Meiosis is the process by which germ cells such as eggs and sperm are produced, resulting in 4 haploid cells with half the normal number of chromosomes. This ensures that when an egg and sperm fuse during fertilization, the offspring has the correct diploid number of chromosomes. Meiosis involves two divisions and stages including prophase I, metaphase I, anaphase I and telophase I, followed by a second round of division.

1. (4 Periods)

2. State Standards
Genetics State Standards
 10.4 In sexually reproducing organism, each offspring
contains a mix of characteristics inherited from both
parents.
Genetics CAPT Strands
 D 34. Describe, in general terms, how the genetic
information of organisms can be altered to make them
produce new materials.
 D 35. Explain the risks and benefits of altering the genetic
composition and cell products of existing organisms.
 D 36. Explain how meiosis contributes to the genetic
variability of organisms.

3. Objectives
 Understand that most organisms have two genes for
each trait.
 Illustrate how meiosis contributes to genetic
variability.
 Create a visual representation of the stages of meiosis.

4. Meiosis
 The process to make cells with half the number of
chromosomes for sexual reproduction
 Occurs in our germ cells that produce gametes
 Sperm & egg
 Results in four cells genetically different from parent
cell and from each other
 Haploid (n) cells

5. Why Do We Need Meiosis?
 It is the fundamental basis of sexual reproduction
 Two haploid (n) gametes are brought together
through fertilization to form a diploid (2n) zygote
 If egg and sperm had the same number of
chromosomes as other body cells then the offspring
would have too many chromosomes.

6. Meiosis
 Meiosis must reduce the chromosome number by half (n)
 Fertilization then restores the 2n number
from mom from dad child
too
much!
meiosis reduces
genetic content
The right
number!

7. Fertilization
2n = 6
n =3

8. Meiosis-Replication
 Replication is the process
of duplicating a
chromosome
 Occurs prior to division
 Replicated copies are
called sister chromatids
 Held together at
centromere

9. Meiosis
Gene Z
Homologous
Sister
chromosomes
Chromatids
(same genes,
(same genes,
different alleles)
same alleles)
 Homologous chromosomes are the same size & shape
 Carry genes for the same trait but are not identical!

10. Checkpoint
Meiosis results in ____.
1. 4 identical cells
2. 4 genetically different cells
3. 2 genetically different cells
4. 2 identical cells

11. Checkpoint
Homologous chromosomes are _____.
1. Same in size
2. Same in shape
3. Carry genes for the same trait
4. All of the above

12. Checkpoint
Why do we need meiosis?
A: To reduce the chromosome number in gametes,
otherwise offspring would have too many
chromosomes

13. Meiosis
Sister
chromatids
separate
Homologous
chromosomes
separate
Meiosis I Meiosis II
Diploid
Diploid
Haploid

14. Meiosis I
Spindle
Nucleus
fibers
Nuclear
envelope
Early Prophase I Telophase I
(Chromosome Late Prophase I Metaphase I Anaphase I
(haploid)
# doubled)

15. Prophase I
Early prophase Late prophase
 Homologous pair.  Chromosomes condense.
 Crossing over occurs.  Spindle forms.
 Nuclear envelope fragments.

16. Prophase I : Crossing-Over
 Synapsis – the pairing of homologous chromosomes
 Group of 4 chromatids
Homologous chromosomes Join to form a
(each with sister chromatids) TETRAD

17. Prophase I : Crossing-Over
 Homologous
chromosomes in a tetrad
cross over each other
 Pieces of chromosomes
or genes are exchanged
 Produces genetic
recombination in the
offspring

18. Prophase I : Crossing-Over
 Homologous chromosomes during crossing-over

19. Metaphase I
 Homologous pairs of
chromosomes align along
the equator of the cell

20. Anaphase I
Homologous chromosomes separate
and move to opposite poles.
Sister chromatids remain
attached at their centromeres.

21. Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.

22. Meiosis II
Only one of the homologous pair
Gene Z of chromosome is present in the
cell.
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with one copy of each
chromosome and thus one copy of each gene.

23. Meiosis II
Prophase II Metaphase II
Anaphase II Telophase II 4 different
haploid cells

24. Prophase II
Nuclear envelope fragments.
 Spindle forms.

25. Metaphase II
Chromosomes align
along equator of cell.

26. Anaphase II
Pole
Sister chromatids separate
and move to opposite poles.
Equator
26

27. Telophase II
Nuclear envelope assembles.
Chromosomes unwind.
Spindle disappears.
Cytokinesis divides cell into
two.

28. Results of Meiosis
 Gametes (egg & sperm) form
 Four haploid cells (n) with one
copy of each chromosome
 One allele of each gene
 Different combinations of
alleles for different genes along
the chromosome

29. Checkpoint
Which best describes crossing-over?
1. Pieces of genes are exchanged
2. Allow genetic variation
3. Occurs in Prophase I
4. All of the above

30. Checkpoint
During anaphase I in meiosis what is pulled apart?
1. Centromere
2. Homologous chromosomes
3. Sister chromatids
4. Spindle fibers

31. Checkpoint
Which best describes a haploid cell?
1. n
2. Half the genetic information
3. Gametes
4. All of the above

32. Spermatogenesis
 Occurs in the testes
 Two divisions produce 4
spermatids
 Spermatids mature into
sperm
 Men produce about
250,000,000 sperm per day

33. Spermatogenesis
Spermatid

35. Oogenesis
 Occurs in the ovaries
 Two divisions produce 3 polar bodies that die and 1
egg
 Polar bodies die because of unequal division of
cytoplasm
 Immature egg called oocyte
 Starting at puberty, one oocyte matures into an ovum
(egg) every 28 days

36. Oogenesis

37. Oogenesis Polar body a
(haploid) X
a Polar
Bodies
X a (haploid)
a X
X
Mitosis Meiosis I Meiosis II
A X
Oogonium A
(diploid) Primary
X
oocyte
A X
(diploid) Ovum Mature
Secondary A Egg (haploid)
oocyte X
(haploid) Second
polar body
(haploid)

38. Checkpoint
How many chromosomes would a sperm or egg contain
if either one resulted from the process of mitosis?
A: 46 Chromosomes

39. Checkpoint
Oogenesis results in______.
1. 4 polar bodies
2. 3 polar bodies
3. 1 ovum
4. Both 2 & 3

40. Checkpoint
Why is the sperm so small compared to the egg?
A: The egg has all the starting nutrients, organelles and
building blocks. The sperm only provides DNA. That
is why oogenesis only produces one ‘good’ egg.

41. Mitosis Meiosis
Number of 2
1
divisions
Number of
2 4
daughter cells
Genetically
Yes No
identical?
Chromosome # 46 23
Where Somatic cells Germ cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction

42.  STUDENT

43. Meiosis
The process to make cells with ___________ the
number of chromosomes for sexual reproduction
Occurs in our germ cells that produce gametes
___________ & ______________
Results in ____________ cells genetically different
from parent cell and from each other
Haploid (____) cells

44. Why Do We Need Meiosis?
It is the fundamental basis of sexual reproduction
Two ____________ (n) gametes are brought together
through fertilization to form a _______________
(2n) _______________
If egg and sperm had the same number of
chromosomes as other body cells then the offspring
would have too many chromosomes.

45. Meiosis must ____________ the chromosome number by
____________ (n)
_____________________ then restores the 2n number
from mom from dad child
too
much!
meiosis ____________
genetic content
The right
number!

46. Fertilization
2n = ____
n =____

47. Meiosis-Replication
Replication is the process of
duplicating a
______________________
Occurs prior to division
Replicated copies are called
sister chromatids
Held together at
__________________

48. Gene
Sister
Homologous Chromatids
chromosomes (same ________,
(same _________, same _________)
different ________)
Homologous chromosomes are the same ________ &
___________
Carry genes for the same trait but are _______ identical!

49. CHECKPOINT √
Meiosis results in ____.
1. 4 identical cells 2. 4 genetically different cells
3. 2 genetically different cells 4. 2 identical cells
Homologous chromosomes are _____.
1. Same in size 2. Same in shape
3. Carry genes for the same trait 4. All of the above
Why do we need meiosis?

50. Sister
chromatids
separate
Homologous
chromosomes
separate
Meiosis I Meiosis II
______________
______________
______________

51. Early prophase I Late prophase I
Homologous pair. Chromosomes condense.
Crossing over occurs. Spindle forms.
Nuclear envelope fragments.

52. Prophase I : Crossing-Over
_____________ – the pairing of homologous
chromosomes
Group of 4 __________________
Homologous chromosomes Join to form a
(each with sister chromatids) ________________

53. Homologous
chromosomes in a tetrad
____________________
each other
Pieces of chromosomes or
genes are
____________________
Produces genetic
recombination in the
____________________

54. Metaphase I Anaphase I Telophase I
________________ Homologous Nuclear envelopes
________________ chromosomes reassemble.
of chromosomes _______________
align along the Spindle disappears.
equator of the cell Sister chromatids
Cytokinesis divides
remain attached at
cell into ________.
the _____________

55. Meiosis II
Only _______ of the homologous
pair of chromosome is present in
the cell.
Gene Z
Sister chromatids carry
_________________ genetic
information.
Meiosis II produces _______________ with one
copy of each chromosome and thus one copy of
each _________.

56. Prophase II Metaphase II Anaphase II
Nuclear envelope Chromosomes ________________
fragments. align along ________________
________________ separate and move
Spindle forms. of cell. to opposite poles.

57. Telophase II
Nuclear envelope assembles.
Chromosomes
____________.
Spindle disappears.
Cytokinesis divides cell into
________.

58. Results of Meiosis
_____________(egg & sperm)
form
Four haploid cells (___) with one
copy of each chromosome
One ________ of each gene
________________ combinations
of alleles for different genes
along the chromosome

59. CHECKPOINT √
Which best describes crossing-over?
1. Pieces of genes are exchanged 2. Allow genetic variation
3. Occurs in Prophase I 4. All of the above
During anaphase what is pulled apart?
1. Centromere 2. Homologous chromosomes
3. Sister chromatids 4. Spindle fibers
Which best describes a haploid cell?
1. n 2. Half the genetic information
3. Gametes 4. All of the above

60. Spermatogenesis
Occurs in the ___________
Two divisions produce 4
__________________
Spermatids mature into
__________________
Men produce about
250,000,000 sperm per day

61. Spermatid

62. Oogenesis
Occurs in the _________________
Two divisions produce:
_______ __________ bodies that die because of
unequal division of cytoplasm
_____ egg- Immature egg called _____________
Starting at puberty, one oocyte matures into an ovum
(egg) every 28 days

64. CHECKPOINT √
How many chromosomes would a sperm or egg contain
if either one resulted from the process of mitosis?
Oogenesis results in______.
1. 4 polar bodies 2. 3 polar bodies
3. 1 ovum 4. Both 2 & 3
Why is the sperm so small compared to the egg?

65. Mitosis Meiosis
Number of divisions
Number of daughter
cells
Genetically
identical?
Chromosome #
Where
When
Role