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ICSE Class X Notes 2020 : Biology (HVB Academy, Mumbai) : Notes on cell division and structure of chromosome

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Soham Multani
HVB Academy, Mumbai
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BIOLOGY CLASS 10 Chapter 2 Cell cycle, cell division and structure of chromosomes 1) Cell division is an important process in all living things. State any four reasons to support your answer. New cells need to be produced in every living organism for a. Growth of the organism b. Replacement of old and worn out cells. c. Repair of damaged cells d. Reproduction in prokaryotes and eukaryotes. 2) Name the two types of cell division. State their region of occurrence and importance. The two types of cell division area. Mitosis takes place in the somatic cells of a living organism. Hence important for growth, repair and replacement of cells. b. Meiosis- occurs in the reproductive cells of a living organism. Hence important for the production of gametes. 3) Define cell cycle. Cell cycle is a genetically controlled series of changes that occur in a newly formed by which it duplicates its contents, undergoes growth and division to form daughter cells.. 1 4) Draw a flow chart showing the different phases in a cell cycle. CELL CYCLE INTERPHASE KARYOKINESIS PROPHASE METAPHASE ANAPHASE TELOPHASE CYTOKINESIS 5) Interphase is often called the resting phase . Why? It is called so, as during this phase, no distinct activity related to cell division is visible, but it is in this phase that DNA is synthesized. 6) Name the three phases in interphase. State the significant changes that occur in each stage. The three phases in interphase are First growth phase or the G1 phase here RNA and proteins are synthesized. Cytoplasm volume increases. Synthesis phase or the S phase DNA is synthesized and chromosomes are replicated. Second growth phase or G2 phase more RNA and proteins are synthesized. 7) Which is the longest phase in the cell cycle? The longest phase in the cell cycle is the interphase. 8) Define karyokinesis. Karyokinesis is the division of the nucleus. The events occurring nuclear division are grouped as prophase, metaphase, anaphase and telophase. 2 Describe the changes that occur in the cell during prophase of mitosis PROPHASE sister chromatid centromere e centriole 1. 2. nuclear membrane e spindle fibres asters 3. 4. Chromatin threads condense into visible short and thick threads called chromosomes. Each chromosome appears to be made up of two identical threads, each called a chromatid. The pair of sister chromatids are connected at a particular point called the centromere., The pair of centrioles divides into two pairs. These appear as star-like structures with radiating fibres called asters. Each pair of asters moves towards the opposite poles of the cell. Between the pair of centrioles and the asters at the opposite poles, spindle fibres begin to develop. The nucleoli and the nuclear membrane disappear. 3 Describe the changes that occur in a cell during metaphase of mitosis. METAPHASE Chromosomes are shortest and thickest during this phase. Chromosomes line up on the equator of the spindle with their centromeres attached to it. Describe the changes that occur in a cell during anaphase of mitosis. ANAPHASE The centromere of each chromosomes divides and the sister chromatids separate as daughter chromosomes. The daughter chromosomes start moving towards the opposite poles of the cell. It is the contraction of the spindle fibres that pull the daughter chromosomes apart. 4 Describe the changes that take place in a cell during telophase of mitosis. TELOPHASE Two sets of daughter chromosomes reach the opposite poles of the cell. The spindle fibres disappear. Each daughter chromosome or chromatid lengthens, become thinner and form the chromatin network. The nuclear membrane reappears. Nucleoli reappear. A furrow appears in the cell membrane in the middle. What is cytokinesis? Cytokinesis is the division of the cytoplasm occurring at the end of telophase. This results in the formation of two daughter cells. Differentiate between cytokinesis in a plant and animal cell. Animal cell cytoplasm divides by a constriction on the cell membrane in line with the equatorial axis. This gradually deepens and converges in the centre and finally splits the cytoplasm, thus resulting in the formation of two daughter cells. (Centripetal) Plant cell- a cell plate is formed at the middle of the equator of the spindle. This cell plate starts to extend towards the periphery (centrifugal). This results in the formation of two daughter cells. Plate formation Cytokinesis furrow 5 Write down the differences between mitosis in plant and animal cells. MITOSIS IN ANIMAL CELLS MITOSIS IN PLANT CELLS Centriole is present and divides into two in early prophase. Centrioles absent Aster is formed from each centriole. No formation of asters and astral rays. Cytokinesis occurs by furrowing of cytoplasm inwards. Cytokinesis is by cell plate formation extending to periphery. Occurs in somatic cells of animal body Occurs in meristematic regions of the plant body. State one region in the plant body and one in the human body where mitosis occurs rapidly? Plant body growing root and shoot tips (meristems) Animal body bruised region of skin, hair bulb, nail matrix. What is meiosis? Meiosis is called reduction division because the chromosome number is reduced to half the number (n) of the original number of chromosomes (2n). This occurs in the germ cells of the gonads. State the region in the plant and human body where meiosis occurs? Plant body occurs in the anthers of the flower to produce pollen grains and in the ovary to produce ovules. Human body occurs in the testis for production of sperms and in the ovaries to produce the ovum. What is the significance of meiosis? 1. The most significant part of meiosis is that the number of chromosomes in the gametes is halved (haploid). This is important as during fertilization when the male and female gamete fuse, the original number (diploid) is restored. 2. Exchange of genetic material occurs during meiosis. This results in genetic recombination and variation. 6 Name two cell organelles that have their own DNA? What is the significance of this? The mitochondria and the chloroplast are two cell organelles that have their own DNA and hence contain genes. So these cell organelles can divide by mitosis to produce two daughter cells. What is the cell life of the following cells? 1. Nerve cells and muscle cells - last a life time. 2. Skin cells 2 week 3. RBC 120 days. 4. Bone cells 10 years 5. Liver cells 300-500 days 6. Epithelial cells of gut 5 days. Define the following terms: 1. Homologus chromosomes 2. Synapsis 3. Crossing over 4. Chiasma 1. Homologus chromosomes are a pair of chromosomes, one from each parent and having similar genes for a characteristic on the same loci. 2. Synapsis is a lateral association between identical pairs of chromosomes. 3. Crossing over exchange of genetic material takes place between chromatids. 4. Chiasma - site of crossing over of the homologus chromosomes. 7 Differentiate between the following terms: 1. Chromosome and chromatid: 2. Centrosome and centromere 3. Aster and spindle fibre 1. Chromosome Chromatid They are condensed chromatin fibres visible just prior to cell division. Each chromosome is made up of two identical thread like structures, each called chromatid. Centrosome Centromere 2. Cell organelles made up of a pair of centrioles, found close to the nucleus in the animal cell. 3. Asters The point of attachment of the sister chromatids. Spindle fibres Centrioles divide in prophase stage and develop star like structures with radiating fibres called asters. They are fibres extending between the asters at the opposite poles. Structure of DNA The DNA molecule is described as a macromolecule as it is one single large molecule. DNA is usually a double-helix and has two strands running in opposite directions. Each single DNA strand is composed of nucleotides. 8 Each nucleotide is made up of three components phosphate and sugar arranged lengthwise along the strand and a nitrogenous base attached inwards. Each nitrogenous base extends to join its complimentary nitrogenous base from the opposite strand. The two nitrogenous bases are attached by a hydrogen bond. Thus the two strands together give a ladder like appearance, with the nitrogenous bases forming the rungs of the ladder. The four nitrogenous bases are Adenine (A), Guanine (G), Cytosine (C), Guanine (G) phosphate sugar Hydrogen bond Board question 1. Study the diagram given below and answer the following questions: E a) Is this a plant or animal cell? Give a reason to support your answer b) Identify this stage of mitosis. State a reason for your identification. c) Label parts E and F. 9 d) Name the stage prior to this stage. e) What is the chromosome number of this cell? f) Draw a diagram showing the next stage after the stage seen in the above diagram. ************************************************************* 10

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