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Chapter 21 Cell Division 21.1 How do cells divide? All organisms will eventually die ... Home Back 2 Next 21.1 How do cells divide? However, life continues because organisms are able to produce offspring ... Home Back 3 Next 21.1 How do cells divide? The process of producing offspring is called ... Reproduction (生殖作用) which involves the processes of cell division Home Back 4 Next 21.1 How 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 Home Back 5 Next 21.1 How do cells divide? Chromosome Genetic information is carried in … Deoxyribonucleic acid (去氧核糖核酸) DNA Home Back 6 Next 21.1 How do cells divide? Chromosome DNA molecule one chromosome when highly coils special protein Home Back 7 Next 21.1 How do cells divide? Chromosome chromatid (染色單體) one chromosome chromatid Home Back 8 Next Brief Revision ~ Word corner ~ Chromosome ** Chromo- Coloured Home Back 9 Next 21.1 How do cells divide? Chromosome exist in pairs homologous chromosome (同源染色體) Home Back 10 Next 21.1 How do cells divide? Chromosome number of pairs varies with different species e.g. 23 pairs humans Home Back 11 Next 21.1 How do cells divide? Chromosome number of pairs varies with different species e.g. 39 pairs dogs Home Back 12 Next 21.1 How do cells divide? Chromosome number of pairs varies with different species e.g. 7 pairs peas Home Back 13 Next 21.1 How do cells divide? Chromosome with 2 sets of chromosomes : diploid (二倍體) (2n) with 1 set of chromosomes : haploid (單倍體) (n) Home Back 14 Next Brief Revision ~ Word corner ~ Diploid, Haploid ** Di Two ** Haplo- Single Home Back 15 Next Extension 21.1 How do cells divide? Mitotic cell division two identical cells produced same chromosome no. as parent Mitotic cell division mitosis (nuclear division) cytoplasmic division Home Back 16 Next Extension Mitosis Just before mitosis • DNA molecules are replicated chromosomes nuclear membranes cell membrane Home Back 17 Next Extension Mitosis Stage 1 Chromosomes become visible Nuclear membranes disintegrate Home Back 18 Next Extension Mitosis Stage 2 Chromosomes line up at the middle of the cell Home Back 19 Next Extension Mitosis Stage 3 Chromatids separate and move to the opposite ends Home Back 20 Next Extension Mitosis Stage 4 New nuclear membranes are formed Chromosomes become invisible again Mitosis is now complete! Home Back 21 Next Extension Cytoplasmic division In animal cells cell membrane constricts at the middle Home Back 22 Next Extension Cytoplasmic division In plant cells new cell walls and cell membranes form between two nuclei Home Back 23 Next Extension Meiotic cell division four identical cells produced reduce the chromosome no. by half consists of two divisions Meiotic cell division meiosis (nuclear division) cytoplasmic division Home Back 24 Next Extension Meiosis First meiotic division 1) Chromosomes shorten and thicken nuclear membranes cell membrane Home Back 25 Next Extension Meiosis First meiotic division 2) Homologous chromosomes pair up Home Back 26 Next Extension Meiosis First meiotic division 3) Nuclear membranes disintegrate Each chromosome consists of two chromatids Home Back 27 Next Extension Meiosis First meiotic division 4) Homologous pairs line up at the middle of the cell Home Back 28 Next Extension Meiosis First meiotic division 5) Members of each homologous pairs separate and move to the opposite ends of the cell Cytoplasm starts to divide Home Back 29 Next Extension Meiosis First meiotic division 6) New nuclear membranes are formed Home Back 30 Next Extension Meiosis Second meiotic division 7) Chromosomes line up again Home Back 31 Next Extension Meiosis Second meiotic division 8) Chromatids separate Cytoplasm starts to divide Home Back 32 Next Extension Meiosis Second meiotic division 9) 4 daughter cells are formed (each has a haploid number of chromosomes) Nuclear membranes are formed Home Back 33 Next Significance of cell division Mitotic cell division 1) Growth 3) As a way of asexual reproduction 2) Repairing damage Home Back 34 Next Significance of cell division Meiotic cell division 1) Formation of haploid gametes 2n meiosis sperm (n) + egg (n) n (in gametes, i.e. sperm and egg) fertilization zygote (2n) the diploid number of chromosomes can be restored after fertilization Home Back 35 Next Significance of cell division Meiotic cell division A a 2) Creation of genetic variations among gametes B b A a b B A a homologous chromosomes arrange B b randomly at the middle of the cell 2 pairs of homologous chromosomes Home Back 36 Next A a B b A a b B Independent assortment (獨立分配) members of each homologous pair separate OR A a B b A a b B OR Gametes with different genetic combinations Home Back 37 Next Extension Brief Revision What is the difference between mitosis and meiosis? 1 Place of occurrence 2 Pairing of homologous chromosomes 3 Separation of homologous chromosomes 4 Number of divisions Mitosis Meiosis Body cells Sex organs Does not occur Occurs Does not occur Occurs 1 2 Home Back 38 Next Extension Brief Revision What is the difference between mitosis and meiosis? 5 Number and types 1 Place of occurrence of daughter cells 2 Pairing of 6 Chromosome homologous number of each chromosomes daughter cell 3 Separation of homologous chromosomes 7 Significance Mitosis Meiosis 2, body cells 4, gametes Diploid (2n) Haploid (n) Forms cells for growth, repair and asexual reproduction Forms haploid gametes so that the diploid condition can be restored in the zygote after fertilization Home Back 39 Next