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Meiosis

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Meiosis

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  1. Meiosis

  2. Heredity • Heredity • Passing of genetic traits from parent to offspring • Chromosome theory of heredity • Chromosomes carry genes • Gene = unit of heredity

  3. What Meiosis is all About • Meiosis allows the creation of unique individuals through sexual reproduction.

  4. In The Beginning Two • Reproduction in which there is a re-mixing of the genetic material is called sexual reproduction • Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops • Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved

  5. Gametes Are Haploid • Gametes must have half the genetic material of a normal cell • If the genetic material in the gametes was not halved, when they combined the zygote would have more genetic material than the parents! • Gametes have exactly one set of chromosomes, this state is called haploid (1n) • Regular cells have two sets of chromosomes, this state is called diploid (2n)

  6. Egg 1n Haploid nucleus Sperm 1n Haploid nucleus Fertilization Results in a Diploid Zygote

  7. Fertilization Results in a Diploid Zygote Egg 1n Haploid nucleus Sperm 1n Haploid nucleus

  8. Haploid nucleus Fertilization Results in a Diploid Zygote Egg 1n Haploid nucleus Sperm 1n

  9. Haploid nucleus Fertilization Results in a Diploid Zygote Egg 1n Haploid nucleus Sperm 1n

  10. Diploid Zygote 2n From Zygote to Embryo

  11. From Zygote to Embryo Cleavage

  12. From Zygote to Embryo Cleavage

  13. From Zygote to Embryo Cleavage

  14. From Zygote to Embryo Cleavage

  15. From Zygote to Embryo Morula

  16. Meiosis 1 Prior to division, amount of DNA doubles

  17. Stages of Meiosis • http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

  18. Independent Assortment during Prophase I • The key difference between Mitosis and Meiosis is the way chromosomes uniquely pair and align in Meiosis Mitosis The first division of Meiosis

  19. Synapsing • UNLIKE in mitosis, homologous chromosomes line up next to each other during prophase • This process is called synapsing • Lined up homologues are called tetrads

  20. Anaphase 1 • During anaphase 1, each homologous chromosome is pulled to opposite sides of the cell. Unlike mitosis, THE CENTROMERES DO NOT BREAK. • Nuclei may or may not reform following division. • Cytokenesis may or may not occur.

  21. Check out the Tetrads

  22. Meiosis I

  23. In Telophase 1, two daughter cells areformed. They are NOT identical! (Why?)

  24. Meiosis II Chromosomes are NOT duplicated again between Meiosis 1 and Meiosis 2 Why not?

  25. Meiosis II • Prophase 2: spindle reforms and chromosomes move toward the metaphase plate • Metaphase 2: sister chromatids lined up on the metaphase plate • Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell • Telophase 2 and Cytokinesis: nuclei form at either pole and each cell is finally divided into two identical daughter cells

  26. All Together Now

  27. Independent Assortment

  28. Independent Assortment Example: 2 chromosomes in haploid n = 2 2n = 22 = 4 possible combinations

  29. In Humans Example: 23 chromosomes in haploid n = 23 2n = 223 = ~ 8 million possible combinations!

  30. Prophase 1 – Crossing Over • Homologous chromosomes come together • Areas of homologous chromosomes connect at areas called chiasmata

  31. Crossing Over • Segments of homologous chromosomes break and reform at similar locations. • Results in new genetic combinations of offspring. • This is the main advantage of sexual reproduction

  32. Crossing-Over

  33. Random fertilization • At least 8 million combinations from Mum and another 8 million from Dad … • >64 trillion combinations for a diploid zygote!!!

  34. X chromosome Boy or Girl? The Y Chromosome Decides Y chromosome

  35. Oogenesis • The female gamete is called an ovum • At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. • A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.

  36. Oogenesis • Only one ovum is produced during meiosis • Oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop. • All the cytoplasm and organelles go into one egg for nourishment for the young organism that will develop after fertilization.

  37. Spermatogenesis • The male gamete is called a sperm • Four spermatocytes are formed during meiosis • Men are busy - meiosis produces roughly 250,000 sperm a day.

  38. Any Questions?

  39. References Slides, information and images were taken from the following presentations: • cchs.churchill.k12.nv.us/marshk/Notes/meiosis.ppt • bioweb.wku.edu/Faculty/Bowker/120/mitosis.html • edweb.sdsu.edu/ltca/Mitosis_Meiosis_files/slide0001.htm • waukesha.k12.wi.us/South/Bio1/MEIOSIS.htm • my-ecoach.com/coaching/meiosis_files/meiosis.ppt • www.grisda.org/tstandish/teachers/presentations/High%20School/Meiosis.ppt • www.biology.usu.edu/courses/biol1010-podgorski/PPpage.htm