Cell Cycle and Cell Division Notes

Cell Cycle and Cell Division INTRODUCTION * CELL CYCLE INVOLVES TWO STAGES : * Rudolf Virchow stated that new cell develops from (1) Interphase or I-phase or Intermitosis or the division of pre-existing cell. Nondividing phase or Energy phase. Van Beneden and Flemming discovered mitosis in animal cell. The term ‘Mitosis’ coined by Flemming. * The term meiosis coined by Farmer and moore. * Some substances stimulate cell division these are called mitogen - Eg: Cytokinins, Epidermal growth Factor or EGF, Platelet Derived Growth Factor or PDGF, Lymphokines. * Some substances inhibit cell division these are called mitotic poison Eg: Cyanides, Azides, Chalones, Colchicine. CELL CYCLE (HOWARD & PELC 1953) : * It involves programmed cyclic changes in the cell that leads to change in cellular components and ultimately cell divides to form two cells. This process is genetically controlled. The duration between two cell cycles is generation time. It is longest phase. It is differentiated into three stages. 95 (A) G1-phase or first gap phase or first Growth phase or pre synthetic phase or post mitotic phase * Its duration is 42% of total time of cell division. Cell Cycle and Cell Division (2) M-Phase (Mitotic phase) : Its duration is 4% of total time. It divides into two stages (A) Karyokinesis (B) Cytokinesis * In this phase some changes occur like synthesis of RNA, proteins, Enzymes for DNA synthesis, Amino MITOSIS : (A) Karyokinesis (Schneider 1887) : acids for Histone formation. It can be divided into four stages for sake of * Some time, G1-phase arrest at the middle and cell convinience undergoes differentiation it is called G0 phase. The Deciding factors are energy producing (I) Prophase (II) Metaphase substances and mitogens this point is called chack (III) Anaphase (IV) Telophase point. (I) Prophase : It is longest phase. Its duration is 2·1% * Once a cell is passed from check point this stage is of total time. It involves three phases. called Antephase (Bullough) Now cell will divide (a) Early prophase : even in unfavourable conditions. Chromatin undergoes dehydration to form long thin * Size of cell and Nucleus becomes doubled in G1 & coiled chromosomes that are called spiralization phase. and their ends are indistinct they look like a boll of (B) S-phase (Synthetic phase) : wool this stage is called spireme stage. It is invisible phase of Mitosis. Its duration is 33% Centrosomes move away to each other and move of total time. Replication of DNA of chromosome towards opposite poles. takes place in S-phase Histone protein synthesizes in this phase. Replication of centrioles (centrosome) starts in S- phase. (C) G2-phase (Second Gap phase or second Growth phase or postsynthetic phase or pre-mitotic phase) : * * Synthesis of RNA and proteins takes place. Replication of centrioles (centrosome) completed in G2 - phase. Tubulin protein is formed. Its duration is 21% of total time. Cyclin protein : Cyclin dependent protein Kinases (CDK) regulate the cell cycle. (b) Mid prophase : Chromosomes are thickened and shifted in the peripheral part of nucleus. Formation of Astral rays starts. Nuclear membrane starts breakdown. Nucleolus reduces in size. 96 Cell Cycle and Cell Division (c) Late prophase : Chromosome are comparatively thickened. Centrosomes are reached to the opposite poles. In this phase nuclear membrane is disappeared. Nucleolus & ER are also disappeared. Aster is formed at each pole by a pair of centrioles and astral rays. Centrioles start formation of spindle fibres. Centrosome helps in the initiation of cell division in animal cells by forming spindle. In plant cells centrosome is absent. The spindle fibre arise (III) Anaphase : from tubulin protein by gelation in the cytoplasm thus * Its duration is 0·3%. Anaphase promoting complex in plant cell centriole and asters have no role in the or APC becomes activated. It causes equatorial formation of spindle apparatus. The middle broad division in each chromosome as a result the no. of part of spindle apparatus is called equator. chromosomes becomes doubled. (II) Metaphase : * Now half chromosomes move towards one pole and remaining half chromosomes move towards opposite pole by the constriction of chromosomal fibre. It is called Anaphasic movement. * Its duration is 0·4%. Chromosomes are thick, shorten and distinct. * The shape, size and structure of chromosomes * can be studied in the metaphase stage. * The tightening of chromosomal fibres brings * 20–25 ATP are consumed by a chromosome during chromosomes at the equator. This process is called anaphasic movement. congression or metakinesis. * Each chromosome has single chromatid & it is The centromeres of all the chromosomes arrange in V-shaped / L-shaped / J-shaped / I-shaped. linear sequence at the equator to form single metaphase plate. (IV) Telophase : * * Kinetochore of chromosomes connect chromosomes to the spindle fibre. Each chromosomes has two distinct chromatids. The arms of each chromosome lie in different direction and the centromere lie on equator. At this time centromere of each chromosome lies towards poles and its arms behind it. It is reverse of prophase. Chromosomes are reached on the opposite poles and now they become uncoiled and converted into long thin chromatin network. Nuclear membranes is reappeared NOR forms Nucleolus. Thus each pole has a nucleus in telophase stage. Spindle fibres are disappeared. 97 Cell Cycle and Cell Division MEIOSIS : It is essential for sexual reproduction. It involves meiosis-I and meiosis-II. Interphase involves - G1, S, G2 Phase. Out of them G2 phase is short. Centrosome replicates in this stage. Meiosis-I : M-phase-It involves karyokinesis & cytokinesis. Karyokinesis : It involves prophase-I, MetaphaseI, Anaphase-I, Telophase-I. (A) Prophase-I : It is longest phase. It can be differentiated into five stages. (B) Cytokinesis (Whitemen) : Laptotene : Chromatin is condensed to form It is a division of cytoplasm. It starts in late anaphase chromosomes. All the chromosomes converge & Completed at the end of telophase. towards a common point near centrosome and form Cytokinesis comprises two types. basket like arrangement this stage is called boquet stage. (i) Cell furrow or cell cleavage method : The chromatids of chromosomes are not distinct due A viscous fibrous dense Mid body is formed in the to nucleoprotein. centre. Constriction of peripheral microfilaments takes place toward centre (Centripetal) and finally both sides meet and two daughter cells are formed Ex: Animal cell. (ii) Cell plate method : In case of plant cell, some spindle fibres exist that form phragmoplast. Golgi vesicles and some ER vesicles deposite in the central part of phragmoplast and their deposition proceeds towards peripheral part (centrifugal) and finally a cell plate is formed followed by the formation of middle lamella. Now Primary wall deposites at each side of middle lamella and ultimately both cells get seperated to form two daugther plant cells. Zygotene : Chromosomes are comparatively small & thick. Homologous chromosomes perform pairing. One homolgous chromosome is maternal and second is paternal. This pairing of two homolgous chromosomes is called synapsis / Syndesis (Mont Gomery 1901). It forms Bivalent. 98 Cell Cycle and Cell Division Diakinesis : Separation of Homologous chromosomes proceeds towards terminal part (terminalization) by the shifting of chiasmata towards chromsomal ends. Centrioles are reached on opposite poles and form aster with astral rays spindle is formed that is amphiastral in animal cell and anastral in plant cell. Pachytene : In this stage Recombination nodule (Zinkler-1977) is formed between two nonsister chromatids. The exchange of segments of nonsister chromatids between two homolgous chromosomes of a Bivalent is called crossing over (Morgan). Recombinase enzyme catalyzes this process.Syneptinemal complex starts dissolving. (B) Metaphase-I : Bivalents arrange in two raws on equator to form Double metaphasic plate. The arms of homologous chromosomes lies parallel to the equator and their centromeres are projected outwardly towards poles. Diplotene : Syneptonemal complex is completely dissolved and chromatids of bivalents are distinct. Each Bivalent has four chromatids and two centromeres. Each chromosome in a bivalent is connected to the spindle pole of its side by a single kinetochore microtubule (chromosomal tractile fibre) instead of two as in metaphase of mitosis. Homologous chromosomes of each Bivalent start (C) Anaphase-I : seperation. They are connected at certain points these In this stage reduction division takes place & are called chiasmata (janssen 1909) and the segregation of Mendelian factors occurs. Bivalents are now called tetrad. Chiasmata are the The chromosome no. becomes half (2n–n). Maternal results of Crossing over. Nuclear membrane starts and paternal chromosomes of each bivalent are disintigration. Nucleolus decreases in size. completely seperated it is called disjunction. They move in opposite direction towards opposite poles due to constriction of chromosomal fibres. It is called anaphasic movement. Thus Bivalents are converted in univalents. Each univalent has divergent two chromatids. At the end of this phase chromosomes are reached at opposite poles. 99 Cell Cycle and Cell Division (D) Telophase-I : Regrouping of Dyad chromosomes occur at each pole Chromosomes undergo despirilization, become thin & elongated. Nucleolus is rarely formed. Nuclear membrane reappears and each pole has a haploid nucleus. Cytokinesis may or may not occur after meiosis-I At the end of meiosis-II four haploid cells are formed that are genetically different. Interkinesis : Significance of meiosis : It is called intrameiotic interphase. In this phase some (a) Variations : deficient substance form but synthesis of DNA does (i) Independent assortment of chromosomes not occur. (ii) Crossing over Meiosis-II : It is also called homotypic or Homoeotypic or equational division. It is just like mitosis but occurs in halpoid nuclei. (iii) Irregular disjunction (iv) Gene mutation during replication & nicking for crossing over. (b) Polyploidy : Failure of chromosomes to separate during anaphase-I leads to polyploidy. (c) Maintenance of chromosome number (d) Sexual reproduction 100 Cell Cycle and Cell Division S.No. 1. 2. 3. 4. 5. 6. Mitosis takes place in the somatic cells Mitosis Meiosis Mitosis takes place in the somatic cells. It occurs either in the reproductive cells or at the time of germination of zygote or zygospore. The cells undergoing mitosis may be The cells undergoing meiosis are always haploid or diploid. diploid. It is a single division which produces two Meiosis is a double division. It gives rise to cells. four cells. Subsequent mitotic divisions are similar to The two divisions of meiosis are not one is the earlier ones. similar. The first heterotypic or reductional while the second one is homotypic or equational like mitosis. Each chromosome replicates in the The chromosomes replicate only once, interphase before every division. prior to meiosis. The number of chromosomes remains the The number of chromosomes is reduced to same after mitosis. one half after mitosis. Prophase Prophase is of shorter duration. 7. 8. 9. 10. 11. 12. 13. Prophase I is of longer duration while prophase II is very brief. Each chromosome has two distinct Chromosomes of prophase I do not show chromatids. distinct chromatids. No bouquet stage is recorded. Chromosomes of animals and some plants show con vergence towards one side during early prophase I. It is known as bouquet stage. Pairing of chromosomes does not occur in Pairing or synapsis of homologous mitosis chromosomes takes place during zygotene of prophase I and continues upto metaphase I. A synaptinemal complex is absent. Synapsed homologous chromosome develop a synaptinemal complex. Crossing over is absent. Crossing over or exchange of similar segments between nonsister chromatids of homologous chromosomes usually takes place during pachytene stage. Chiasmata are absent. Chiasmata or visible connections between homologous chromosomes of bivalents are observed during diplotene, diakinesis 101 Cell Cycle and Cell Division M etaphas e 14. Centrom eres produce a single m etaphasic plate. 15. 16. 17. O nly the centromeres lie at the equator. The limbs o f chrom osom es are oriented in various direct ions. A centrom ere is connected w ith both the spindle poles. Tw o chrom atids of a chromosom e are genetically sim ilar. A n aphase A centromere splits length -w ise to form 18. tw o centrome res in the beginning of anaphase. 19. A naphasic chrom osom es are single stranded. T el ophase 20 Telophase is longer and produces interphase nucl ei. C ytoki nesis 21. Cytokinesis follow s ever y m itosis. It produces two cells. A double m et aphasic plate is form ed by cen trom eres in metaphase I but only one in metaphase II. Lim bs of the chrom osom es m ostly lie at the equator w hil e the centromeres project towards the poles in m etaphase I. A centrom ere is connected to on e spindle pole in m etaphase I but both in m etaphase II. The tw o chrom atids of a chromosom e are often genetical ly dissimilar due to crossing over. Centrom eres do not divide during anaphase I but do s o in anaphase II. Chrom osom es are d ouble stranded in anaphase I but single stranded in anaphase II. Telophase I is shorte r and nuclei never enter t he interphase. Cytokinesis often does not occur after the first or reductio nal division. It is then sim ultaneous after second division to result in four new cells. 102