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Cell Divison Introduction 1 CELL DIVISON INTRODUCTION 1. In unicellular organism like bacteria, W.Flemming at first studied mitotic division protozoans, asexual reproduction takes place in Salamander. by Strasburger discovered meiosis. (A) binary fission Strasburger gave the form prophase, (B) mitosis metaphase, anaphase, telophase. (C) meiosis Term meiosis was given by Farmer and (D) sporic meiosis Moore. Ans. (A) It is important for development, regeneration Solution. In unicellular organisms like bacteria, and reproduction. protozoans, asexual reproduction takes Hormone cytokinin increases rate of cell place by binary fission division. 2. In amitosis which of the following is true TYPES OF DIVISION (A) no differentiation of chromosomes and Amitosis spindle Mitosis (B) chromosomes are equally distributed Meiosis (C) nuclear envelope does not degenerate & Amitosis (No spindle, No chromosome) remains intact It is the simplest mode of cell division at (D) distinct chromosomes do not appear in first described by Remak (1841). division This type of division starts with elongation of Ans. (A) nucleus. Solution. In amitosis no differentiation of Nucleus becomes dumbbell shaped, and gets chromosomes and spindle takes place, so it divided into two daughter nucleus. is a direct division Nuclear division is followed by the division of EDU CATALYSERS cytoplasm and results in the formation of 3. The division of mitochondria and chloroplasts two daughter cells. occurs in In this division, no spindle formation and no (A)mitosis distinct chromosome formation occurs. (B)meiosis Nuclear envelope remains intact. The (C)amitosis daughter cells are approximately (D)M phase the two equal halves of a parental cell. E.g. PPLO, Blue-green algae, Bacteria and 4. In mammals, the growth of foetal membranes Eukaryotic cells. Examples are (amnion, chorion, allantois, yolk sac), yeast-budding occurs by amitosis. cartilage of mammals takes place by Amoeba multiple fission occurs by amitosis. (A) mitosis Paramecium division of meganucleus. (B) meiosis Mammals-growth of foetal membranes (C) M phase (amnion, chorion, allantois, yolk sac) (D) amitosis Division of mitochondria and chloroplasts. MITOSIS Mitosis was discovered by Flemming in 1879 in animal cell and in plants cells by Strasburger in 1875. Mitosis is a cell division in which parent cell divides to form two daughter cells, in which number of chromosome, amount of DNA, number and types of gene are equal to EXAMPLES parent cell. It occurs in somatic cell (n, 2n, polyploid). It is called indirect division. 1 Mitosis results in increase in the number of Enzymes for DNA synthesis and amino acids cells in the body. for histone formation are CELL CYCLE formed. Nucleolus produces rRNA, mRNA and tRNA. Cell division can be divided into two phases. Metabolic rate of the cell becomes very high. It may be called pre DNA synthesis phase. Interphase Division phase or M phase [Resting Phase] [Mitotic Phase] S-PHASE(SYNTHESIS PHASE) Invisible phase of Mitosis. Replication of DNA takes place. (DNA gets doubled) Protein molecules called histones are synthesized that cover each strand of DNA. INTERPHASE OF CELL CYCLE Centrosome (Centriole) replicate is late Interphase is the phase between two cell S-phase in cytoplasm divisions, divided into three sub stages G1, S G2-PHASE and G . 2 [Step-II Phase or Pre Mitotic Phase] In this phase, cell prepares itself for division Tubulin protein synthesis starts for spindle and hence called as preparatory phase. formation. It is the longest phase of cell cycle. It gets This phase may be called post DNA synthesis completed approximately in 19-22 hours. phase. It is also called resting phase (previously) Cell division involves enormous expenditure In this phase, cell remains metabolically of energy, thus cell stores ATP in G phase extremely active. 2 In this phase synthesis of protein, enzyme, Cell organelles like golgibodies, chloroplast DNA and RNA takes place and mitochondria are formed. After G phase cell enters the division or M- Centrosome (Centriole) duplicates into two. 2 Thus two centrosomes (4centriole) are phase formed. EXAMPLES 1. The time period required for interphase is Cell cycle can remain arrested only in G1 phase. Then G is called as G phase. about 1 0 G phase is found in the cells of permanent (A)60% (B)95% 0 tissue. (C)30% (D)50% Cell cycle runs by a group of special proteins Ans. (B) “Cyclins and Cdks (MPF). Solution. Cell cycle has two phases, Interphase THREE SUB STAGES OF INTERPHASE and M-phase in which interphase takes more G1 PHASE than 95% of time and M-phase takes less Step-1 Phase or Post Mitotic Phase]- Most than 5%. variable phase with respect to duration. 2. Interphase is divided into three sub phases Cell growth occurs in this phase, so most except– probably it is longest stage of interphase. (A)G1 phase (B)S phase Most of the cell organelles duplicate. (C)G2 phase (D)G0 phase Structural and functional proteins are Ans. (D) formed. Solution. Interphase is divided into three sub phases i.e G1 phase, S-Phase and G2 Phase. 2 G0 phase is not a part of interphase They are activated when they are combined EDU CATALYSERS with a key protein called cyclin. 3. Decision of cell division occurs during ….. Kinase is an enzyme that removes a phase of cell cycle. phosphate group from ATP & add it to another protein. (A) G1 (B) G2 At some check points a kinase enzyme (C) S phase (D)G0 combines with cyclin & this moves the cell 4. All of the following takes place in S phase cycle forward. except S-kinase is capable of starting the (A)Amount of DNA unchanged replication of DNA after it combines with S- (B)Amount of DNA becomes double in cyclin (G - Cyclin). After some time S-cyclin 1 nucleus is destroyed &S-kinase is no longer active. (C)Centriole duplicates in the cytoplasm M-kinase is capable of turning on mitosis (D)Histone proteins are synthesized after binding with M-cyclin, (G -cyclin). CAUSE OF MITOSIS 2 The detail of cell cycle varied from organism Kern plasm theory : Hertwig, says that to organism & different time in an organism. mitosis occurs due to disturbance in However certain characteristics are universal karyoplasmic index (KI) of cell. component of cell cycle control. V KI n Division Phase VVC n Here Vn = Volume of nucleus V = Volume of cell c V – V = Volume of cytoplasm c n Karyokinesis Cytokinesis 1 KI (Nuclear division) (Cytoplasmic division) Volume of cytoplasm MITOSIS CELL DIVISION CONTROL (A) Karyokinesis A cell reproduces by performing an orderly It can be divided into four stages for the set sequences of irreversible events in which sake of convinience it duplicates its contents & then divides into (I) Prophase (II) Metaphase two. These events are known as cell cycle. (III) Anaphase (IV) Telophase Mitosis promoting factor M-Cdk Prophase : It is the longest phase. Its duration is 2·1% of the total time. It involves Degenerating M-cyclin three phases. M-cyclin (A) Early prophase : M G2 Cdk Cdk G 2 Chromatin undergoes dehydration to G S 1 S form long thin & coiled chromosomes that is called spiralization and their Degenerating S-cyclin ends are indistinct. They look like a ball S-cyclin S-Cdk of wool. This stage is called spireme DNA replication promoting factor stage. Molecular biologists identify the Centrosomes move away from each biomolecules that control or drive the cell other and move towards opposite poles. cycle. Many biologists, worked with invertebrate or frog egg’s and others, with yeast cell or cell culture. Scientists concluded that the activity of enzymes, known as cyclin dependant kinases (Cdk’s) regulates the cell cycle. 3 fungi and Algae, formation of Intranuclear spindle takes place. Nuclear membrane exists. (B) Metaphase : Chromosomes are thick, shorten and distinct. The shape, size and structure of chromosomes can be studied in the metaphase stage. (B) Mid prophase : Spindle apparatus consists of Spindle fibres. Chromosomes are thickened and shifted in It consists of 93-95% tubulin protein, 3-5% the peripheral part of nucleus. RNA and trace amount of actin, myosin & Formation of Astral rays starts. Nuclear lipid. membrane starts to breakdown. Nucleolus reduces in size. SPINDLE FIBRES ARE OF THREE TYPES. (C) Late prophase : (A) Continuous fibres : That connect two Chromosomes are comparatively poles. thickened. Centrosomes have reached (B) Discontinuous fibres : They originate from the opposite poles. a pole and do not reach at other pole. In this phase, nuclear membrane is (C) Chromosomal fibres : They connect disappeared. Nucleolus & ER are also chromosome and poles. disappeared. The tightening of chromosomal fibres brings Aster is formed at each pole by a pair of chromosomes at the equator. This process is centrioles and astral rays. Centrioles called congression or metakinesis. start the formation of spindle fibres. The centromeres of all the chromosomes Centrosome helps in the initiation of arrange in linear sequence at the equator to cell division in animal cells by forming form single metaphase plate. spindle. In plant cells centrosome is Kinetochore of chromosomes connect absent. chromosomes to the spindle fibre. Each The spindle fibre arise from tubulin chromosome has two distinct chromatids. protein by gelation in the cytoplasm, The arms of each chromosome lie in thus in plant cell, centriole and asters different direction and the centromere lies have no role in the formation of spindle on the equator. apparatus. The middle broad part of spindle apparatus is called as equator. Points to remember : (1) Prometaphase : Some scientists considered prometaphase stage between prophase and metaphase. In this stage aster formation is completed. Formation of spindle fibres is also completed. Nuclear (III) Anaphase : membrane and nucleolus are Anaphase Promoting Complex or APC completely disappeared. becomes activated. It causes equatorial (2) Eumitosis : Formation of division in each chromosome.
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