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ICSE BIOLOGY CELL DIVISION NOTES

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Tanay Nayak
Avalon Heights International School, Mumbai

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Cell division Q1. Difference between Mitosis and Meiosis Where it occurs What for When it occurs Number of daughter cells produce Number of chromosomes passed on to each daughter cell Number of nuclear division Identity of chromosomes and genes in daughter cell Mitosis In the somatic (body) cells. To provide for growth and replacement. Continuously throughout life. Two daughter cells. Full set of chromosomes is passed on to each daughter cell. This is the diploid (2n) number of chromosomes. A single nuclear division after chromosome duplication. Identical. Meiosis In reproductive cells. Only for gamete formation. Only in reproductively active age. Four daughter cells. Only half the number of chromosomes is (only one member from each pair). This is haploid (n) number of chromosomes. Followed by two nuclear divisions after chromosomes duplication. Randomly assorted between the gametes produce. This results in genetic variations. Q.2 Mention the phases of mitosis. (i) Prophase Centrioles start moving apart and reach opposite poles. Chromosomes because distinct. Chromosomes are already duplicated as paired chromatids. 1 Sister chromatids attached to each other at a small region called centromere. Spindle fibers appear between daughter centrioles. Nuclear membrane and nucleolus disappear. (ii) Metaphase Each chromosome gets attached to spindle by its centromere. Chromosomes lined up in one plane at equator. (iii) Anaphase Centromere attaching the two chromatids divides. The two sister chromatids of each chromosome separate and are drawn apart towards opposite poles pulled by shortening of spindle fibers. A furrow starts in the cell membrane at the middle in animal cell. (iv) Telophase Two sets of daughter chromosomes reach opposite poles. Spindle fibers disappear. Chromatids thin out in the form of chromatin fiders. Nuclear membrane is formed. 2 The cleavage furrow starts deepening in the animal cell. Nucleoli reappear. Cytokinesis Cleavage furrow deepens totally in animal cell and separates the two daughter cells. Chromosomes become arranged in a horizontal plane at the equator. Chromosomes become visible as fine, long threads. Daughter chromosomes move to opposite poles of the spindle Chromosomes lose their distinctiveness and gradually become transformed into chromatin network. Metaphase Prophase Anaphase Telophase Note: Animal cell have sun shaped structure called as ester; which is absent in plant cell. Q.3 Significance of mitosis 1. Growth or increase in the body size due to formation of new cells in the tissues. 2. Repair of damaged and wounded tissues by renewal of the lost cells. 3. Replacement of old and dead cells such as the replacement of the blood cells and the epidermal cells of the skin. 3 4. Asexual reproduction in which the unicellular organisms, such as amoeba or the yeast cell, divide into two. Q.4. Significance of Meiosis 1. Chromosome number is halved in gametes, so that on fertilization, the normal number (2n) is restored. 2. It provides for mixing up of genes which occurs in two ways: (i) The maternal and paternal chromosomes get mixed up during the first (reduction) division as they separate from the homologous pairs. (ii) Cross joining. While the maternal and paternal chromosomes are separating, the chromatid material very often gets exchanged between the two members of homologous pair resulting in genetic recombination. Q.5. Differentiate between mitosis in plant cell and animal cell Mitosis in animals Mitosis in plants Asters are formed Asters are not formed Cytokinesis by furrowing the cytoplasm. Cytokinesis by cell plate formation Occurs in most tissues throughout the body ( for growth and replacement) Occurs mainly at the growing tip and sides Q.5. Differentiate between karyokinesis and cytokinesis Karyokinesis Cytokinesis It is nuclear division during mitosis It is cytoplasmic division during mitosis or meiosis or meiosis 4 It involves stages like prophase, metaphase, anaphase and telophase It involves cytoplasmic furrow (animal cell) or cell plate (plant cell) Two daughter nuclei are formed Two daughter cells are formed. Note : Centrosome initiates cell division in animal cell Centromere : It joints duplicated chromosome at a point. Q.6. Mention the phases of cell cycle Ans. G1- phase S- phase G2- phase (cell cycle) Karyokinesis Cytokinesis. Q.7. Describe cell cycle? Ans. The following are the stages of cell division; (i) First growth phase (G1): Proteins are synthesized and volume of cytoplasm increases. Mitochondria in all cells and chloroplast in plant cell divide-these organelles have their own DNA. 5 In late G1 Phase all cells must follow one of the two paths. They may either withdraw from the cell or enter a resting phase or start preparing for the next division by entering the next synthesis phase (S) (iI) Synthesis phase (S): More DNA is synthesized, the chromosomes are duplicated (iii) First growth phase (G2): This is a shorter growth phase, in which proteins necessary for cell division are synthesized. Now the cell is ready to start next cell division and thus the cell cycle goes on. Q.8. What is Nucleosome ? Ans. DNA is surrounded by group of eight histone molecule , this complex structure is called as nucleosome. Q.9. Describe the structure of DNA? Ans. (1) DNA is a large molecule, a macro molecule consisting of millions of nucleotides. Hence it is called also called polynucleotide 6 (2) DNA is a double helix structure, where sugar and phosphate form the backbone of the polynucleotide and paired nitrogenous bases forms the centre. (3) Nucleotide consist of three components: Sugar, phosphate and nitrogenous base (4) There are four types of nitrogenous base: Adenine, Guanine, Cytosine and Thymine. (5) Pairing between nitrogenous base is such that : -- Adenine (A) Always pairs with Thymine (T) --Guanine (G) Always pairs with Cytosine (C) ****************************************************************** 7

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Additional Info : ICSE Biology Notes by Bhomik Acharya (bhomik1018)
Tags : icse bio cell division, ICSE Notes and Tips.

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