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1. LET US REVIEW!

2. The human
body
contains
100 trillion
cells.
There is a
nucleus inside
each human
cell (except red
blood cells).
Each nucleus
contains 46
chromosomes,
arranged in
23 pairs.
One
chromosome
of every pair is
from each
parent.
The
chromosomes
are filled with
tightly coiled
strands of
DNA.
Genes are
segments
of DNA that contain
instructions to
make
proteins— the
building blocks
of life.

3. 1

4. Fertilization 2
Fertilization occurs when the nucleus of a male
reproductive cell combines with the nucleus of a
female reproductive cell
The reproductive cells are called GAMETES
In animals, the male gamete is the
SPERM cell and the female gamete is the
OVUM
When the male and female gametes combine,
the resulting cell is called a ZYGOTE

5. sperms
ovum
nuclei combine cell division (mitosis)
embryo
formed
ANIMAL
3

6. This sperm will
fertilize the ovum
4

7. Gamete Formation

8. 7
Meiosis reduces the number of chromosome sets
from diploid to haploid.

9. STAGES OF MEIOSIS

10. • Meiosis is preceded by the replication of
chromosomes.
• Meiosis takes place in two sets of cell
divisions, called meiosis I and meiosis II
• One replication followed by the two cell
divisions result in four daughter cells.
• Each daughter cell has only half as many
chromosomes as the parent cell.

12. • In the first cell division (meiosis I),
homologous chromosomes (each with two
sister chromatids) separate from each other
and go to daughter cells
• In the second cell division (meiosis II), sister
chromatids separate.
• Each division has prophase, metaphase
anaphase and telophase. The interphase
between Meiosis I and II is usually very short
or even nonexistent.

13. egg
SPERMATOGENESIS a b OOGENESIS
polar
body
spermatogonium
primary
spermatocyte
secondary
spermatocyte
oogonium
primary
oocyte
secondary
oocyte
meiosis l
polar bodies
(will be degraded)
spermatids
meiosis ll

14. • Meiosis I is preceded by interphase, in
which chromosomes are replicated to
form sister chromatids
• The sister chromatids are genetically
identical and joined at the centromere
• The single centrosome replicates,
forming two centrosomes.

15. Prophase I
• Occupies more than 90% of the time required for
meiosis
• Chromosomes begin to condense
• In synapsis, homologous chromosomes loosely pair
up, aligned gene by gene

16. • Each pair of chromosomes forms a tetrad, a
group of four chromatids joined together
• In crossing over or recombination ,
nonsister chromatids exchange DNA
segments
• Each tetrad usually has one or more
chiasmata, X-shaped regions where crossing
over occurred
• Crossing over allows exchange of genetic
information between homologous
chromosomes

17. Tetrad

18. Metaphase I
• In metaphase I, tetrads line up at the
metaphase plate, with one homologous
chromosome facing each pole
• Microtubules attach homologous
chromosomes to opposite spindle poles
• The alignment of homologous chromosomes
is random. There is a 50:50 chance they will
align in any one direction

19. Cartoon
Version
Of
Prophase I
And
Metaphase I
Prophase I Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

20. Anaphase I
• In anaphase I, pairs of homologous
chromosomes separate-synapsis is over.
• Spindle fibers pull the tetrads apart towards
the opposite side of the cell.
• Cytokinesis begins.

21. Telophase I
• In the beginning of telophase I, each half of
the cell has a haploid set of chromosomes;
each chromosome still consists of two
sister chromatids
• Cytokinesis usually occurs simultaneously,
forming 2 haploid daughter cells
• There is no S phase between Meiosis I and
II
• New nuclear and nucleolus form.

22. Cartoon
version of
Anaphase I
and
Telophase I
Anaphase I Telophase I and
Cytokinesis
Sister chromatids
remain attached
Homologous
chromosomes
separate
Cleavage
furrow

23. 2 haploid daughter cell

24. END OF FIRST MEIOTIC
STAGE

25. MEIOSIS II
• No chromosomes
replication between
meiosis I and
meiosis II.
• Similar to the
events of mitosis.

26. Prophase II
• In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each still
composed of two chromatids) move toward the
metaphase plate
Metaphase II
• In metaphase II, the sister chromatids are
arranged at the metaphase plate
• Because of crossing over in meiosis I, the two
sister chromatids of each chromosome are no
longer genetically identical.

27. Cartoon
Version of
Prophase II
and Metaphase
II
Prophase II Metaphase II

28. Anaphase II
• In anaphase II, the sister chromatids separate
• The sister chromatids of each chromosome now
move as two newly individual chromosomes toward
opposite poles
Telophase II and Cytokinesis
•In telophase II, the chromosomes arrive at opposite
poles
•Nuclei form, and the chromosomes begin
decondensing

29. Cartoon
Version of
Anaphase II
and Telophase
II
Anaphase II Telephase II and
Cytokinesis
Sister chromatids
separate Haploid daughter cells
forming

30. Meiosis 6 13
Cell division is completed,
forming four gametes
each with half the number
of chromosomes of the
parent cell gametes

31. 23
46
46
23
23
23
23
23
23
46
sperm
mother
cell
ovum
mother
cell
sperms produced
by meiosis
fertilization
zygote
ova produced by meiosis
but only one develops to
maturity
15

32. Cell division continues by
mitosis, so all the cells will
contain 46 chromosomes early embryo
46 46
46 46
46 46
46 46 46 46
46 46 46 46
46 46
46 46
16

33. Genes 17
gene for brown eyes
Genes for any one characteristic
occupy corresponding positions
on homologous chromosomes
But they do not necessarily control
the characteristic in the same way
For example, one of the gene pair
responsible for eye colour might
determine brown eyes and its
partner determine blue eyes*
gene for
blue eyes
gene for
curly hair
gene for
straight
hair

34. Usually only one of a gene pair will be expressed in an
individual
A person inheriting the gene for brown eyes and the gene
for blue eyes will have brown eyes
The gene for brown eyes is said to be dominant to
the gene for blue eyes. The gene for blue eyes is not
expressed in this individual
The gene for blue eyes is said to be recessive to the
gene for brown eyes
18

35. Gene combinations 19
In the first stage of meiosis, the illustration (slide 10)
showed one ‘red’ and one ’blue’ chromosome
going to each daughter cell
One gamete will receive the
gene combination for brown
eyes and curly hair. The other
will receive the genes for blue
eyes and straight hair B
B
b
C
c
c = gene for straight hair
C = gene for curly hair
b = gene for blue eyes
B = gene for brown eyes

36. It is just as likely that both ‘blue’ chromosomes
will go to one daughter cell and both ‘red’
chromosomes go to the other
B
b
c
C
One gamete will receive the
genes B and c (brown eyes
and straight hair)
The other gamete will receive
genes b and C (blue eyes and
curly hair)
20

37. 21
So, there could be 4 types of gamete with different
combinations of the genes
BC brown eyes, curly hair
bc blue eyes, straight hair
Bc brown eyes, straight hair
bC blue eyes, curly hair

38. Variation 22
Meiosis not only halves the number of
chromosomes but can also rearrange
the genes
This is one cause of the variations that occur
in members of the same species

39. Rearrangement of genes can also take
place at fertilization
A sperm may carry a gene for brown eyes (B)
or a gene for blue eyes (b)
An ovum may carry a gene for brown eyes (B)
or a gene for blue eyes (b)
At fertilization, four possible combinations
can occur
23

40. sperm ovum 4 Possible
BB
Bb
bB
bb
B B
b
fertilization
b
24
combinations
Although there are 4 possible combinations of genes
BB, Bb and bB have the same effect of producing brown eyes
Only bb gives rise to blue eyes

41. Question 1
Which of the following are gametes ?
(a) sperms
(b) dividing cells
(c) ova
(d) nuclei

42. Question 3
What is the correct sequence of events in meiosis ?
(a) (b) (c) (d) (e) (f)
(a) a, b, d, c, e, f
(b) b, a, d, c, e, f
(c) b, d, a, c, e, f
(d) a, b, d, c, e, f

43. Question 4
Which of the following represent variation
within a species ?
(a) black cats and tabby cats
(b) collie dogs and dachshunds
(c) goldfinch and greenfinch
(d) shire horses and race horses

44. Answer
Correct

45. Answer
Incorrect