Step 4 of Meiosis • During anaphase the homologous partners pull apart to opposite sides of the cell. • Show this by pulling the homologous chromosomes apart • Draw what you see on your worksheet.

  9. Telophase I/Cytokinesis: Step 5 • During telophase I and cytokinesis the cell pinches and separates into two cells. • The two chromosomes will separate from each other and go into separate cells. • Put one copy of EACH homologous chromosome into each cell. • How does the chromosome number in each cell compare to the chromosome number in the original cell? Draw what you see. This is the end of meiosis I.

  10. Prophase II & Metaphase II : Step 6 • The cells actually go back into interphase and then prophase II, but we will skip that for now. • During metaphase II the chromosomes (made up of two arms still attached to each other) line up down the middle of the cell. • How is this stage similar to what you saw in mitosis? How is it different?

  11. Anaphase II: Step 7 • During anaphase II, sister chromatids in each cell pull apart from one another. One arm moves to one end of the cell and the other arm moves to the opposite end of the cell. Separating these chromatids allow each sex cell to get 1 version of the gene (allele). This ensures that each child will get one copy of each gene from mom, and one of each from dad. This is why genotypes consist of 2 letters for each gene!

  12. Telophase II & Cytokinesis: Step 8 • During telophase II and cytokinesis, the sister chromatids from anaphase have been separated and 4 cells are made. • Put the chromosomes into their cells.

  13. Comparing cells • Look at the beginning and end pictures made from meiosis. Explain how the number of chromosomes and number of cells are different between from start to end.

  14. What is made at the end of Meiosis… • At the end of meiosis, 4 sex cells are made. • Sex cells are sperm or egg cells

  15. Why does meiosis need to happen? • These are the sex cells of a human. How many chromosomes are in each sex cell? • How are the number of chromosomes different between the egg/sperm and the baby’s cell that is made? • KEY POINT: Why does meiosis need to happen when making sex cells?

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