1. Cellular Reproduction of Reproductive Cells
MEIOSIS
Cellular Reproduction of Reproductive Cells

2. Vocab you should know…
Meiosis: cell division where one body cell produces four gametes, each containing half the number of chromosomes as a parent’s body cell
Homologous Chromosomes: paired chromosomes w/ genes for the same traits arranged in the same order
Gametogenesis: process by which male & female gametes are formed
Spermatogenesis: process by which male gametes form

3. More Vocab…
Oogenesis: production, growth & maturation of an egg (ovum)
Zygote: diploid cell formed when a sperm fertilizes an egg
Crossing Over: exchange of genetic material b/t nonsister chromatids from homologous chromosomes during prophase I of meiosis; results in new allele combinations

4. More Vocab…
Genetic Recombination: major source of genetic variation among organisms caused by reassortment or crossing over during meiosis
Nondisjunction: failure of homologous chromosomes to separate properly during meiosis; results in gametes with too many or too few chromosomes
Fertilization: fusion of male & female gametes
Independent Assortment: random distribution of the pairs of genes on different chromosomes to the gametes

5. More Vocab… Gamete: male & female sex cells; sperm & eggs
Tetrad: 4 chromatids in a pair of homologous chromosomes that come together as a result of synapsis during meiosis
Polar Body: a short-lived product of the formation of gametes by meiosis
Gene: short segment of DNA containing instructions for a single trait

6. More Vocab…
Synapsis: the pairing of homologous chromosomes during meiosis
Homologue: something homologous
Ovum: a mature egg cell (female reproductive cell)
Spermatid: immature sperm cell that has almost completed development
Spermatazoa: mature male reproductive cell

7. Cell Cycle for Reproductive Cells
Interphase
Meiosis I: prophase I, metaphase I, anaphase I, telophase I
Cytokinsis I / interkinesis
Meiosis II: prophase II, metaphaseII, anaphase II, telophase II
Cytokinesis / Cytokinesis II

9. Cell Cycle of a Reproductive Cell
Interphase – long first stage of cell grow & DNA duplication (same as in mitosis)
Meiosis I – similar to mitosis, results in production of 2 2n (diploid) cells
Interkinesis or Cytokinesis I – division of cytoplasm to create 2 separate cells
Meiosis II – similar to mitosis also, except that 2 cells are undergoing it at the same time, resulting in 4 n (haploid) cells
Cytokinesis II – division of cytoplasm, similar to mitosis, except results in 4 cells

10. Interphase
Just as in mitosis, prior to division, the cell must duplicate all of its contents, including its DNA and must grow in size in order to be able to accommodate all of the duplicate material
Humans = 46 chromosomes, after interphase = 46 x 2 = 92

11. There are four stages to meiosis I Prophase I Metaphase I Anaphase I
Telophase I

12. Prophase I DNA coils into chromatids Sister chromatids pair up
Spindle fibers appear
Nucleolus & nuclear membrane disappear
**synapsis occurs: each pair of sister chromatids lines up next to its homologue forming a tetrad (the two pairs of sister chromatids join together instead of a pair of just a pair of sister chromatids)

13. Prophase I

14. Prophase I cont’d
In the tetrad, chromosomes are aligned lengthwise, so genes of one chromosomes are adjacent to the same genes on the other chromosome

15. Prophase I – crossing over
During synapsis, chromosomes twist around each other
Sometimes they twist so tightly they exchange genetic material = crossing over
In humans this happens 2-3 times for every pair of chromosomes
Crossing over can result in new traits w/in the organism = genetic recombination
After prophase I = 92 chromosomes

16. Crossing Over

17. Metaphase I Tetrads line up randomly at the equator
Spindle fibers from the poles (centrioles) attach to the centromere of each pair of the homologous chromosomes
After metaphase I = 92 chromosomes

18. Metaphase I

19. Metaphase I

20. Anaphase I
Homologous chromosomes (sister chromatids) move to the poles, each pair of the tetrad to the opposite pole
Independent assortment = the random separation of homologous chromosomes resulting in genetic variation (this is what is responsible for the differences in individuals)
After anaphase I = 92 chromosomes

21. Anaphase I

22. Independent Assortment

23. Telophase I Homologous chromosomes reach the opposite poles
Spindle fibers break down
Nuclear envelopes forms around each set of chromosomes, nuclei reappear
After telophase I = 92 chromosomes, 46 in each new nucleus

24. Telophase I

25. Interkinesis or Cytokinesis I
Interkinesis, like cytokinesis, divides the cells cytoplasm resulting in 2 2n (diploid) cells, each having a complete set of DNA
Meiosis I starts w/ 1 2n cells, at the end of Meiosis I, we have 2 2n cells
Each new cell has 46 chromosomes

26. Interkinesis

27. Meiosis II
After Meiosis I is complete, the two cells go directly into Meiosis II
***No Interphase*** this means no duplication of chromosomes
Meiosis II has four phases
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis II – division of cytoplasm of both cells to create 4 separate cells

28. Prophase II Nuclear membrane breaks down, nucleoli disappears
Spindle fibers form
Chromatin condenses
Sister chromatids pair up
After prophase II = 46 chromosomes in each cell

29. Prophase II

30. Pairs of sister chromatids move to equator in each cell
Metaphase II
Pairs of sister chromatids move to equator in each cell
After metaphase II = 46 chromosomes in each cell

31. Metaphase II

32. Chromatids separate & move towards opposite poles in each cell
Anaphase II
Chromatids separate & move towards opposite poles in each cell
After anaphase II = 46 chromosomes in each cell

33. Anaphase II

34. Telophase II Nuclear membranes form around all 4 sets of chromosomes
Spindle fibers break down in all cells
Nucleoli appear in the nucleus of all 4 cells
Chromatids uncoil into chromatin
After telophase II = 46 chromosomes in each cell, 23 in each new nucleues

35. Telophase II

36. Cytokinesis II
Cleavage furrow deepens until it pinches off each cell, creating 4 new cells
All 4 cells are haploid (n) (contain 1 set of chromosomes)
After cytokinesis II = 23 chromosomes in each new cell

37. Cytokinesis II

38. Gametes & Gametogenesis
Meiosis results in the production of gametes
In humans:
Females: oogenesis
Males: spermatogenesis

39. Oogenesis
Diploid reproductive cell divides meiotically to form 1 mature ovum (egg)
But…meiosis results in the formation of 4 cells…how is it that we only get 1 mature ovum?
Ova are very large, so when cytokinesis occurs, one cells get the majority of the cytoplasm, this becomes the ovum
the other 3 cells do not receive enough cytoplasm to survive, they are called polar bodies & will degenerate (disappear)

40. Oogenesis

41. Spermatids develop into mature spermatozoa
Spermatogenesis
Diploid reproductive cell divides meiotically to form 4 haploid spermatids
Spermatids develop into mature spermatozoa

42. Spermatogenesis

43. Fertilization
The process of fusing a male and female gamete to produce offspring
Only occurs b/t two gametes, haploid (n) cells
Results in an offspring that has diploid (2n) cells (n x n = 2n)

44. Fertilization

45. Cancer
Uncontrolled cell division that may be caused by environmental factors &/or changes in enzyme production in the cell cycle
Basically, when the internal timing of a cell causes it to repeatedly divide at a faster than normal rate

46. Meiosis Quiz
What happens to the DNA of a reproductive cell during interphase?
Synapsis occurs during which phase of Meiosis (be sure to note I or II)?
What does synapsis result in?
During what phase does crossing over occur in?

47. Meiosis Quiz cont’d What is crossing over?
The random separation of homologous chromosomes resulting in genetic variation is ____ _____?
Interkinesis/Cytokinesis I results in the formation of 2 ________ cells.
Cytokinesis II results in the formation of 4 _______ cells.

48. Meiosis Quiz Cont’d
Oogenesis results in the formation of _____ mature egg cells & 3 ______ ______.
Spermatogenesis results in the formation of ________ spermatids.

49. http://faculty. clintoncc. suny. edu/faculty/Michael