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Meiosis: A Play in Three Acts - Chromosome Drama Explained

Join the captivating drama of meiosis with this educational presentation. Discover how chromosomes pair up, cross over, and separate to create genetic diversity. Learn about the role of homologous chromosomes, crossing over, and the significance of X and Y chromosomes in this crucial cell division process. Dive into the world of meiosis with engaging visuals and informative content.

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Meiosis: A Play in Three Acts - Chromosome Drama Explained

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  1. Supplemental File 3: Lecture Presentation Slides Meiosis: A Play in Three Acts Starring DNA Sequence Dina L. Newman and L. Kate Wright Thomas H. Gosnell School of Life Sciences Rochester Institute of Technology Rochester, NY

  2. Act I

  3. Actors needed! • 6 student volunteers to be chromosomes in a diploid cell

  4. What is the value of “n” in this diploid cell? A. 2 B. 3 C. 6 D. 12 E. other n = the number of unique chromosome types

  5. How many chromosomes?

  6. When does “n” change for an organism? • When DNA is replicated • When sister chromatids are separated • When homologous chromosomes are separated • A and B • Never

  7. How do we define “n”? • The number of chromosomes that makes up a set of information (all the genes) • The complete set of chromosomes originally inherited from one parent • The number of chromosomes in the haploid genome

  8. What has to happen to these chromosomes before cell division can begin? • DNA replication • Chromosomes condense and nuclear membrane breaks down

  9. How do the new DNA sequences compare with the old ones?

  10. How many chromosomes are in the cell now? A. 3 B. 6 C. 12 D. other

  11. What is the value of “n” in this cell?

  12. What is the next step? • Homologous chromosomes pair together

  13. What are homologous chromosomes? By National Human Genome Research Institute, http://www.genome.gov/Images/EdKit/bio1c_large.gif, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2132905

  14. How similar are homologous chromosomes to each other? • Every human genome is 99.5% identical to every other human genome by DNA sequence • 3 billion bp in the human genome • Only 15 million bp are different • How similar is maternal chromosome #1 to paternal chromosome #1? • ALMOST IDENTICAL!

  15. Act 2

  16. How do homologous chromosomes find their mates? • Once you have found your mate, link arms to show that you are physically paired

  17. How similar are the homologous chromosomes? • Why is sequence similarity important?

  18. Crossing over: a physical link between homologous chromosomes A T C C G T C T C C C A C T A G G C A G A G G G T G A T C C G T C T C C C A C T A G G C A G A G G G T G A T C C G T A G G C C A C C C T C T A G A G G G T G G T G GG A G A T C T C CC A C C T C C C A C G A G G G T G To see in 3D and more detail: http://www.dnatube.com/video/929/Honologous-recombinationof-DNA-Holliday-Junction

  19. Homologues, cross over! • Crossing over is also known as meiotic recombination • This can occur anywhere there is a stretch of identical sequence

  20. What is the purpose of crossing over? • To produce genetic variation that can be acted on by natural selection. • To allow proper segregation of chromosomes during meiosis.

  21. What is the effect of crossing over? • Recombinant chromosomes • Genetic variation isan outcome, not the underlying reason we do it

  22. What about X and Y chromosomes? • They do not form homologous pairs • They pair with each other

  23. Act 3

  24. Okay, our homologous chromosomes are paired. What’s next? • What causes chromosomes to “line up”?

  25. Need more actors • 2 student volunteers to be centrosomes

  26. “Line up” and “Pull apart” • What is really going on in the cell here? • What is going to be separated? • What needs to be broken in order for the chromosomes to separate?

  27. How many chromosomes in each cell? A. 3 B. 6 C. 12 D. other

  28. What is the value of “n” in each cell? A. 2 B. 3 C. 6 D. 12 E. other

  29. What do we count? • To determine ploidy? (is cell haploid or diploid?) • To determine n?

  30. What has to happen next? • The sister chromatids have to separate • IF we were REALLY going to stay true to our model, what would we have to do to our students?

  31. 6 more actors needed! Student volunteers are no longer chromosomes, they are now sister chromatids! Each PAIR is a chromosome.

  32. How many chromosomes in each cell? A. 3 B. 6 C. 12 D. other

  33. What is the value of “n” in each cell? A. 2 B. 3 C. 6 D. 12 E. other

  34. How many genetically different cells do we have?

  35. Key Concepts • Meiosis is a special type of cell division that produces gametes for potential use in fertilization. • n is a constant for any given organism, does not change during life cycle • Chromosomes may be replicated or unreplicated—thus, DNA content or number of “arms” is not a good way to determine number of chromosomes. Instead, look at number of independent units. • Homologous chromosomes pair during meiosis I. This is facilitated by matching DNA sequence. This is essential for proper segregation that ensures each cell receives one of each type of chromosome. • X and Y chromosomes pair together so that they can segregate appropriately during meiosis. This is facilitated by identical sequence in the pseudoautosomal regions at the tips. • Meiosis II is like Mitosis, where sister chromatids are separated.

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