Modeling Meiosis Essential Question: How do our cells determine who we are? Due Monday! b Background Information: Today we are going to do a step by step simulation of meiosis. We will be working with the chromosomes of a Puffle, an imaginary organism. 1. If the Puffle has a diploid chromosome number of four, what will the haploid chromosome number be? __________________ 2 2. Place a large paper plate in the center of your work space. Imagine that this is one sex cell in a Puffle. The boundary of the plate is the cell membrane. 3. You are going to create a diploid nucleus containing two pairs of chromosomes. First, divide each of your four balls of colored clay in half. 4. Take one-half of each ball and roll it between your hands to form four elongated, snakelike chromosomes. Make the red and blue chromosomes as long as your index finger. Make the green and yellow ones half that length. Do the same thing with the other half of clay. Paper plate 5. Remember that a chromosome is made of tightly coiled strands of DNA. Within each chromosome there are many, many genes. The chromosomes within each pair are said to be homologous, meaning similar but not necessarily identical. Homologous chromosomes contain the same genes but not necessarily the same alleles. For instance, two homologous chromosomes might contain the gene which codes for eye color, but the alleles might be different--like blue eye allele on one chromosome and brown eye allele on the other. Blue eye allele Brown eye allele Homologous Chromosomes (1 from mom, other from dad) 6. We are going to examine four traits in the Puffle, specified by four genes. Remember that genes are segments of chromosomes which code for proteins that cause observable traits (phenotypes). Table II describes some traits of our imaginary creature and their locations on each chromosome. Green Chromosome Yellow Chromosome Table II -- Imaginary Traits of the Puffle Genes on PAIR 1 Fur Color Fur Type G (green fur) c (straight fur) g (yellow fur) C (curly fur) Genes on PAIR 2 Blue Chromosome Red Chromosome Eye Color B (blue eyes) b (red eyes) Eyelash length L (long eyelashes) l (short eyelashes) Table II -- Imaginary Traits of the Puffle Genes on PAIR 1 Fur Color Fur Type Green Chromosome G (green fur) c (straight fur) g (yellow fur) C (curly fur) Yellow Chromosome Genes on PAIR 2 Eye Color Eyelash length B (blue eyes) L (long eyelashes) b (red eyes) l (short eyelashes) Blue Chromosome Red Chromosome g G C c B b L l 7. Note that we are looking at only two genes on each chromosome, and ignoring hundreds of other genes. Also, note that the Puffle we are studying is completely heterozygous for all four genes examined (has both dominant and recessive alleles - Gg, Bb, etc.). This does not always happen. 8. Label the location of each gene by cutting out the paper labels provided and sticking them on to the clay as shown above. Cut the paper as close to the letters as you can. INTERPHASE 9. During interphase, each chromosome is replicated by making another complete set of DNA. Simulate this replication by creating a matching chromosome (same shape and color) using the extra bits of clay you set aside earlier. Label each chromatid with genes so they are exact copies. 10. Connect sister chromatids together at the centromere (center point) by pinching them together as shown below: g g G G C C c c B B b b L L l l centromere 8 11. How many chromatids are present at this stage?_________ How many 4 chromosomes? ____________ PROPHASE I 12. Pair up each newly replicated chromosome with its homologous partner, forming two tetrads (2 pairs of chromosomes, total of 4 chromatids). 13. Now that homologous chromosomes are near each other, crossing-over can occur. In each tetrad, place one chromatid over another. Then trade alleles so the colors are mixed. B B L b b l L B B b b L l L l l g g GG g g GG C C c c C c C c METAPHASE I 14. Imagine that the nuclear membrane has now broken down in your imaginary Puffle cell. Line the two tetrads (pairs of chromosomes) end to end across the center of the cell. Attach a piece of yarn stretching from the centromere (center point) of each chromosome to the spindle fibers end of the cell nearest to it. What do the pieces of yarn represent? ____________________ centromere Large paper plate yarn 15. In a real cell, would the red chromosome always line up on the same side as the green No. Chromosomes line up in random order, chromosome? Why or why not? _______________________________________________ like shuffling cards. __________________________________________________________________________ ANAPHASE I 16. Separate each tetrad and use the spindle fibers to move the homologous chromosomes to opposite poles of the cell as shown below: TELOPHASE I 17. Cytokinesis (cell division) occurs to form two daughter cells. These daughter cells are not haploid or diploid, but some kind of intermediate stage. Arrange your clay chromosome models on two large paper plates as shown below: GG g g BB Cc Ll bb Cc Ll 18. Are the two daughter cells identical (in genetic makeup)? Why or why not? ______ No. One chromosome came from mom, other from dad. In ________________________________________________________________________ _________________________________________________________________________ crossing over, they also exchanged parts. MEIOSIS II PROPHASE II & METAPHASE II 19. Line up the chromosomes in each cell end to end along the center line. Attach spindle fibers to the centromeres (center points) as shown below: centromere yarn ANAPHASE II 20. Separate the two sister chromatids and use the spindle fibers to move them to opposite poles as shown below: 21. In anaphase II, what must happen to the centromere region? _______________________________________________________________________ Has to break so chromatids can separate _______________________________________________________________________ TELOPHASE II 22. Cytokinesis occurs again, producing a total of four daughter cells. You can show this by separating the 8 sets of chromosomes on to 4 small paper plates as shown below: gCBL Yellow, curly fur blue eyes long lashes gcBl Yellow, straight fur blue eyes short lashes GCbL Green, curly fur red eyes long lashes Gcbl Green, straight fur red eyes short lashes 23. Are these cells haploid? How do you know this? _____________________________ Yes. Each have 1 set of chromosomes, half as many as parent cell _______________________________________________________________________ different 24. Are the four daughter cells genetically the same as each other or different?_________ different Are they genetically the same as the parent cells or different? ___________________ Explain why this is so. _______________________________________________________ Chromosomes got mixed up in random order and went to different cells. _______________________________________________________________________ _______________________________________________________________________ In crossing over, chromosomes traded bits of DNA. 25. Write the genotype and phenotype beneath each of the daughter cells shown in #22.