Meiosis is a process of nuclear division that reduces the chromosome number from diploid to haploid. It occurs in two divisions, meiosis I and meiosis II, producing four haploid daughter cells. In meiosis I, homologous chromosomes separate, reducing the number by half. Meiosis II then separates sister chromatids, similar to mitosis, resulting in four haploid cells each with half the original chromosome number. This ensures genetic variation and maintains the diploid number after fertilization.

1. BIOLOGY FORM 4
CHAPTER 5 PART 2
CELL DIVISION - MEIOSIS

2. REFRESHER MITOSIS
Mitosis – HD
Mitosis

3. ????????
• What is the name of the process shown in
the video?
• Where does it occur?
• What is the significance of the process?
• Name the stages.
• What happens during each stage?
• What is the difference between plant and
animal cell?

4. MITOSIS VIDEO

5. MITOSIS & MEIOSIS
ACTIVITY
• Arrange the cards in the correct sequence

6. HOTS : Please explain what has
this got to do with cell division?
Planaria

7. SET INDUCTION
Meiosis+-+The+Continuation+of+Life-SD

8. Meiosis
• What does this
picture tell us
about meiosis?
• How is the
purpose of
meiosis different
from the purpose
of mitosis?

9. Where does meiosis
occur in a male and
a female human?
Inside testes in
a male and
ovaries in a
female.

10. Meiosis in Plant
Occurs in the
• ovary (female)
Ovary
Occurs in the
• anther (male)
Anther

11. Meiosis produces
reproductive cells
(gametes)
Sperm
&
Ovum
Pollen
grains
& Egg
cells
Meiosis is the process of nuclear division
that reduces the number of chromosomes
in new cells to half the number of
chromosomes in the parent cell.

12. Significance of meiosis:
• Reduce chromosome number from
diploid (2n) to haploid (n).
• Promote genetic diversity (variation).
Unique phenotypes may
give a reproductive
advantage to some
organisms.

13. Which process restores the diploid
number of chromosomes?

15. Meiosis reduces the chromosome number;
Fertilisation restores the diploid number.
egg
sperm
meiosis
46
fertilisation
23
23
46
23
46
23

16. Features of Meiosis
Meiosis includes two rounds of division – meiosis I and
meiosis II.
Meiosis I Meiosis II
Prophase I Prophase II
Metaphase I Metaphase II
Anaphase I Anaphase II
Telophase I Telophase II
Short interphase Haploid cells

17. During meiosis I,
homologous chromosomes
separate.
2n n
During meiosis II,
sister chromatids separate.
n n
[just like mitosis]
How is the HAPLOID
condition achieved?
Chromosomes replicate
ONCE but divide TWICE

18. Meiosis I: Reduction Division
Nucleus Spindle
fibers
Nuclear
envelope
Early Prophase I
(Chromosome
number doubled)
Late
Prophase I Metaphase I Anaphase I Telophase I
(diploid)

19. Prophase I
Prophase I may last a long time.
Human males:
 about 1 week for prophase I
 1 month for entire meiotic cycle
Human females:
 prophase I begins before birth, and
ends up to decades later during the
monthly ovarian cycle

20. Prophase I
Interphase Prophase I
(Bivalents)

21. Tetrads Form in Prophase I
(each with sister chromatids)
Homologous
chromosomes join
to form a TETRAD
Synapsis

22. Crossing over: genetic recombination
between non-sister chromatids
Plural (chiasmata)
- sites of crossing
over

23. Crossing-Over
Crossing over produces variation in gametes

24. Prophase I
Early prophase
Homologous
chromosomes pair up
- synapsis.
Crossing over occurs.
Late prophase
Chromosomes short & thick.
Spindle fibre forms.
Nuclear membrane &
nucleolus fragments.
Centrioles move to opposite
poles, aster forms

25. The longer the chromosome, the more
chiasmata that can form

26. PROPHASE I

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

28. METAPHASE I

29. Anaphase I
Homologous pair
saparate and move to
opposite poles.
Sister chromatids remain
attached at their
centromeres.

30. ANAPHASE I

31. Telophase I
Nuclear membrane &
nucleolus reappear.
Spindle fibres disappears.
Cytokinesis divides cell into
two.
2 Daughter cells = n (haploid)

32. TELOPHASE I

33. MEIOSIS I

34. Meiosis I:
 chromosome number
has been cut in half
 end result: 2 haploid
cells
Meiosis II:
 Similar to mitosis
 DNA does not
replicate

35. Meiosis II:
Prophase II
Metaphase II
Anaphase II
Telophase II 4 Identical
haploid
cells
Meiosis II is
similar to mitosis

36. Prophase II
Nuclear membrane &
nucleolus fragments.
Spindle fibres forms.
Centrioles move to
opposite poles.

37. PROPHASE II

38. Metaphase II
Chromosomes align
along equator of cell.

39. METAPHASE II

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

41. ANAPHASE II

42. Telophase II
Nuclear membrane &
nucleolus reappears.
Chromosomes become
long & thin (decondense).
Spindle fibre disappears.
Cytokinesis divides cell
into two.
4 haploid (n) daughter cells

43. TELOPHASE II

44. MEIOSIS II

45. Results of Meiosis
Gametes (egg &
sperm) form
Four haploid cells with
one copy of each
chromosome

47. Centromere remains
intact
Anaphase I is the
reduction division.

48. Meiosis II No
interphase II
(or very short -
no more DNA
replication)
Remember:
Meiosis II is
similar to mitosis
http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/c
hapter3/animation__stages_of_meiosis.ht
ml

49. What phase of meiosis?

50. Telophase I or Telophase II?

51. Which stage?

52. VIDEO
How Meiosis Works

53. Genetic variation is almost infinite as a
result of meiosis

54. HOTS Question:
Name and describe TWO main advantages of
meiosis relative to mitosis. (4)
1) Creates new combinations of genes which
may result in the offspring being able to
survive in a changing environment.
2) Halves the number of chromosomes so that
after fertilisation, the diploid number is
restored.

55. ACTIVITY
• Show the sequence of Meiosis I and
Meiosis II using plasticine on manila card.
• Check your arrangement of
mitosis/meiosis

56. Comparing
Meiosis I and
Meiosis II
Activity

57. Comparing
Mitosis and
Meiosis
Activity

58. VIDEO
2d Difference between mitosis and meiosis

63. The importance of meiotic cell
division
• Ensure the diploid number of chromosomes
is maintained
• Provides genetic variation

64. Appreciating the movement of
chromosomes during mitosis and meiosis
• Asexual reproduction (mitosis) produces
offspring that are identical to the parent
• Sexual reproduction produces genetic
variation
• If meiosis does not occur properly, the
gametes formed will have abnormal number
of chromosomes eg Down Syndrome &
Klinefelter Syndrome (47 chromosomes) and
Turner Syndrome (45 chromosomes).

65. Down Syndrome: A meiotic error
Cause: Nondisjunction during either:
anaphase I
anaphase II

66. anaphase I anaphase II
Homologous
chromosomes fail to
separate
Sister
chromatids fail
to separate

67. Down Syndrome: trisomy 21
• In humans, if both chromosome 21 homologues
go to the same pole, and the resulting egg is
fertilised, it will be trisomic for chromosome 21.
Egg
cell
Sperm
cell
n + 1
n (normal)
Zygote
2n + 1

68. DOWN SYNDROME
KARYOTYPE

69. The chance of having a Down syndrome child
goes up with maternal age

70. Egg is in center of picture
Many cumulus cells from
the ovary are seen around
the egg. This is a low
quality, oocyte from a
woman 41 years old.
Egg is irregularly shaped
and dark.
A "good" egg from a
32 year old woman.
Photos from Advanced Fertility Clinic in Chicago

71. Question:
The diagram below represents the karyotype of
a human.
a) Why does the
structure shown
in Figure 2 have
an X-shaped
form? (2)
Two sister chromatids
are joined at the
centromere.

72. Diagrams A, B and C below show the same stage
in mitosis, meiosis I and meiosis II in a plant cell.
a)Identify the stage shown giving your
reason.
Metaphase – chromosomes aligned at equator.
Question:

73. b) The cell in diagram A
has 20 units of DNA.
How many units of DNA
would there be in a cell
from this plant at the
end of:
Cell A has 8 chromatids =
20 units.
At end of mitosis, 4
chromosomes present.
So = 10 units
ii) meiosis?
2n for Cell A is 4
chromosomes = 10 units.
At end of meiosis, 2
chromosomes present.
So = 5 units
ii) mitosis?

74. Question:
The simplified diagram below shows a nucleus
from a cell of an insect.
a) Draw diagrams to show the two nuclei which
would be formed if the cell would divide by
mitosis. (2)

75. b) Draw diagrams to show three different
nuclei which could be formed if the
cell divided by meiosis. (Assume that
no crossing over has taken place). (3)
Any 3 from:

76. THE END

77. Genetic Variation in meiosis occurs in
three ways:
1. Crossing over: prophase I
2. Random orientation of bivalents:
metaphase I
3. Random orientation of sister
chromatids : metaphase II

78. Crossing over does not create
new alleles, it simply recombines
existing ones in new ways.
Meiosis I
Meiosis II
NEW COMBINATIONS
1. Crossing Over

79. 2. Random orientation of bivalents
No. of different combinations: 2n
2n = 4
n = 2
No. of
combinations:
22 = 4

80. Suggest explanations for the following
observation.
Meiosis, unlike mitosis, is not considered to
form part of a cell cycle. (2)
When cells divide by mitosis, cytokinesis and
interphase follow and then the same phases
are repeated. However, a cell divides by
meiosis only once. There is no repetition of
phases.
Question:

81. 106
Gametogenesis
Oogenesis
or
Spermatogenesis

82. 107
Spermatogenesis
Occurs in the
testes
Two divisions
produce 4
spermatids
Spermatids mature
into sperm
Men produce about
250,000,000
sperm per day

83. 108
Spermatogenesis in the
Testes
Spermatid

84. 109

85. 110
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

86. 111
Oogenesis in the Ovaries

87. 112
Oogenesis
Oogonium
(diploid)
Mitosis
Primary
oocyte
(diploid)
Meiosis I
Secondary
oocyte
(haploid)
Meiosis II
(if fertilization
occurs)
First polar body
may divide
(haploid)
Polar
bodies
die
Ovum (egg)
Second
polar body
(haploid)
a
A
X
X
a
X
A X
a
X
a
X
Mature
egg
A
X
A
X

88. Question:
Read the passage below and insert the appropriate
terms from the table below in the blank spaces
provided. Note that each term in the table may be used
once, more than once or not at all.
Fertilisation Two Crossing-over Four Forty-six
Genetic variation Haploid Homologous Chromatids Eight
Gametes Diploid Twenty-three Chromosomes Nucleus
Meiosis is an important biological process that reduces
the chromosome number by half during the formation of
…………… in animals. During the first meiotic
"reduction division," the chromosomal pairs are
partitioned so that each gamete contains one of each
chromosomal pair that is it becomes ………… .
gametes
haploid

89. When haploid gametes unite during ……………… the
resulting zygote is ………… having received one
chromosome of each pair from each parent.
Meiosis involves ……… successive nuclear divisions
that produce …… haploid daughter cells. The first
division (meiosis I) is the reduction division; the
second division (meiosis II) separates the
……………….. .
Meiosis increases ……………………… in the
population. Each diploid cell undergoing meiosis can
produce 2n different chromosomal combinations,
where n is the haploid number.
fertilisation
diploid
two
four
chromatids
genetic variation