Cell Division PDF

Cell Division  Cell cycle Total or sum of changes that occur between one mitosis to another mitosis cell division. OR It is all those changes which occur during cell growth and cell division. Eg. bacterial cell has 20 min, epithelial cell has 8-10min etc. It is divide...

Cell Division  Cell cycle Total or sum of changes that occur between one mitosis to another mitosis cell division. OR It is all those changes which occur during cell growth and cell division. Eg. bacterial cell has 20 min, epithelial cell has 8-10min etc. It is divided into: Interphase, Karyokinesis and cytokinesis. 1.Interphase It is longest phase. Also known as preparatory phase. Divided into 3 sub-phases: a. Gap one(G1 phase):Cell size increases. Carbohydrate, lipids ,structural and functional protein are formed. RNAs are formed. b. Synthetic phase(S-phase):DNA replicates. Histone protein are formed.Each chromosome has two chromatid with centromere. c. Gap two(G2 phase):Duplication of cell organelles takes place, cell store energy, protein for spindle fibre are formed. 2.Karyokinesis:Division of nucleus. 3.Cytokinesis:Division of cytoplasm.  Cell Division The process of formation of more than one daughter cells from pre-existing mother cell is called cell division. It occurs by three ways. 1. Amitosis 2. Mitosis 3. Meiosis  Amitosis(Direct cell division) Cell division by simple cleavage of the nucleus and division of the cytoplasm without spindle formation or appearance of chromosomes. Discovered by Robert remak (1841), described by Flemming (1882). Cell division in protozoa, bacteria and cyanobacteria.  Mitosis Mitosis is a process where a single cell divides into two identical daughter cells have same number of chromosomes. Also known as Somatic cell division/Equational/Indirect cell division. Observed by Strasburger (1870) in plant cells and Boveri and Flemming (1879) in animals. It occurs only in Vegetative/Somatic cells. Equal number of chromosomes are formed in daughter cells as in mother cell. Mitosis process is necessary for growth and development of an organism.  It is completed in three stages: 1.Interphase 2.Karyokinesis 3.Cytokinesis 1.Interphase: Preparatory phase Replication of chromosome, formation of proteins etc. Appear in thread like structure called chromatin fibre. 2.Karyokinesis(Karyon:nucleus,kinesis:movement) It is completed in four phases: a. Prophase b. Metaphase c. Anaphase d. Telophase a. Prophase Shortening and thickening of chromosome. Each chromosome has two chromatids joined by common centromere. Chromosome are arranged irregularly in nucleoplasm. Nuclear membrane, nucleolus start to disappear and spindle fibre start to appear. b. Metaphase All chromosomes are arranged at equatorial plane. Formation of spindle fibre is complete and attached to centromere from opposite pole. Chromosomes lies close to each other so form apparent plate called metaphasic plate. c. Anaphase: The centromere of each chromosome divide so chromatid have its own chromosome. Spindle fibre contracts so each chromatid move towards opposite pole. During anaphasic movement centromere leads the path and limb trail behind. So, it attains V,U,L,J,I shaped structure. True chromosome is appeared in anaphase. d. Telophase: Two groups of chromosomes are organized into two nuclei. The chromosomes elongate and overlap each other to form chromatin. Nuclear membrane and nucleolus start to reappear. 3.Cytokinesis (Gr.kytos :hollow, kinesis :movement) Division of cytoplasm to form two daughter cells. Cell organelles are equally distributed in both cells. It takes place by two methods: a. Cell plate method: In plant cell b. Cleavage or constriction method: In animal cell Mitosis cell division  Significance of Mitosis(IMP) Responsible for growth and development of multicellular organism. Mitosis produce genetically identical cells. It maintain number of chromosomes in cells. Repair and regeneration of cells. Regeneration of lost body parts is also possible due to mitosis. Replace old or worn out cells. Wound or injury is healed by repeated mitosis division.  Meiosis Cell division in which a mother cell divide into four daughter cells having half number of chromosomes. Also known as reductional cell division as number of chromosome is reduced to half. Cells in which meiosis takes place are called meiocytes. First studied by Van Benedin (1887).Term was coined by Farmer and Moore in 1905. Meiosis takes place only in reproductive cells for gamete formation.  It is also completed in 3 stages: 1. Interphase 2. Karyokinesis *Meiosis I-Prophase I, Metaphase I, Anaphase I , Telophase I Interkinesis *Meiosis II-Prophase II, Metaphase II, Anaphase II , Telophase II 3. Cytokinesis 1. Interphase: All necessary cell organelles are formed. Chromosomes replicates in S-phase. S-phase of meiosis is longer than mitosis. Long thread like chromatin fibre are present. 2. Karyokinesis: Meiosis I First nuclear division. Homologous chromosomes are separated into two nuclei. Divided into 4 phases: a. Prophase I:It is complicated and longest phase. Has 5 sub phases(IMP): i. Leptotene ii. Zygotene iii. Pachytene iv. Diplotene v. Diakinesis i. Leptotene: Size of nucleus increases. Shortening and thickening of chromosome. Chromosome posses swollen area called chromomeres. Replicated chromosomes appear in single chromatid due to presence of nucleoprotein between two sister chromatids. ii. Zygotene Homologous chromosomes start pairing along their length. Pairing of homologous chromosome is called Synapsis (1 mark). Paired chromosomes are called as bivalents. Bivalents are held together by ribonucleoprotein forming synaptonemal complex. iii. Pachytene: Nucleoprotein between sister chromatids dissolves so it appears as 4 chromatids called tetrad. The exchange of genetic materials between non- sister chromatids of homologous chromosome takes place. The process is called as crossing over. Shortening and thickening of chromosomes takes place. iv. Diplotene: Synaptonemal complex start to dissolve so non- sister chromatids start separating. They start to separate except at the point of attachment called Chaismata. Nuclear membrane and nucleolus start to disappear. v. Diakinesis: Chaismata slide towards end of the chromosome and finally disppear separating non-sister chromatids.The process is called terminalisation. Nuclear membrane and nucleolus finally disppear. Spindle fibre start appearing. b. Metaphase I: Formation of spindle fibre is complete and converse towards opposite end called poles. Bivalents are arranged at equatorial plane in two planes. Metaphasic plates are formed. Centromere attaches with spindle fibre by tractile fibrils. C. Anaphase I: Homologous chromosomes gets separate from one another and move towards opposite poles by the process called disjunction that forms two haploid group of chromosomes. Centromeres donot break so each chromosome bears two chromatids called dyads. They attains different shape like V,U,L,J,I. Centromere lead path and arms trail behind. D. Telophase I Nuclear membrane and nucleolus reappear at two groups of chromosomes on the opposite poles. Chromosomes elongates. Two haploid nuclei are formed from single diploid nucleus (In some cases Telophase is totally absent). Interkinesis: similar to Interphase between Meiosis I & Meiosis II but DNA synthesis donot occur. Meiosis II Number of chromosomes remain same as in meiosis I, similar to mitosis. So, called as Meiotic mitosis or homotypic division. It is also completed in 4 phases: a. Prophase II b. Metaphase II c. Anaphase II d. Telophase II a. Prophase II: Shortening and thickening of chromosomes. Nuclear membrane and nucleolus disappear and spindle fibre appear. b. Metaphase II: Dyad chromosomes arrange themselves in an equatorial plane. Centromere attaches with spindle fibre,with one metaphasic plate c. Anaphase II: Centromere of dyad chromosome break down so two chromatids are separated. They move towards opposite pole. d. Telophase II: Four groups of chromosomes arrange themselves into four haploid nuclei. Reappearance of nuclear membrane and nucleolus. Length of chromosome elongates. 3.Cytokinesis:Formation of four haploid daughter cells by division of cytoplasm. a. Successive method: In this method each karyokinesis is followed by cytokinesis. So, after meiosis I two haploid cells are formed. b. Simultaneous method: In this method, cytokinesis take place once after meiosis II. Four haploid cells are formed at last.  Significance of Meiosis: (IMP) 1. Responsible for gamete formation. 2. Maintain fixed number of chromosome in sexually reproducing organism. 3. Crossing over takes place during meiosis produce new combination of genes. It lead to variation. 4. Meiosis is also important for production of spores in many organisms. 5. Sometimes mutation may occur due to irregularities in meiosis during Anaphase I.  Mitosis Meiosis (IMP) 1.Division of a mother cell into two 1.Division of a mother cell into four daughter cells having same number of daughter cells having half number of chromosomes. chromosomes. 2.It may take place in haploid or diploid 2.It take place only in diploid reproductive somatic cells. cells. 3.Produce genetically identical cells. 3.Produce different cells due to crossing over. 4.Nucleus divide once. 4.Nucleus divide twice. 5.Help in multiplication of cells for growth 5.Involves in production of gametes. and development. 6.Doesnot cause variation. 6.It cause variation.  Thank you!!!