1. KEY CONCEPT
Meiosis creates sex cells that are haploid

2. The Process of Meiosis
One parent cell produces 4 gametes/sex cells
Sex cells have half the chromosome number (haploid) as the parent cell (diploid)
Daughter cells are genetically different than parent cell

3. Meiosis is also known as Reduction Division because as the cell divides, its chromosome number is reduced

4. Why must the genetic material be reduced?
n (sperm) + n (egg) = 2n (zygote)
from mom
from dad
child
meiosis reduces
genetic content
Too
many!
Just right!

5. Meiosis occurs in 2 Stages:
Meiosis I and Meiosis II

6. In Meiosis I homologous chromosome pairs separate

7. In Meiosis II sister chromatids separate

8. Homologous Chromosomes – * One inherited from each parent
WHAT’S THE DIFF????
Homologous Chromosomes –
* One inherited from each parent
* Similar gene segments but NOT identical
Sister Chromatids
Sister Chromatids
Sister chromatids –
* Made in Interphase “S”
* Genetically identical

9. : Meiosis I (ANAPHASE I ) – Homologous Chromosome Pairs separate
Haploid n
Diploid 2n .

10. : Meiosis I (ANAPHASE I ) – Homologous Chromosome Pairs separate
Haploid n
Diploid 2n .

11. : Meiosis I Meiosis II Diploid 2n Haploid n Homologous chromosomes
separate
Sister chromatids
separate .

12. Interphase I Cell carries out job Chromosomes stringy chromatin
Chromosomes duplicated in “S” apart .

13. Prophase I (Early) Chromosomes condense (coil)
Nuclear membrane breaks apart
Spindle fibers form .

14. Prophase I Late
Homologous chromosomes synapse and exchange DNA segments – crossing over
Increases genetic diversity cause each
chromosome now has genes from both parents mixed together

15. Crossing Over – Increases genetic diversity
This increases genetic diversity important for survival and evolution

16. Metaphase I
Homologous chromosomes line up side by side on the equator and attach to the same spindle fiber

17. Anaphase I Homologous chromosomes separate and move to opposite poles.
Each pole receives only 1 copy of each chromosome

18. Telophase I and Cytokinesis
Nuclear envelopes reforms
Spindle fibers disappear
Cytokinesis divides the
cytoplasm into two cells

19. End of Meiosis I Results of Meiosis I 2 Haploid Cells
No doubling of chromosomes occurs before Meiosis II

20. Meiosis II Prophase II (haploid) Metaphase II Anaphase II Telophase II
Four
Non-identical
haploid
daughter cells

21. Prophase II
Nuclear envelope breaks apart
Spindle forms.

22. Metaphase II
Chromosomes line up on cell equator

23. Anaphase II
Sister chromatids separate
and move to opposite poles.

24. Telophase II and Cytokinesis
Nuclear envelope forms
Spindle fibers disappear
Chromosomes unravel.
Cytokinesis divides the cell’s cytoplasm into two cells

25. Each cell is genetically unique
End of Meiosis II
4 Cells with Haploid #
Each cell is genetically
unique

26. Gametogenesis – Process of Haploid cells developing into mature gametes
In males meiosis occurs in the testicles and forms sperm In females meiosis occurs in the ovaries and forms eggs

27. SPERMATOGENESIS Sperm get flagella and become swimmers.
Sperm contribute DNA to an embryo.

28. OOGENESIS Eggs contribute DNA, cytoplasm, and organelles to an embryo.
Cytoplasm divides unevenly
- the egg gets most of the contents; the other cells form 3 polar bodies which can’t be fertilized.

29. “Putting It All Together” - Fertilization

30. What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction.
Meiosis Video Link

31. Meiosis Video Link #1
Meiosis Video Link #2
Meiosis Video Link#3

32. Problems in Cell Division

33. Effects of Mistakes in Cell Division
If a mutation occurs during mitosis the outcome will affect the organism the cell is in
If a mutation occurs during meiosis the outcome will affect the organism’s offspring

34. Mistakes in Meiosis usually occurs during Anaphase
Anaphase I - homologous chromosomes fail to separate or
Anaphase II - sister chromatids fail to separate.

35. Problems in Meiosis Failure of chromosomes to separate –Nondisjunction
Typically occurs during Anaphase I or II
Results in too many or too few
chromosomes in sex cells

37. Trisomy Monosomy (lethal)
After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none.
The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21.
Trisomy
Monosomy (lethal)

38. Nondisjunction can occur with either autosomes or sex chromosomes

40. Human Disorders due to Nondisjunction of Autosomes (#1-22)
Trisomy 21 – Down’s Syndrome
Trisomy 13 – Patau’s Syndrome
Trisomy 18 – Edward’s Syndrome
Trisomy = cell has 3 copies of a chromosome

41. Down’s Syndrome – Trisomy 21

42. Examples:
Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21

43. Patau Syndrome - Trisomy 13

44. Edward’s Syndrome Trisomy 18

45. Nondisjunction - Sex Chromsomes
Turner’s Syndrome – X (only 1)
Klinefelter’s Syndrome - XXY

48. Turner’s syndrome – X
Only 45 chromosomes, missing a second X sex chromosome
Girls affected – short, slow growth, heart problems

49. Klinefelter’s syndrome XXY –
47 chromosomes, extra X chromosomes
Boys affected – low testosterone levels, underdeveloped muscles, sparse facial hair

50. Parental origin of meiotic error leading to aneuploidy

51. Problems in Cell Division

52. If a mutation occurs during mitosis the outcome will affect the organism the cell is in
If a mutation occurs during meiosis the outcome will affect the organism’s offspring through sex cell inheritance

53. Usually the mistake occurs during anaphase I when either homologous chromosomes either fail to separate or during Anaphase II when the sister chromatids fail to separate.

54. Results in too many or too few chromosomes in offspring
Problems in Meiosis
Nondisjunction – Failure of chromosomes to separate during Anaphase I or II
Results in too many or too few
chromosomes in offspring

56. Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1)
After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none.
The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21.
Trisomy
Monosomy (lethal)

57. Nondisjunction of Autosomes
Trisomy 21 – Down’s Syndrome
Trisomy 13 – Patau’s Syndrome

58. Down’s Syndrome – Trisomy 21

59. Examples:
Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21

60. Patau Syndrome - Trisomy 13

61. Nondisjunction - Sex Chromsomes
Turner’s Syndrome – X
Klinefelter’s Syndrome - XXY

64. Turner’s syndrome – only 45 chromosomes, missing a sex chromosome (X)
Girls affected – short, slow growth, heart problems

65. Klinefelter’s syndrome – 47 chromosomes, extra X chromosomes (XXY)
Boys affected – low testosterone levels, underdeveloped muscles, sparse facial hair

66. Parental origin of meiotic error leading to aneuploidy

67. Evertything past this point is just a duplicate – still double check

68. Results in too many or too few chromosomes in offspring
Problems in Meiosis
Nondisjunction – Failure of chromosomes to separate during Anaphase I or II
Results in too many or too few
chromosomes in offspring

70. Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1)
After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none.
The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21.
Trisomy
Monosomy (lethal)

71. Nondisjunction of Autosomes
Trisomy 21 – Down’s Syndrome
Trisomy 13 – Patau’s Syndrome

72. Down’s Syndrome – Trisomy 21

73. Patau Syndrome - Trisomy 13

74. Nondisjunction - Sex Chromosomes
Turner’s Syndrome – X
Klinefelter’s Syndrome - XXY

77. Meiosis differs from mitosis in significant ways.
Meiosis has two cell divisions while mitosis has one.

78. Mitosis Meiosis Number of divisions 1 2
Number of daughter cells made through process 4
Are the new cells Genetically identical?
Yes No
# of Chromosomes compared to parent cell
Same as parent
Half of parent
What cell go thru this division process
Somatic cells
Sex cells
When do cells go thru this division process
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction

79. MITOSIS MEIOSIS Occurs in somatic (body) cells
Occurs only in gonads (sex organs: ovary/testes)
Produces cells for repair,
maintenance, growth, asexual reproduction
Only produces gametes (sex cells: egg/sperm)
Results in identical diploid (2n) daughter cells
Reduction division results in haploid (n) cells

80. Mitosis or Meiosis? 2 diploid 4 haploid
Crossing over between nonsister chromatids
Reduction division
Increases genetic variability vs. clones
Meiosis 1 Homologous chrom separate
Meiosis 2 Sister chromatids separate

81. Mitosis or Meiosis? 2 diploid 4 haploid
Crossing over between nonsister chromatids
Reduction division
Increases genetic variability vs. clones
Meiosis 1 Homologous chrom separate
Meiosis 2 Sister chromatids separate

82. Parental origin of meiotic error leading to aneuploidy