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
19
(No Transcript)
20
Metaphase I

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

21

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

22

• 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

23

• 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

24

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

25

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

26

• 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

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








29
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
Chromosome number remains the same Chromosome number is halved
Homologous chromosomes do not pair Homologous chromosomes pair to form bivalents in prophase I







30
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
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






31
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
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





32
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
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




33
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
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)



34
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
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


35
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
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

36
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
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

45

• 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

50

• 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

51
(No Transcript)
52
(No Transcript)
53
(No Transcript)
54

• 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