Cell Division PDF

Cell Division 1. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. //120 2. DNA synthesis occurs only during one specific stage in the cell cycle. //120 3. A typical eukary...

Cell Division 1. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. //120 2. DNA synthesis occurs only during one specific stage in the cell cycle. //120 3. A typical eukaryotic cell divide once in approximately every 24 hours. //121 4. Yeast can progress through the cell cycle in only about 90 minutes. //121 5. The cell cycle is divided into two basic phases: Interphase and M Phase (Mitosis phase). //121 6. The interphase lasts more than 95% of the duration of cell cycle. //121 7. The M Phase represents the phase when the actual cell division or mitosis occurs. //121 8. The interphase represents the phase between two successive M phases. //121 9. The interphase is divided into three further phases: G1 phase (Gap 1) , S phase (Synthesis) , G2 phase (Gap 2). //121 10. If the initial amount of DNA is denoted as 2C then it increases to 4C after S phase. //121 11. there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, it will be same 2n after S phase. //121 12. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. //121 13. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues. //121 14. cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. //122 15. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so. //122 16. In animals, mitotic cell division is only seen in the diploid somatic cells. //122 17. there are few exceptions where haploid cells divide by mitosis, for example, male honey bees. //122 18. the plants can show mitotic divisions in both haploid and diploid cells. //122 19. Since the number of chromosomes in the parent and progeny cells is the same in M phase ,it is also called as equational division. //122 20. mitosis has been divided into four stages of nuclear division (karyokinesis)- prophase , metaphase , anaphase , telophase. //122 21. Prophase is marked by the initiation of condensation of chromosomal material. //122 22. Prophase which is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase. //122 23. Cells at the end of prophase, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope. //123 24. The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis that is metaphase. //123 25. the chromosomes are spread through the cytoplasm of the cell. //123 26. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. //123 27. metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. //123 28. Small disc-shaped structures at the surface of the centromeres are called kinetochores.//123 29. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. //123 30. Spindle fibres attach to kinetochores of chromosomes. //123 31. Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles. //123 32. anaphase stage is characterised by Centromeres split , separate and Chromatids move to opposite poles. //124 33. In telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. //124 34. Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei. //124 35. Nucleolus, golgi complex and ER reform. //124 36. the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis. //124 37. In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. //124 38. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. //124 39. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells. //124 40. In some organisms karyokinesis is not followed by cytokinesis. //124 41. It results in multinucleate condition arise leading to the formation of syncytium (e.g., liquid endosperm in coconut). //124 42. Mitosis or the equational division is usually restricted to the diploid cells only. //125 43. In some lower plants and in some social insects haploid cells also divide by mitosis. //125 44. Mitosis usually results in the production of diploid daughter cells with identical genetic complement. //125 45. The growth of multicellular organisms is due to mitosis. //125 46. The kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells this division is called meiosis. //125 47. Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication. //125 48. Four haploid cells are formed at the end of meiosis II. //125 49. prophase 1 has been further subdivided into the following five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. //126 50. During leptotene stage the chromosomes become gradually visible under the light microscope. //126 51. During zygotene chromosomes start pairing together and this process of association is called synapsis. //12652. synapsis is accompanied by the formation of complex structure called synaptonemal complex. //126 53. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. //126 54. During pachytene stage , the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. //126 55. the appearance of recombination nodules occur, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. //126 56. Crossing over is the exchange of genetic material between two homologous chromosomes.//126 57. Crossing over is also an enzyme-mediated process. //126 58. enzyme involved in crossing over is called recombinase. //126 59. The beginning of diplotene is recognised by the dissolution of the synaptonemal complex. //126 60. the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. //12661. X-shaped structures, are called chiasmata. //126 62. In oocytes of some vertebrates, diplotene can last for months or years. //126 63. The final stage of meiotic prophase I is diakinesis marked by terminalisation of chiasmata. //126 64. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. //126 65. Metaphase I: The bivalent chromosomes align on the equatorial plate. //126 66. The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes. //126 67. Anaphase I: The homologous chromosomes separate, while sisterchromatids remain associated at their centromeres. //127 68. Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as dyad of cells. //127 69. The stage between the two meiotic divisions is called interkinesis and is generally short lived. //127 70. There is no replication of DNA during interkinesis. //127 71. Interkinesis is followed by prophase II, a much simpler prophase than prophase. //127 72. The nuclear membrane disappears by the end of prophase II and the chromosomes again become compact. //127 73. Metaphase II: At this stage the chromosomes align at the equator and spindle get attached to the kinetochores of sister chromatids. //127 74. Anaphase II: It begins with the splitting of the centromere of each chromosome. //127 75. Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope. //128 76. cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells. //128 77. Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms. //128 78. It also increases the genetic variability in the population of organisms from one generation to the next. //128