Chapter 21

Cell Division

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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 ...

ReproductionReproduction((生殖作用生殖作用 ))

which involves the processes of cell divisioncell division

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21.1 How do cells divide?

Cell divisionCell 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?

Genetic information is carried in …

Chromosome

Deoxyribonucleic acidDeoxyribonucleic acid

((去氧核糖核酸去氧核糖核酸 ))

DNADNA

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21.1 How do cells divide?

Chromosome

when highly coilsDNA

molecule

special protein

one chromosome

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one chromosome21.1 How do cells divide?

Chromosome

chromatid (染色單體 )

chromatid

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~ Word corner ~

** Chromo- Coloured

Chromosome

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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

humanshumans

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21.1 How do cells divide?

Chromosome

number of pairs varies with different species

e.g.39 pairs

dogsdogs

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21.1 How do cells divide?

Chromosome

number of pairs varies with different species

e.g.7 pairs

peaspeas

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21.1 How do cells divide?

Chromosome

with 2 sets of chromosomes : diploid (二倍體 ) (2n2n))

with 1 set of chromosomes : haploid (單倍體 ) (nn))

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** Haplo- Single

~ Word corner ~

** Di- Two

Diploid, Haploid

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Mitotic cell division

mitosis(nuclear division)

cytoplasmic division

21.1 How do cells divide?

Mitotic cell division

two identical cells produced same chromosome no. as parent

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• DNA molecules are replicated

MitosisJust before mitosis

cell membrane

nuclear membranes

chromosomes

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MitosisStage 1

Nuclear membranes disintegrate

Chromosomes become visible

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MitosisStage 2

Chromosomes line up at the middle of the cell

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MitosisStage 3

Chromatids separate and move to the opposite ends

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MitosisStage 4

New nuclear membranes are formed

Chromosomes become invisible again

Mitosis is now complete!

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Cytoplasmic divisionIn animal cells

cell membrane constricts at the middle

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Cytoplasmic divisionIn plant cells

new cell walls and cell membranes

form between two nuclei

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Meiotic cell division

meiosis(nuclear division)

cytoplasmic division

Meiotic cell division four identical cells produced

reduce the chromosome no. by half

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consists of two divisions

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1) Chromosomes shorten and thicken

MeiosisFirst meiotic division

cell membrane

nuclear membranes

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2) Homologous chromosomes pair up

MeiosisFirst meiotic division

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Each chromosome consists of two chromatids

MeiosisFirst meiotic division

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3) Nuclear membranes disintegrate

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MeiosisFirst meiotic division

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4) Homologous pairs line up at the middle of the cell

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MeiosisFirst meiotic division

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5) Members of each homologous pairs separate and move to the opposite ends of the cell

Cytoplasm starts to divide

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MeiosisFirst meiotic division

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6) New nuclear membranes are formed

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MeiosisSecond meiotic division

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7) Chromosomes line up again

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MeiosisSecond meiotic division

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8) Chromatids separate

Cytoplasm starts to divide

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MeiosisSecond meiotic division

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9) 4 daughter cells are formed (each has a haploid number of chromosomes)

Nuclear membranes are formed

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1) Growth1) Growth

Significance of cell division

3) As a way of asexual3) As a way of asexual reproduction reproduction

2) 2) Repairing damageRepairing damage2) 2) Repairing damageRepairing damage

Mitotic cell division

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Significance of cell divisionMeiotic cell division

1) Formation of haploid gametes

2n n (in gametes, i.e. sperm and egg)

meiosis

the diploid number of chromosomes can be restored after fertilization

sperm (n) + egg (n) fertilization zygote (2n)

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aA

b B

Significance of cell divisionMeiotic cell division

2) Creation of genetic variations among gametes

Aa

B b

2 pairs of homologous chromosomes

homologous chromosomes arrange randomly at the middle of the cell

aA

B b

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aA

b B

aA

B b

A

B

a

b

A

b

a

B

OR

OR

members of each homologous pair separate

Gametes with different genetic combinations

Independent aIndependent assortmentssortment((獨立分配獨立分配 ))

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Brief RevisionBrief RevisionWhat is the difference between mitosis and meiosis?

Mitosis Meiosis

11 Place of occurrence

22 Pairing of homologous

chromosomes

33 Separation of homologous chromosomes

44 Number of divisions

Body cells

Does not occur

Does not occur

1

Sex organs

Occurs

Occurs

2

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Brief RevisionBrief RevisionWhat is the difference between mitosis and meiosis?

Mitosis Meiosis

11 Place of occurrence

22 Pairing of homologous

chromosomes

33 Separation of homologous chromosomes

2, body cells

Diploid (2n)

Forms cells for growth, repair and

asexual reproduction

4, gametes

Haploid (n)

Forms haploid gametesso that the diploidcondition can berestored in the zygoteafter fertilization

55 Number and types of daughter cells

66 Chromosome number of each daughter cell

77 Significance

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