Ch 8: Cell Division

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Ch 8: Cell Division

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Title: Ch 8: Cell Division

1
Ch 8 Cell Division

Modern cell theory states that
'all new cells are derived from other cells'.
Human body (1014 cells) ?? A ?? B ?? parental
cells ?? ancestral cell
where A zygote B gametes
Mitosis give daughters cells having the ______
number of chromosomes as parent
Meiosis give daughter cells having only ______
number of chromosomes as parent

same
half
2
8.1 Chromosomes

8.1.1 Chromosome Structure
?Chromosome consists of DNA, proteins and RNA
? only visible during cell division
? as invisible chromatin in resting cells.
? each chromosome consists of 2 threads
(chromatids) joined at a point (centromere)
?chromosomes vary in shape and size,
both within and between species

3
8.1.2 Chromosome Number

chromosome number varies from one species to
another but is always the same for normal
individuals of the same species
majority of organisms have 10 to 40 chromosomes
in their cells

4
8.2 Mitosis

cell cycle a regular pattern of events taking
place in a
dividing cell
Interphase nucleus is
mechanically inactive
although chemically very active

Mitosis when the nucleus is
mechanically active

5
Mitosis
6
Mitosis

Interphase or (resting phase) chromosomes are
not visible but DNA content doubles
duplication of organelles takes place

7
Mitosis

Prophase
initially chromosomes occur
as long, thin tangled thread
then shorten thicken to
form two chromatids joined
at the centromere
centrioles migrate to opposite poles of cell,
developing an aster and microtubules to form the
spindle
nucleolus disappears nuclear envelope
disintegrates

not occurring in higher plants

8
Mitosis
9
Mitosis

Metaphase
Chromosomes arrange themselves
at equator of spindle, become
attached to spindle fibres at
centromeres

10
Mitosis

Anaphase shortening of spindle
fibres causes sister chromatids to
separate and move to opposite poles,
using energy from mitochondria

11
Mitosis

Telophase
Chromatids reach their
respective poles
new nuclear envelope
forms around each group
chromatids uncoil,
lengthen and become
invisible again
spindle fibres disintegrate
and nucleoli form in each
new nucleus

12
Mitosis
13
8.2.1 Differences between Mitosis in Plants and
Animal Cells

1. No centrioles thus no aster in higher plants
2. Animal cells cell division by constriction of
centre of
parent cell
Plant cells cell wall formation across
equator of
parent cell
3. Most animal cells are capable of mitosis
Plant cells only carry out mitosis in
meristematic cells

14
8.3 Meiosis (Reduction Division)
15
8.3 Meiosis (Reduction Division)

One division of the chromosomes followed by
two divisions of nucleus and cell
A diploid (2n) parent cell give rise to 4 haploid
(n)
daughter cells
Forms gametes (sperms and ova) or spores in some
plants

1st meiotic division similar to mitosis except
for a highly modified prophase
2nd meiotic division a typically mitotic
division

16
Meiosis

Prophase I
Chromosomes become visible, shorten and fatten

17
Meiosis

Prophase I
Chromosomes become visible, shorten and fatten

Homologous chromosomes pair together (synapsis)
to
form a bivalent

18
Prophase I - chromosomes become visible, shorten
and fatten - homologous chromosomes pair together
(synapsis) to form a bivalent

- sister chromatids join at chiasmata where
crossing over can occur
- nucleolus disappear,
nuclear membrane
breaks down
- centrioles migrate to poles to form a spindle

- nucleolus disappear, nuclear membrane breaks
down - centrioles migrate to poles to form a
spindle
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Metaphase I

Bivalents arrange
themselves on equator
with each of a pair of
homologous chromosomes
orientated to opposite
poles randomly

Anaphase I
Spindle fibres attaching to the centromeres
contract and
pull sister chromosome of homologous chromosomes
to
opposite poles

Telophase I
Chromosomes reach opposite poles
Nuclear envelopes form
Spindle fibres disappear
Chromatids uncoil
Cell divides into two
Nucleus may enter into interphase
but no DNA replication OR
Cell enters prophase II directly

Prophase II
Nucleolus disappears
nuclear envelope breaks down
Centrioles divide and move to
opposite poles
Spindle develops at right angle
to spindle of 1st meiotic
division

Metaphase II
Chromosomes arrange
themselves on equator of
spindle with spindle fibres
attached to centromeres of
each chromosome

Anaphase II
- centromeres divide and pulled by spindle fibres
to opposite poles,
carrying the chromatids with them

Telophase II
- Upon reaching poles, chromatids unwind
nuclear envelope and nucleolus reform
- Spindle disappears, 2 cells divide to give
4 cells (a tetrad)

27
8.4 Comparison of Mitosis and Meiosis

Table 8.2 Differences between mitosis and meiosis

Mitosis Meiosis
A single division of chromosomes nucleus A single division of chromosomes but a double division of the nucleus

28
8.4 Comparison of Mitosis and Meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Table 8.2 Differences between mitosis and meiosis

Mitosis Meiosis
A single division of chromosomes nucleus A single division of chromosomes but a double division of the nucleus
Chromosome number remains the same Chromosome number is halved
Homologous chromosomes do not pair Homologous chromosomes pair to form bivalents in prophase I
Chiasmata do not form Chiasmata form
No crossing over Crossing over occurs
Daughter cells are identical as parents Daughter cells are genetically different from parents
2 daughter cells are formed 4 daughter cells are formed (in females, only 1 is functional)
Chromosomes shorten thicken Chromosomes coiled but longer
Chromosomes form a single row at equator Chromosomes form double rows at equator during metaphase I
Chromatids move to opposite poles Chromosomes move to opposite poles during 1st meiosis
37
Differences between Nuclear Division and Cell
Division
Nuclear Division Cell Division

38
Differences between Nuclear Division and Cell
Division
Nuclear Division Cell Division
similar in plants and animals different in plants and animals

39
Differences between Nuclear Division and Cell
Division
Nuclear Division Cell Division
similar in plants and animals different in plants and animals
involves duplication of chromosomes involves duplication of organelles

40
Differences between Nuclear Division and Cell
Division
Nuclear Division Cell Division
similar in plants and animals different in plants and animals
involves duplication of chromosomes involves duplication of organelles
daughter nuclei may be similar (mitosis) or dissimilar (meiosis) daughter cells are always similar

41
Differences between Nuclear Division and Cell
Division
Nuclear Division Cell Division
similar in plants and animals different in plants and animals
involves duplication of chromosomes involves duplication of organelles
daughter nuclei may be similar (mitosis) or dissimilar (meiosis) daughter cells are always similar
spindle formation occurs no spindle formation

42
Differences between Nuclear Division and Cell
Division
Nuclear Division Cell Division
similar in plants and animals different in plants and animals
involves duplication of chromosomes involves duplication of organelles
daughter nuclei may be similar (mitosis) or dissimilar (meiosis) daughter cells are always similar
spindle formation occurs no spindle formation
often, but not always followed by cell division always preceded by nuclear division
43
8.5 The Significance of Cell Division

8.5.1 Significance of Mitosis

Mitosis produce daughter cells which are exact
copies of the parental cell for
a) Growth
b) Repair
c) Asexual reproduction

44
8.5.2 Significance of Meiosis

The long-term survival of a species depends on
its ability to adapt to a constantly changing
environment
and to colonize new environments

Therefore it is necessary for offspring to be
different from their parents as well as different
from each other.
There are 3 ways variation is brought about
a) Production and fusion of haploid gametes
b) Random distribution of chromosomes during
metaphase I and consequent independent
assortment
c) Crossing over between homologous chromosomes

a) Production and fusion of haploid gametes

46
b) Random distribution of chromosomes during
metaphase I and consequent independent
assortment
47
c) Crossing over between homologous chromosomes
48

MEIOSIS ??? VARIATION
??? EVOLUTION of the fittest !

49
Cancer - a breakdown of control of cell division

In the absence of control, the tissue grows,
invading and crowding out other tissues,
sometimes destroy them. Other causes of cancer
1. mutation by chemicals or radiations
2. viral infections

1. Compare and contrast the process of mitosis
and meiosis. Discuss the roles and significance
of mitosis and meiosis in the life of flowering
plants and mammals, illustrating your answer with
examples where appropriate. (20 marks)
96-II-6

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2. Describe and explain which stage of mitosis is
most suitable for determining chromosome
number. (2 marks) 94-I-4

3. (a) Distinguish between (i) codon and
anticodon, 93-II-1(a)
(ii) continuous and discontinuous variation
(4 marks)
e.g. tongue rolling, blood groups

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