Answers for Ch 5 Cell Division

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Discovering Biology 1A – Chapter 5

Chapter 5 Cell Cycle and Division Exercise (p. 157 – 161) Multiple Choice Questions 1. B The nuclei nuclei of of all all body body cell cellss norma normally lly contain contain a fixed fixed number number of chrom chromosom osomes. es. Hence, Hence, all all human body cells, including the human liver cells, have 46 chromosomes. 2. B The paired paired homolog homologous ous chromo chromosom somes es migrat migratee towar towards ds the the middl middlee of the cell and become become attached to the spindle fibres. This stage is known as metaphase I of meiosis. Phase Phase A is the second second growth growth phase phase (G2 phase) phase) which which come comess after after the S phase. phase. 3. B At the beginni beginning ng of of mitot mitotic ic cell cell divisi division, on, the chromos chromosome omess repl replicat icatee and and doubl doublee 4. C themselves. As a result, 52 chromosomes can be formed. At 15 15 minute minutess after after the the previo previous us cell cell divis division, ion, the dist distance ance between between the chrom chromati atids ds of of the 5. B chromosome starts to increase from 0, which means the chromosomes start to split into chromatids. At the same time, the distance between the chromosomes and the poles begins to to decrease, decrease, which means that that the chromatids chromatids begin to to migrate migrate towards towards the poles. poles. Only Only choice choice C corre correctl ctly y repres represents ents the arrangem arrangement ent of the the chrom chromoso osomes mes at 10 10 minut minutes. es. 6. C 7. A Befor Beforee mit mitos osis is,, the the chro chromo moso some mess dup dupli lica cate te thems themsel elves ves.. 8. D Egg cell contains contains a haplo haploid id numbe numberr of chromo chromosome somess in the nucleus nucleus as it is is a gamete gamete resulted from meiotic cell division. 9. A Homolog Homologous ous chromo chromosom somes es will will be paired paired up in meiosi meiosiss but but not not in mitosis mitosis.. 10. D At the the end end of the firs firstt and second second meiot meiotic ic divi divisio sion, n, the the daught daughter er cell cell will will have have only only half half of the original number of chromosomes of the parent cell. At the end of the second meiotic division, the daughter cell will remain haploid. Short Questions 11.

12. (a) (b) (c) 13.

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Meiotic cell division. Anther (or other acceptable answers). Testis (or other acceptable answers). Feature

Site of division Number of daughter cells produced produced Chromosome number in each parent cell Chromosome number in each daughter cell

Mitotic cell division root tip/shoot tip (0.5) 2 (0.5) 20 (0.5) 20 (0.5)

Meiotic cell division ovary/anther (0.5) 4 (0.5) 20 (0.5) 10 (0.5)

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20 chromosomes. (1) 40 chromosomes. (1) 20 chromosomes. (1) Vincr ncrist istine ine pr prevent ventss the the form ormation of of sp spindl ndle fi fibres du during mitos itosiis. (1) Without spindle fibres, the chromatids in each chromosome cannot be pulled towards towards the opposite opposite ends ends of the the cell. (1) No daughter daughter chromosome chromosomess can be formed. formed. (1) The cancer cell ther herefore cannot divide to form new cancer cells. (1) (ii) Apart from affecting the cancer cells, vincristine will also affect other actively dividing body cells such as stem cells. (1) Vincristine can prevent the formation of spindle fibres in stem cells during mito mitosi sis. s. This This pre preve vents nts ste stem m cells cells from from carr carryin ying g out mit mitoti oticc cell cell divis divisio ion. n. (1) (1) As a result, as old blood cells are constantly broken down and new blood cell cellss can canno nott be be pro produ duce ced, d, this this lead leadss to to a low low cou count nt of bloo blood d cel cells ls.. (1) (1) A, D, C, B. (2) Type of cell division – mitotic cell division. (1) Site of occurrence – shoot tip. (1) It provides new cells for the plant to grow. (1) Bone marrow and skin (or other acceptable answers). (1) In animal animal cells cells,, at the end of of mitosis mitosis,, constri constrictio ction n across across the midd middle le of cytopl cytoplasm asm,, (1) leading to the separation of cytoplasm into two equal halves. (1) In plant cells, after the completion of mitosis, cell walls will be formed across the middle of the parent cell, (1) leading to separation of cytoplasm into two equal halves. (1) (i) Mitotic cell division. (1) Two diploid daughter cells are formed. (1) (ii) Their number and kinds of chromosomes are identical to those in the parent cell. (1) (i) Each daughter cell contains the same number and kinds of chromosomes as the other daughter cell. (2) (ii) Each daughter cell contains the same number and kinds of chromosomes as the parent cell. (2) The nucleus of the pollen cell of couch grass contains only half of the chromosomes as the parent cell while the daughter cells shown in the diagram have the same number of chromosomes as the parent cell. (1) Chromosomes in the nucleus of pollen cell of couch grass may not be identical to each other and the parent cell while chromosomes from the daughter cells shown in the diagram are identical to each other and the parent cell. (1) (i) 46. (1) 23. (1) (ii) (iii) 23. (1) (iv) 46. (1) 46. (1) (v) Meiotic cell division. (1)

(c)

Meiotic cell division produces haploid gametes, which contain only half the

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15. (a) (b) (c) (d) (e)

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(d) (e) 18. (a)

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number of chromosomes of the body cells. (1) When male and female gametes fuse together to form zygote, the normal diploid number of chromosomes is restored. (1) Meiotic cell division is therefore important for a species to maintain a constant num number ber of of ch chromosomes in in the the offsp fspring ing pr produc oduceed by by se sexual ual re reproduc oducttion. (1) Ovary and testis. (2) Fertilisation. (1) X – Mitosis. (1) It is because the amount of DNA of the daughter cells is just the same as parent cell at the end of cell division. (1) Y – Meiosis. (1) It is because the DNA content is only half of the parent cell at the end of cell division. (1) A – DNA replication is occurring in the nucleus. (1) B – The nucleus divides into two daughter nuclei at the end of mitosis. (1) C – The nucleu nucleuss divides divides into into two two daughter daughter nucle nucleii during during the first first meioti meioticc divisio division. n. (1) D – Fusio Fusion n of two haploid haploid nuclei nuclei duri during ng fert fertili ilisati sation on prod produces uces a diplo diploid id nucle nucleus. us. (1) At stage S – diploid daughter cells (e.g. body cells). (1) At stage T – haploid daughter cells (e.g. gametes). (1)

Essay-type Questions 19. Structure 19. Structure of chromosomes: chromosomes: Chromosomes are thread-like structures inside the cell nuclei. When a cell is not dividing, the chromosomes cannot be distinguished under the microscope as they extend into very fine threads. When When a cel celll sta start rtss to to divi divide de,, the the chro chromo moso some mess bec become ome much much shor shorte terr and and thicke thickerr. (1) (1) They can then be seen under the microscope as darkly stained threads after treating with a special stain. The number of chromosomes in the body cells of organisms of the same species is always the same. The chromosomes in a cell always occur in pairs. The members of a pair of chromosomes are known as homologous chromosomes. Function of chromosomes: They hey car carrry gene genettic mate materi rial alss whi which ch dete determ rmiine the the cha chara ract cter eris isttics ics of of the the or organis anism. m. 20. Similariti 20. Similarities: es: Both processes result in the formation of daughter cells. Differences: Differences: Mitotic cell division will form two daughter cells while meiotic cell division will form four daughter cells. (1) Mitotic cell division involves one nuclear and cytoplasmic division while meiotic cell division involves two nuclear and cytoplasmic divisions. Pairing of homologous chromosomes is absent in mitotic cell division while pairing of homologous chromosomes is present in mitotic cell division. Number Number of chromosome chromosomess in daughter daughter cells cells is the same as parent cell (diploid) (diploid) in mitotic mitotic cell division while number of chromosomes in daughter cells is only half of the parent 101273209.doc

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cell (haploid) in meiotic cell division. Genetic materials in daughter cells are identical to parent cell in mitotic cell division while genetic materials in daughter cells may not identical to parent cell in meiotic cell division. Mitotic cell division is important in growth, repair and asexual reproduction while meiotic meiotic cell cell divisi division on is importa important nt in the the produc productio tion n of gamete gametess for sexual sexual reprod reproducti uction. on. Mitotic cell division takes place at active growing regions in the body while meiotic cell division takes place at sex organs.

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Public Examination Questions (p. 162 – 165) Notes:

Answers for the questions of HKCEE and HKALE are not available in this publication due to copyright restrictions.

Multiple Choice Questions 4. B As the the chrom chromosom osomee pairs pairs in the the cell cell do do not align align in the the middle middle of the the cell, cell, the the cell cell is is not in metaphase I. The cell is in prophase I and its diploid chromosome number is 6. Hence, the haploid number of chromosomes in this cell is 6/2 = 3. (IBO Biology, Higher Level, Paper 1, TZ1, May 2005, Q.30) Structured Questions 5. During prophase, chromosomes become shortened and thickened. They become visible under the microscope. Each of them is composed of two chromatids. (1) During metaphase, chromosomes arrange at the middle of the cell. (1) Each of them attaches to thread-like structures called the spindle fibres (1) During anaphase, the spindle fibres contract and pull the two chromatids in each chromosome towards the opposite ends of the cell. (1) Chromatids will reach their respective poles. Each chromatid is now called daughter chromosome. (1) Eventually, the daughter chromosomes gradually extend into tiny threads and become invisible. (1) (UCLES 06 A Level Bio. 9284/2 Nov. Q.9(b)) 8. (a) (1) During interphase, DNA replicates itself inside the nucleus. (1) This ensures that all the genetic information is retained during the replication process. (1) (2) (2) Duri During ng prop prophas hase, e, homolo homologo gous us chrom chromos osome omess wil willl not not lie side side by side side of each other to form pairs. (1) This ensures that the chromosomes will not exchange their genetic materials at this stage. (1) (3) (3) Duri During ng anap anaphas hase, e, the two two chro chroma mati tids ds of the homolo homologo gous us chrom chromos osom omee wil willl be pulled pulled apart apart towards towards the opposite opposite poles by by spindle spindle fibres. fibres. (1) As the DNA content of each chromatid is the same as its sister chromatid, this ensures that the daughter cells produced are identical in their genetic content. (1) (b) During metaphase I of meiosis, homologous chromosomes arrange themselves randomly ©


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Solution guide

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in the middle of the cell. (2) Each homologous chromosomes are separated randomly and independently to the other pairs of homologous chromosomes. (2) Therefore, each of the gametes formed contain different combinations of genetic materials. This is known as independent assortment. (2) The fusion of gametes during fertilisation is also a random process. (1) It leads to different combinations of genetic materials in the zygote. (1) (UCLES 02 A Level Bio. 9284/2 Nov. Q.11(a)(b)) Telophase I. (2) After telophase I, the nuclear membrane disappears. (1) When the second meiotic division begins, each chromosome appears as a pair of chromatids. In metaphase II, chromosomes will line up in the middle of the cell and become attached to the spindle fibres. (1) The spindle fibres then contract and the chromatids are pulled to opposite ends of the cell in anaphase II. (1) At telophase II, each chromatid becomes a daughter chromosome. A nuclear membrane is formed around each group of chromosome. (1) The cytoplasm divides to form four haploid daughter cells, which may develop into gametes. (1) In metaphase I of meiosis, (1) the random distribution and independent assortment of homologous chromosomes in the middle of the cell (2) enable the chromosomes to be arranged in various combinations. As a result, new genetic variations can be obtained. (1) (UCLES 03 A Level Bio. 9284/4 Jun. Q.4) Haploid means a cell contains only one set of chromosomes, (0.5) which ich is hal half of the total tal numbe umberr of chro hromoso osomes of the parent rent cel cells. ls. (0.5) Diploid means a cell contains both sets of chromosomes, (0.5) in which the number of chromosomes is same as those in parent cells. (0.5) Cell A – metaphase. (1) Cell B – anaphase. (1) At the end of metaphase, spindle fibres contract, (1) causing the chromatids to separate from each other (1) and move towards the opposite ends of the cell. (1) At the the end of pr prophase hase in mi mitosis, is, the the nucl nuclea earr env enveelope disinte ntegrate ates. (0.5) The The nucl nuclea earr mem membr bran anee rema remain inss disi disint nteg egra rate ted d duri during ng met metap apha hase se and and ana anaph phas ase. e. (1) (1) During telophase, nuclear envelope will form around each group of daughter chromatids, producing two daughter nuclei. (0.5) (UCLES 03 A Level Bio. 9284/2 Nov. Q.2(a)(i)(b)(c)(d))

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The chromosomes in a body cell always occur in pairs. (1) The membe members rs of a pair pair of of chromo chromosom somes es are are known known as homolog homologous ous chrom chromosom osomes. es. (1) (b) First meiotic division: During prophase I, the chromosomes replicate and become visible as long and coiled threads. These threads then become shortened and fattened. (1) Homologous chromosomes will lie side by side with each other to form pairs. Each chromosome appears as a pair of chromatids. (1) During metaphase I, the paired homologous chromosomes move towards the middle of the cell and become attached to the spindle fibres. (1) During anaphase I, each of the two members of the homologous chromosomes is separated and pulled apart through the contraction of spindle fibres towards the opposite poles of the cell. (1) During telophase I, the chromosomes reach poles and the chromatids begin to uncoi ncoill. (1) Second meiotic meiotic division: division: During metaphase II, the chromosomes in each of the two daughter cells line up in the middle of the cell. (1) During anaphase II, the spindle fibres contract. The pair chromatids separate from each other and move towards opposite ends of the cell. (1) During telophase II, the chromatids reach their respective poles and each chromatid becomes a daughter chromosome. (1) (c) Meiotic cell division is a kind of cell division which produces four haploid daughter cells from one diploid parent cell. (1) When male and female gametes fuse together to from zygote, the normal diploid number of chromosomes is restored. (1) Meiotic cell division is very important for a species to maintain a constant number of chromosomes in the offspring produced by sexual reproduction. (2) Without the production of diploid gametes through meiosis, (1) a zygote possessing double the number of chromosomes as the parent will be resulted after fertilisation. (1) (UCLES 04 A Level Bio. 9284/2 Nov. Q.8) Q.8) 12. (a) (i) Centromere. (1) It is the site at which the two chromatids are attached to each together in a (ii) chromosome. (1) It is also the site at which chromosomes become attached to the spindle fibres during nuclear division. (1) Type of nuclear division – meiosis. (0.5) (b) (i) Stage – anaphase I. (0.5) (ii) Type of nuclear division – mitosis. (0.5) Stage – anaphase. (0.5) (c) In Fig. 1, the paired chromosomes are moving away from each other while their sist sister er chrom chromat atids ids of each each chro chromo moso some me are still still atta attache ched d to the centr centrom omer ere. e. (1) (1) In Fig. 2, the centromere has split so that each chromatid from the separating chromosome is moving towards opposite poles of the dividing cell. (1) (UCLES 05 A Level Bio. 9284/2 Nov. Q.2(a)(b)(c)) 11. (a)

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Answers for Ch 5 Cell Division

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