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ICSE Notes 2017 : Biology (Cell Division and Chromosomes)

13 pages, 93 questions, 2 questions with responses, 2 total responses,

Prathamesh Pai
Vidyodaya Public School, Udupi

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Biology-10 1. Structure of Chromosomes, Cell Division Structure of chromosomes 1. What are Chromosomes? 2. Discovery of chromosomes 3. Structure of chromosomes Cell Division 1. Why new cells need to be produced? 2. Types of cell division 3. Mitosis 4. Cell Cycle 5. Meiosis Definitions: 1. Cell Division Cell division is the process by which a parent cell divides into two or more daughter cells. 2. Interphase Non-dividing stage of the cell, where the quantity of DNA doubles and synthesis of DNA takes place is Interphase. 3. Chromatin fibres A network of very long extremely thin dark-staining fibres, which then forms chromosomes 4. Chromosomes Highly condensed, coiled chromatin fibres are called chromosomes. 5. Chromomere Bead-like accumulations of chromatid material. 6. Centromere A point on a chromosome by which it is attached to a spindle fibre during cell division. 7. Chromatid Each of the two thread-like strands into which a chromosome divides longitudinally during cell division. 8. Spindle fibres Spindle fibres are protein structures that pull apart the genetic material in a cell when the cell divides. 9. Asters Centrioles are surrounded by radiating rays called asters. 10. Chromatid material Region of tightly folded DNA 11. Histones Particular type of proteins found in the chromatin. 12. Nucleosome Groups of histone molecules surrounded by DNA strands. 13. DNA A nucleic acid that consists of genetic material composed of 2 complementary strands wound around each other in a double helix. 14. DNA polymerases The DNA polymerases are enzymes that create DNA molecules by assembling nucleotides. 15. Mitosis Mitosis is the cell division in which one parent cell divides into two identical daughter cells. 16. Meiosis It is a kind of cell division which produces gametes. 17. Karyokinesis All nuclear changes that occur during cell division are collectively called karyokinesis. 18. Cytokinesis Karyokinesis is followed by division of cytoplasm. 19. Poles of the Cell Extremities of axis 20. Equator of the Cell Middle plane dividing the cell into equal halves 21. Stem cells Undifferentiated cells which can undergo unlimited divisions and give rise to one or several different types of cells 22. Chromonemata Gene-bearing portion of chromosome 23. Chromomere Bead-like accumulations of chromatid material Function: 1. Cell Division It enables life to continue generation after generation. 2. Centromere It serves to attach to the spindle fibre during cell division. 3. Meiosis 4. 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 the old and dead cells such as the replacement of the blood cells and the epidermal cells of the skin. 4. Asexual reproduction in which the unicellular organisms, such as amoeba or the yeast cell, divide into two. 5. Maintains same chromosome number in daughter cells. 5. Spindle Fibre Spindle fibres form a protein structure that divides the genetic material in a cell. 6. Asters Their function is to hold the two centrioles in place at the two poles of the cell during cell division. 7. DNA The main role of DNA in the cell is the long-term storage of information. 8. Mitochondria and chloroplasts (in cell division) Both mitochondria and chloroplasts have their own DNA (containing certain genes). They also contain their own Ribosomes which help in producing the particular proteins of these two organelles. Both these divide of their own by simple fission, just splitting into two and are partitioned between the two daughter cells produced by mitosis. Mitochondrial division is also guided by the genes in the nucleus and through the cytoplasmic ribosomes. Location: 1. Centromere It lies within the thinner sediment of the chromosome, the primary constriction. 2. Histones They are found in chromatin. 3. DNA They are found in chromatin and in mitochondria. Characteristics: 1. Prophase 1. The chromosomes have become short and thick and are clearly visible inside the nucleus. 2. Each chromosome has already duplicated to form two chromatids. The two sister chromatids remain attached to each other at a small region called centromere. 3. The centrosome splits into two along with simultaneous duplication of the centrioles contained in it. The daughter centrioles move apart and occupy opposite "poles" of the cell. 4. A number of fibres appear between the two daughter centrioles, which are called the spindle fibres. 5. The nuclear membrane and the nucleolus disappear. 6. The duplicated chromosomes start moving towards the "equator" of the cell. 2. Metaphase 1. The duplicated chromosomes arrange on the equatorial plane. 2. Each chromosome gets attached to a spindle fibre by its centromere. 3. Anaphase 1. The centromere attaching the two chromatids divides and the two sister chromatids of each chromosome separate and are drawn apart towards opposite poles. 2. The drawing action is performed by the contraction of spindle fibres. 3. Furrow starts in the cell membrane at the middle of the cell. 4. Telophase 1. Two sets of daughter chromosomes reach opposite poles. 2. Spindle fibres disappear. 3. Chromatids thin out in the form of chromatid fibres. 4. Nuclear membrane is formed. 5. Cleavage furrow starts deepening in the animal cell. 6. Nucleoli reappear. 7. In plant cell, cell plate is laid down in the cytoplasm at the equatorial plane. 5. Cytokinesis 1. Cleavage furrow deepens totally in the animal cell and separates the two daughter cells. 2. Cell plate grows from the centre to the periphery, resulting in 2 cells. 6. Body Cells (age) 1. Cells of eye lens, nerve cells of cerebral cortex and most muscle cells last life time and cannot be replaced if dead. 2. Skin Cells are continuously lost and replaced by underlying cells in every 2 weeks or so. 3. RBCs last for about 120 days and are replaced. 4. Bone Cells are replaced every 10 years. 5. Epithelial cells lining the gut 5 days 6. Average life of other gut cells 15 years 7. Liver cells are replaced every 300-500 years 8. Brain and nerve cells once formed in the embryo do not divide further. 9. Specialised germinal cells in the ovary and testis in animals and in the ovary and anthers in plants undergo the other type of cell division called meiosis to produce sex cells. 7. Plant Cells (age) The cells at the growing points (meristems) divide very rapidly and produce new leaves, buds and flowers, etc. 8. New cells formed The new cells at the end of mitosis are relatively small, with a full sized nucleus but relatively little cytoplasm. 9. First Growth Phase (G1) 1. RNA and proteins are synthesised, the volume of cytoplasm increases. 2. Mitochondria (in all cells) and chloroplasts (in plant cells) divide - these two organelles have their own DNA. 3. In late G1 phase, all cells must follow one of the two paths. They may either withdraw from the cell cycle and enter a resting phase (R) or start preparing for the next division by entering the next synthesis phase. 10. Synthesis Phase (S) More DNA is synthesised, the chromosomes arc duplicated. 11. Second Growth Phase (G2) This is a shorter growth phase, in which RNA and proteins necessary for cell division continue to be synthesised. Now the cell is ready to start next cell division and thus the cell cycle goes on. 12. Meiosis 1. Chromosome number is halved in gametes (sex cells), 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 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 a homologous pair resulting in genetic recombination. 13. Reduction Division 1. Chromosomes condense 2. Chromosomes arrange in Homologous pairs 3. Chromosomes split longitudinally 4. Chromosome pairs separated- Each daughter cell receives only one chromosome from each pair. (Reduction division) 14. Mitotic Division 1. Centromere splits 2. Chromatids of each chromosome separate to become independent chromosomes Differences: 1. Mitosis Animal Cell 1. Asters are formed 2. Cytokinesis by furrowing of cytoplasm 3. Occurs in most tissues throughout the body Plant cell Asters are not formed Cytokinesis by cell plate formation Occurs mainly at the growing tips and sides. 2. Mitosis Where it occurs? What for? When it occurs? No. of daughter cells produced No. of nuclear divisions Identify of chromoso mes and genes in daughter cell No. of chromoso mes passed on the each daughter Meiosis In the somatic cells In the reproductive cells To provide for growth and replacement Continuously throughout life Only for gamete formation 2 4 A single nuclear division after chromosome duplication. 2 nuclear divisions after chromosome duplication. Identical Randomly assorted between the gametes produced. This results in genetic variations. Full set. This is diploid no. of chromosomes Only half the number. This is the haploid no. of chromosomes. Only in reproductively active age cell Give Reasons: 1. Condensed chromatin fibres are named as chromosomes. Chromosomes readily pick up certain dyes and get coloured, hence the name chromosomes (chroma: coloured; soma= body) 2. New cells need to be produced. New cells need to be produced for (1) growth: Every organism, be it a plant or an animal, begins its life as a single cell. This cell divides repeatedly to form a cluster of cells which start shaping for different functions to form tissues and organs. Thus, cell division is essential for growth. (2) replacement: They is always wear and tear of the cells during normal body functions, which needs to be replaced by new cells formed through a division of their parent cells. (3) repair: When there is accidental injury, it should be repaired through the cells which divide and cover up the gaps. (4) reproduction: Reproduction is also through the activity of the dividing cells. Special cells in the reproductive organs undergo a special kind of cell division (meiosis) to produce sperms and eggs. 3. Spindle fibres are formed in plant cell, even though they do not have centrioles. Due to microtubules, organized by cell wall of the plant cell 4. Mitosis occurs in growing tips and sides of plants For lengthening and for increase in girth 5. Out of the 23 pairs of chromosomes in humans, only single chromosomes i.e. one member of each pair (haploid) is passed on to the sex cells. This is essential because when the male and female gametes fuse during fertilisation, the normal double (diploid) number of chromosomes (in pairs) is reacquired. What? When? How? 1. When do duplicated chromosomes get evenly distributed? During mitosis 2. How body cells are formed? Body cells are the result of a long chain of repeated mitotic divisions, having same type of chromosomes in same number. 3. When do chromatin fibres condense to form chromosomes? During prophase 4. How did Walther Fleming discover chromosomes? He noticed these in rapidly dividing cells of larvae of an amphibian, salamander. He saw minute threads that appeared to be dividing lengthwise. Fleming called their division as mitosis. 5. What is a chromatin made of? It constitutes the fibre made of 2 substances: DNA (40%) and Histones (60%). 6. Explain the composition of DNA. It is composed of repeating nucleotides, which are made up of 3 components: phosphate, sugar (pentose) arranged lengthwise and nitrogenous base. 7. What are nitrogenous bases? Explain their arrangement. Nitrogenous bases are Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). The nitrogenous base extends to join complementary nitrogenous base from the other strand by a Hydrogen bond. Adenine pairs with Thymine with 2 hydrogen bonds. Guanine pairs with Cytosine with 3 Hydrogen bonds. 8. How do the 2 strands look like? What are the rungs ? The 2 strands make a ladder like arrangement, with the nitrogenous bases forming the rungs of the ladder. 9. How new DNAs are formed? During the interphase of cell cycle, each DNA molecule duplicates in readiness for their equitable distribution in the two daughter cells during mitosis. For replication, the DNA double helix opens at one end, making the two strands free to which new strands begin to form and the process continues in a sequence for the whole length of the DNA. 10. Where does meiosis occur? Meiosis occurs in egg producing oocytes (overies) and sperm producing spermatocytes (testis). 11. What are the most important aspects of mitosis and meiosis? Mitosis- Same normal chromosome number is maintained at each cell division. Meiosis- number of chromosomes in the sex cells is halved. 12. What happens before cell division? Nucleus gets prepared and DNA quantity doubles. 13. State the 4 main phases of mitosis 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase 14. Can the cell cycle go endlessly? At some places it stops permanently, at some places temporarily and at others till it is needed. Uncontrolled non-stop cell cycles may lead to tumours that may or may not be cancerous. 15. Where does meiosis take place? It takes place in reproductive organs, testis and ovary in humans to produce sperms and ova. In flowering plants, it takes place in anthers and ovary to produce pollen grains and ovules. 16. How chromosomes are formed? Chromosomes are formed of very long fine chromatin fibres, which are made of DNA and histones. 17. What forms the basis of variations? Meiosis involves crossing over between chromatids of homologous pairs that form basis of variations. 18. What is chromatid material? Region of tightly folded DNA. Fill in the blanks 1. First stage of cell division 2. Chromosome was discovered by ___________. 3. Literally meaning of mitosis 4. DNA full form acid 5. Fundamental unit of chromatin 6. Arrangement of chromatin fibres coiled Prophase Walther Fleming Thread Deoxyribonucleic Nucleosome Coiled and super- 7. Shape of DNA molecule was studied by Rosalind Franklin 8. Structure of DNA was founded by Watson and Crick. 9. Building Blocks of DNA Nucleotides 10. Reproduction in Amoeba is by _________. Mitosis 11. A large portion of house hold dust contains _____cells. Skin 12. The powdery material that comes on scratching the skin are ______. Dead skin cells 13. Phases of mitosis karyokinesis, cytokinesis 14. Chromatids are attached to each other by__________. centromere 15. Chromatid in early stages of condensation chromonema Diagrams: Structure of Chromosome

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Additional Info : ICSE Prelims 2017 : Biology (Vidyodaya Public School, Udupi)

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