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Chapter 21
Cell Division
21.1 How do cells divide?
All organisms will eventually die ...
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21.1 How do cells divide?
However, life continues because organisms are
able to produce offspring ...
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21.1 How do cells divide?
The process of producing offspring is called ...
Reproduction
(生殖作用)
which involves the processes of cell division
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21.1 How do cells divide?
Cell division
Mitotic cell
division
(有絲細胞分裂)
Meiotic cell
division
(減數細胞分裂)
… both involves replication of genetic material
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21.1 How do cells divide?
Chromosome
 Genetic information is carried in …
Deoxyribonucleic acid
(去氧核糖核酸)
DNA
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21.1 How do cells divide?
Chromosome
DNA
molecule
one chromosome
when highly coils
special
protein
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21.1 How do cells divide?
Chromosome
chromatid
(染色單體)
one chromosome
chromatid
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Brief Revision
~ Word corner ~
Chromosome
** Chromo-  Coloured
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21.1 How do cells divide?
Chromosome
 exist in pairs
 homologous
chromosome
(同源染色體)
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21.1 How do cells divide?
Chromosome
 number of pairs varies with different species
e.g.
23 pairs
humans
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21.1 How do cells divide?
Chromosome
 number of pairs varies with different species
e.g.
39 pairs
dogs
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21.1 How do cells divide?
Chromosome
 number of pairs varies with different species
e.g.
7 pairs
peas
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21.1 How do cells divide?
Chromosome
 with 2 sets of chromosomes : diploid (二倍體)
(2n)
 with 1 set of chromosomes : haploid (單倍體)
(n)
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Brief Revision
~ Word corner ~
Diploid, Haploid
** Di Two
** Haplo-  Single
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Extension
21.1 How do cells divide?
Mitotic cell division
 two identical cells produced
 same chromosome no. as parent
Mitotic cell division
mitosis
(nuclear division)
cytoplasmic
division
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Extension
Mitosis
Just before mitosis
• DNA molecules are
replicated
chromosomes
nuclear membranes
cell membrane
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Extension
Mitosis
Stage 1
Chromosomes
become visible
Nuclear membranes
disintegrate
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Extension
Mitosis
Stage 2
Chromosomes line up
at the middle of the cell
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Extension
Mitosis
Stage 3
Chromatids separate
and move to the
opposite ends
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Extension
Mitosis
Stage 4
New nuclear
membranes are formed
Chromosomes become
invisible again
Mitosis is now complete!
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Extension
Cytoplasmic division
In animal cells
cell membrane
constricts at the
middle
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Extension
Cytoplasmic division
In plant cells
new cell walls and
cell membranes
form between two
nuclei
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Extension
Meiotic cell division
 four identical cells produced
 reduce the chromosome no. by half
 consists of two divisions
Meiotic cell division
meiosis
(nuclear division)
cytoplasmic
division
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Extension
Meiosis
First meiotic division
1) Chromosomes
shorten and thicken
nuclear membranes
cell membrane
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Extension
Meiosis
First meiotic division
2) Homologous
chromosomes pair up
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Extension
Meiosis
First meiotic division
3) Nuclear membranes
disintegrate
Each chromosome
consists of two
chromatids
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Extension
Meiosis
First meiotic division
4) Homologous pairs
line up at the middle
of the cell
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Extension
Meiosis
First meiotic division
5) Members of each
homologous pairs
separate and move
to the opposite ends
of the cell
Cytoplasm starts to
divide
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Extension
Meiosis
First meiotic division
6) New nuclear membranes
are formed
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Extension
Meiosis
Second meiotic division
7) Chromosomes line
up again
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Extension
Meiosis
Second meiotic division
8) Chromatids separate
Cytoplasm
starts to
divide
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Extension
Meiosis
Second meiotic division
9) 4 daughter cells are
formed (each has a
haploid number of
chromosomes)
Nuclear membranes
are formed
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Significance of cell division
Mitotic cell division
1) Growth
3) As a way of asexual
reproduction
2) Repairing damage
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Significance of cell division
Meiotic cell division
1) Formation of haploid gametes
2n
meiosis
sperm (n) + egg (n)
n
(in gametes, i.e.
sperm and egg)
fertilization
zygote (2n)
 the diploid number of chromosomes can
be restored after fertilization
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Significance of cell division
Meiotic cell division
A
a
2) Creation of genetic
variations among gametes
B
b
A
a
b
B
A
a
homologous
chromosomes arrange
B
b
randomly at the
middle of the cell
2 pairs of homologous
chromosomes
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A
a
B
b
A
a
b
B
Independent
assortment
(獨立分配)
members of
each
homologous
pair
separate
OR
A
a
B
b
A
a
b
B
OR
Gametes with different
genetic combinations
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Extension
Brief Revision
What is the difference between mitosis and
meiosis?
1 Place of occurrence
2 Pairing of
homologous
chromosomes
3 Separation of
homologous
chromosomes
4 Number of divisions
Mitosis
Meiosis
Body cells
Sex organs
Does not occur
Occurs
Does not occur
Occurs
1
2
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Extension
Brief Revision
What is the difference between mitosis and
meiosis?
5
Number
and types
1 Place
of occurrence
of daughter cells
2 Pairing of
6 Chromosome
homologous
number
of each
chromosomes
daughter cell
3 Separation of
homologous
chromosomes
7 Significance
Mitosis
Meiosis
2, body cells
4, gametes
Diploid (2n)
Haploid (n)
Forms cells for
growth, repair and
asexual reproduction
Forms haploid gametes
so that the diploid
condition can be
restored in the zygote
after fertilization
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