1. Meiosis

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

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

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

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

10. From Zygote to Embryo Diploid Zygote 2n

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 MitosisThe 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. 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. Nuclei may or may not reform following division. Cytokenesis may or may not occur. Cytokenesis may or may not occur.

21. Check out the Tetrads

22. Meiosis I

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

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

27. 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

28. All Together Now

29. Independent Assortment

30. Example: 2 chromosomes in haploid n = 2 2 n = 2 2 = 4 possible combinations

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

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

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

34. Crossing-Over

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

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

37. 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.

38. 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.

39. 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.

40. Any Questions?

41. 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.grisda.org/tstandish/teachers/presentations/High%20School/ Meiosis.ppt www.biology.usu.edu/courses/biol1010-podgorski/PPpage.htm