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Ch 8: Cell Division same half Modern cell theory states that

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1 Ch 8: Cell Division same half 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 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 - sister chromatids join at chiasmata where crossing over can occur
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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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

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

39 Differences between Nuclear Division and 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
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
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
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 sperm (haploid) zygote (diploid) an ovum (haploid)
a) Production and fusion of haploid gametes a sperm (haploid) an ovum (haploid) chromosomes fertilization zygote (diploid)

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
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) II-6

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54 3. (a) Distinguish between (i) codon and anticodon, 93-II-1(a)
2. Describe and explain which stage of mitosis is most suitable for determining chromosome number. (2 marks) 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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