2. Chapter 21 Cell Division

3. BackNext Home 2 21.1How do cells divide? All organisms will eventually die...

4. BackNext Home 3 21.1How do cells divide? However, life continues because organisms are able to produce offspring...

5. BackNext Home 4 21.1How do cells divide? The process of producing offspring is called... Reproduction ( 生殖作用 ) cell division which involves the processes of cell division

6. BackNext Home 5 21.1How 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

7. BackNext Home 6 21.1How do cells divide?  Genetic information is carried in … Chromosome Deoxyribonucleic acid ( 去氧核糖核酸 ) DNA

8. BackNext Home 7 21.1How do cells divide? Chromosome when highly coils DNA molecule special protein one chromosome

9. BackNext Home 8 one chromosome 21.1How do cells divide? Chromosome chromatid ( 染色單體 ) chromatid

10. BackNext Home 9 ~ Word corner ~ ** Chromo-  Coloured Chromosome Brief Revision

11. BackNext Home 10 21.1How do cells divide? Chromosome  exist in pairs  homologous chromosome ( 同源染色體 )

12. BackNext Home 11 21.1How do cells divide? Chromosome  number of pairs varies with different species e.g. 23 pairs humans

13. BackNext Home 12 21.1How do cells divide? Chromosome  number of pairs varies with different species e.g. 39 pairs dogs

14. BackNext Home 13 21.1How do cells divide? Chromosome  number of pairs varies with different species e.g. 7 pairs peas

15. BackNext Home 14 21.1How do cells divide? Chromosome 2n)  with 2 sets of chromosomes : diploid ( 二倍體 ) (2n) n)  with 1 set of chromosomes : haploid ( 單倍體 ) (n)

16. BackNext Home 15 ** Haplo-  Single ~ Word corner ~ ** Di-  Two Diploid, Haploid Brief Revision

17. BackNext Home 16 Mitotic cell division mitosis (nuclear division) cytoplasmic division 21.1How do cells divide? Mitotic cell division  two identical cells produced  same chromosome no. as parent Extension

18. BackNext Home 17 DNA molecules are replicated Mitosis Just before mitosis cell membrane nuclear membranes chromosomes Extension

19. BackNext Home 18 Mitosis Stage 1 Nuclear membranes disintegrate Chromosomes become visible Extension

20. BackNext Home 19 Mitosis Stage 2 Chromosomes line up at the middle of the cell Extension

21. BackNext Home 20 Mitosis Stage 3 Chromatids separate and move to the opposite ends Extension

22. BackNext Home 21 Mitosis Stage 4 New nuclear membranes are formed Chromosomes become invisible again Mitosis is now complete! Extension

23. BackNext Home 22 Cytoplasmic division In animal cells cell membrane constricts at the middle Extension

24. BackNext Home 23 Cytoplasmic division In plant cells new cell walls and cell membranes form between two nuclei Extension

25. BackNext Home 24 Meiotic cell division meiosis (nuclear division) cytoplasmic division Meiotic cell division  four identical cells produced  reduce the chromosome no. by half Extension  consists of two divisions

26. BackNext Home 25 1) Chromosomes shorten and thicken Meiosis First meiotic division cell membrane nuclear membranes Extension

27. BackNext Home 26 2) Homologous chromosomes pair up Meiosis First meiotic division Extension

28. BackNext Home 27 Each chromosome consists of two chromatids Meiosis First meiotic division Extension 3) Nuclear membranes disintegrate

29. BackNext Home 28 Meiosis First meiotic division Extension 4) Homologous pairs line up at the middle of the cell

30. BackNext Home 29 Meiosis First meiotic division Extension 5) Members of each homologous pairs separate and move to the opposite ends of the cell Cytoplasm starts to divide

31. BackNext Home 30 Meiosis First meiotic division Extension 6) New nuclear membranes are formed

32. BackNext Home 31 Meiosis Second meiotic division Extension 7) Chromosomes line up again

33. BackNext Home 32 Meiosis Second meiotic division Extension 8) Chromatids separate Cytoplasm starts to divide

34. BackNext Home 33 Meiosis Second meiotic division Extension 9) 4 daughter cells are formed (each has a haploid number of chromosomes) Nuclear membranes are formed

35. BackNext Home 34 1) Growth Significance of cell division 3) As a way of asexual reproduction reproduction 2) Repairing damage Mitotic cell division

36. BackNext Home 35 Significance of cell division Meiotic cell division 1) Formation of haploid gametes 2nn (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)

37. BackNext Home 36 aA bB Significance of cell division Meiotic cell division 2) Creation of genetic variations among gametes A a Bb 2 pairs of homologous chromosomes homologous chromosomes arrange randomly at the middle of the cell aA Bb

38. BackNext Home 37 aA bB aA Bb A B a b A b a B OR members of each homologous pair separate Gametes with different genetic combinations Independent assortment ( 獨立分配 )

39. BackNext Home 38 Brief Revision What is the difference between mitosis and meiosis? MitosisMeiosis 1 1 Place of occurrence 2 2 Pairing of homologous chromosomes 3 3 Separation of homologous chromosomes 4 4 Number of divisions Body cells Does not occur 1 Sex organs Occurs 2 Extension

40. BackNext Home 39 Brief Revision What is the difference between mitosis and meiosis? MitosisMeiosis 1 1 Place of occurrence 2 2 Pairing of homologous chromosomes 3 3 Separation of homologous chromosomes 2, body cells Diploid (2n) Forms cells for growth, repair and asexual reproduction 4, gametes Haploid (n) Forms haploid gametes so that the diploid condition can be restored in the zygote after fertilization 5 5 Number and types of daughter cells 6 6 Chromosome number of each daughter cell 7 7 Significance Extension