1. Meiosis

2. Genes
Scientists in the 1900s knew that genes carried the characteristics of different organisms.
Where were these genes?
What cellular process was responsible for the different characteristics of genes?

3. Chromosomes
Strands of DNA and protein inside the nucleus in eukaryotic (animals and plants) cells
Carriers of genes
Genes are located in specific positions on chromosomes

4. Diploid Cells Diploid – “two sets”
Haploid
Diploid – “two sets”
The diploid cells of most adult organisms contain two complete sets of inherited chromosomes
Diploid

5. Homologous Chromosomes in Diploid Cells
These two chromosomes are known as homologous or homologues

6. Diploid Number Diploid number in fruit flies is 8 2N = 8
N = the number of chromosomes in the parent sex cell (sperm or egg)
Therefore, how many chromosomes does a fruit fly’s sperm cell contain?

7. Haploid Cells
Haploid
Some cells contain only a single set of chromosomes, and therefore a single set of genes
These cells are called “haploid” cells
Haploid – one set
Diploid

8. Haploid Cells
The gametes (sex cells) of sexually reproducing organisms are haploid
For fruit flies, the haploid number is 4
N = 4

9. Sex Cells are Haploid
If most cells in the body undergo mitosis and are diploid, then the sex cells that are haploid need to undergo a different process.
This process is called meiosis

10. Mitosis Review
Before we start learning the steps of meiosis, let’s review the steps of mitosis

11. Mitosis Review
Prophase – genetic material inside the nucleus condenses and the duplicated chromosomes become visible.
Outside the nucleus spindles start to form

12. Mitosis Review
Metaphase – centromeres of the duplicated chromosomes line up across the center of the cell
Spindle fibers connect the centromere of each chromosome to the pole of the spindle

13. Mitosis Review
Anaphase – chromosomes separate and move along spindle fibers to opposite ends of the cell
Telophase – chromosomes, which were distinct and condensed, begin to spread out into a tangle of chromatin

14. Phases of Meiosis
There are two stages of meiosis, each with four phases
Meiosis I
Prophase I, Metaphase I, Anaphase I and Telophase I
Meiosis II
Prophase II, Metaphase II, Anaphase II, and Telophase II

15. The Stages of Meiosis
Meiosis I
Meiosis II

16. Meiosis I
Just prior to meiosis I, the cell undergoes a round of chromosome replication during interphase I.
Each replicated chromosome consists of two identical chromatids joined at the center.

17. Prophase I
The cells begin to divide, forming a structure called a tetrad, which contains four chromatids

18. Prophase I
As homologous chromosomes (corresponding chromosomes from male and female) pair up and form tetrads, they undergo a process called crossing-over

19. Crossing-over in Prophase I
First, the chromatids of the homologous pairs cross over one another
Then, the crossed sections of the chromatids are exchanged

20. Crossing-over and Alleles
Crossing-over is important because it produces new combinations of alleles in the cell
Alleles – one of a number of different forms of a gene
This is what leads to genetic diversity

21. Crossing-over

22. Metaphase I
As prophase I ends, a spindle forms and attaches to each tetrad
During metaphase I, paired homologous chromosomes line up across the center of the cell

23. Anaphase I
During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell
When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cells

24. Telophase I
During telophase I, a nuclear membrane forms around each cluster of chromosomes
Cytokinesis follows telophase I, forming two new cells

25. Telophase I & Cytokinesis

26. Meiosis I Meiosis I results in two cells, called daughter cells.
These two cells have a haploid number of chromosomes.
The chromosomes are still duplicated.

27. Meiosis II
The two cells produced by meiosis I now enter a second meiotic division
Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II
The steps of meiosis II look just like the steps of mitosis

28. Prophase II
As the cell enters prophase II, their chromosomes become visible

29. Metaphase II
During metaphase of meiosis II, chromosomes line up in the center of each cell.

30. Anaphase II
As the cell enters anaphase, the paired chromosomes separate.

31. Telophase II and Cytokinesis
Each of the four daughter cells formed in meiosis II receives unduplicated chromosomes that are different from the versions in each other cell.

32. Meiosis II
The four new daughter cells now contain the haploid number (N) of unduplicated chromosomes.

33. Gametes to Zygotes
The haploid cells produced by meiosis II are gametes
Gametes – sex cells
Male animals - sperm
Male plants – pollen grains
Female animals – only one of the cells produced in meiosis is involved in reproduction
Egg cells

34. Gametes to Zygotes
Fertilization – the fusion of male and female gametes – generates new combinations of alleles in a zygote
Zygote – fertilized egg
The zygote undergoes cell division by mitosis and eventually forms a new organism

35. LETS MAKE A BABY!!!!

36. LETS MAKE A BABY!!!! X Y

37. LETS MAKE A BABY!!!! X Y

38. LETS MAKE A BABY!!!! X Y

39. LETS MAKE A BABY!!!! X Y

40. LETS MAKE A BABY!!!! Y X

41. LETS MAKE A BABY!!!! Y X

42. LETS MAKE A BABY!!!! Y X

43. LETS MAKE A BABY!!!! Y X

44. LETS MAKE A BABY!!!! Y X

45. LETS MAKE A BABY!!!! Y X

46. LETS MAKE A BABY!!!! Y X

47. LETS MAKE A BABY!!!! Y X

48. LETS MAKE A BABY!!!! Y X

49. Y X
LETS MAKE A BABY!!!!

50. Y X
LETS MAKE A BABY!!!!

51. Y X
Awww! WE MADE A ZYGOTE!

52. Please stand by while the zygote goes through MITOSIS…X

53. Please stand by while the zygote goes through MITOSIS…X Y X

54. Please stand by while the zygote goes through MITOSIS…X Y X Y X Y X

55. Please stand by while the zygote goes through MITOSIS…X Y X Y X Y X Y X Y X Y X Y X

56. Please stand by while the zygote goes through MITOSIS…X

57. Y X Y X
Get the point? Y X

58. Now we’ve got a GROWN-UP BOY…X

59. …WHO MEETS A GIRL!!! Y X X X

60. After dating for 12 years, they decide it is time to get married…X X X

61. Meanwhile, they’ve been going through MEIOSIS…X X X

62. …in their ovaries and testicles!
Watch and learn… X Y X

63. X Y X

64. X Y X

65. X Y X

66. X B D d b a Y X A b D
S Phase!

67. Y X X A b D B D d b a
S Phase! Y X A b D X B D d b a

68. S Phase! Y X A b D X B D d b a
Prophase I Y X X

69. Prophase I Y X X
Metaphase I Y X X

70. Metaphase I Y X X
Anaphase I Y X X X

71. Anaphase I Y X X
Telophase I Y X X X

72. Telophase I X Y
Prophase II X Y

73. Prophase II X Y
Metaphase II X Y

74. Metaphase II X Y
Anaphase II X Y

75. Anaphase II X Y Y
Telophase II X X Y X X X X

76. Now we have sperm and eggs! Can you fill in the alleles?X X X Y X X X

77. It is time to make another baby…X A b D X B D a X d b a Y A b X d a b X A D b X D a B

78. Lets do some bookkeeping…Y X X
DAD
Phenotype:
Genotype:
Possible Sperm:
MOM
Phenotype:
Genotype:
Possible Eggs:
Afro, Bl Eyes
NoFro, Br Eyes
AA, bb
aa, Bb Ab
aB, ab

79. And now for a Punnet’s Square…Y X X
Possible Eggs
aB ab
Phenotypic Ratio:
50% Blue eyed fro
50% Brown eyed fro
AaBb
Aabb Ab
Fro, Br Eyes
Fro, Bl eyes
Possible Sperm