1. THE CELL CYCLE, MITOSIS& MEIOSIS

2. Three Main stages in the Cell Cycle3. Interphase(growth- 90% of the time)2. Cell division(cytokinesis)1. Mitosis(nuclear division) 3. Interphase has three subphases:3. G2(Gap 2)2. S phase(DNA synthesis)1. G1(Gap 1) 4. What happens in each subphase?synthesis andreplication oforganellessynthesis ofproteins and RNADDNNAA ssyynntthheessiisscells thatdo notdivide againenter G0 5. Three organelles replicate:MitochondriaChloroplastsCentrioles 6. The length of the Cell Cycle dependson:The type of cell: usually the morespecialised the cellthe less likely it isto divideExternal factors e.g.:temperaturefoodoxygen suppliesRBC arereplaced ata rate of 2-3million/sec Nerve cellsusually neverdivide 7. Question: [SEP, 2009]Suggest explanations for the followingobservation.Interphase is not a static stage ofthe cell cycle. (2)Proteins and RNA are synthesised,DNA is replicated and organelles aresynthesised or replicated. 8. Question No. 9 Pg. 49: [MAY, 2006]This question is concerned with cell division.Give a brief description of the Cell Cycle. (2)Nuclear division is followed by cytokinesiswhich results in two cells.Each cell enters interphase where proteins andRNA are synthesized during the G1 phase.DNA replicates during the S phase andorganelles are synthesised or replicated duringthe G2 phase. 9. Question: [MAY, 2012]The graph below illustrates how the quantity ofDNA, within a mammalian somatic cell, variesduring different phases of the cells cycle.Additionally the graph shows the two growthphases, G1 and G2, which are separated by anintermediate phase S.a) In terms of thegraph above, explainwhat is happeningwithin the cell duringphase S. (2) 10. DNA replicates. Each chromosome consists of twosister chromatids.b)State ONE process other than cell growth whichoccurs during phase G2. (1)Organelles are synthesised or they replicate. 11. c) Account for the changes in the quantity of DNA inMG1SG2the cell during mitosis. (2)At the end of mitosis, quantity of DNA returns to theoriginal value as sister chromatids separate. 12. Cells divide for three reasons:1.For growth e.g.formation of abud in Hydra2. To producegametes or spores3.To replacedamaged cellsMucor Salmon gametes mixing 13. The cytoplasm and organellesmay be distributed unequallye.g. ovum formationBUTthe genetic materialshould beequal in both cells 14. CHROMOSOMES: carry the genetic code contain all characters of cells 15. Linear chromosomes ineukaryotesComposed of chromatin acomplex of DNA and proteinsCircularchromosomein prokaryotes. 16. DNA is packedThe coiling and condensing allows theDNA to be equally divided up between theold cell and the new one to be created. 17. The total extendedlength of DNA in ahuman cell isnearly 2m, but thismust be fit into anucleus with adiameter of only 5to 10m. 18. There is NO relationship between size of organismand chromosome numberAnt: 2 Fruit Flies: 8 Carrots: 20Shrimp: 254Turkey: 82Dog: 48Adders tongue(Fern): 1260Largest number ofchromosomes 19. HHoommoollooggoouuss cchhrroommoossoommeess Paired chromosomes- both chromosomes ofa pair carry genes- control same inheritedcharacters- homologous = sameinformation 20. Ais:a photo of chromosomes in a cell, arranged inhomologous pairsthe set of chromosomes 21. PPrreeppaarriinngg aa kkaarryyoottyyppee 22. PPrreeppaarriinngg aa kkaarryyoottyyppeeKaryotypes can be usedto determine if there is anabnormality inchromosome number orstructure.Pictures of thechromosomes are cut outand arranged in pairsaccording to size andbanding patterns. 23. The human genome (the total number ofchromosomes) in humans is:23 homologous chromosomal pairsSet 23:Sex chromosomesSet 23:Sex chromosomes 24. Autosomes(The Autosomes code for most of the offsprings traits)In Humans theAutosomesare sets 1 - 22 25. HHuummaann KKaarryyoottyyppeeWhy are chromosomesX shaped?MMaallee oorr ffeemmaalleekkaarryyoottyyppee??MMaallee 26. Spot the error in thefollowing karyotypes: 27. A missing X Chromosome (X0)Turner syndromeSterile female 28. An extra X Chromosome (XXY)Klinefelter syndromeSterile male 29. TRISOMY 21- Down Syndrome 30. TWO types of cells in the bodyNon-reproductiveor somatic cellsNon-reproductiveor somatic cellsGametes &SporesGametes &Sporesdiploid numberof chromosomes (2n)phagocyteneuroneshaploid number (n)spermatozoapollen grains(microspores) 31. Which process restores the diploidnumber of chromosomes? 32. Meiosis reduces the chromosome number;Fertilisation restores the diploid number.eggspermmeiosis46fertilisation /syngamy232346 2346 23 33. Why are chromosomes drawn in adifferent colour?Chromosomes are of eitherMATERNAL or PATERNAL origin.from Mum from Dad 34. Advantages of having two sets ofchromosomes:1) Genetic variation isincreased. Eachindividual will have amixture ofcharacteristics fromboth parents.1) Genetic variation isincreased. Eachindividual will have amixture ofcharacteristics fromboth parents.2) If a gene on one chromosome of a pairis faulty, the second chromosome mayprovide a normal back-up.2) If a gene on one chromosome of a pairis faulty, the second chromosome mayprovide a normal back-up. 35. Question: [SEP, 2009]Suggest explanations for the followingobservation.Meiosis, unlike mitosis, is not considered toform part of a cell cycle. (2)When cells divide by mitosis, cytokinesis andinterphase follow and then the same phases arerepeated. However, a cell divides by meiosis onlyonce. There is no repetition of phases. 36. Meiosis & Mitosis Alternate in LifeCyclesthe life cycles of all sexually reproducingorganisms follow a pattern of alternationbetween diploid and haploid chromosomenumbersbut there is some variation in the life cycles 37. Three types of Life Cycles:1) Haploid dominant algae & fungiAlgae 38. 2) Diploid dominant - Life cycle ofanimals 39. 3) Adults alternate - Life cycle of plantsDIPLOIDHAPLOIDMoss plant 40. types of nuclear division 41. - the process by which a cell nucleusdivides to produce two daughternuclei containing identical sets ofchromosomes to the parent cellMitosis inroot tip 42. Question: [MAY, 2006 Paper3]A researcher is provided with onion bulbs which aresprouting following dormancy. She intends to use thesebulbs to study mitosis in onion cells.Which parts of the bulb are suitable for this purpose?2-3 mm from the root tips 43. - the process by which a cell nucleusdivides to produce four daughter nucleieach containing half the number ofchromosomes of the original nucleus- also called reduction divisionMeiosis forms the ovum andspermatozoa in animals. 44. SSiiggnniiffiiccaannccee ooff mmiittoossiiss [[55 rroolleess]]2. Regeneration andcell replacement1. Growth3. Genetic stability 45. Binary fissionin AmoebaBudding in Yeast4. Asexual reproduction Single-celled eukaryotes reproduceasexually Simple multicellular eukaryotesreproduce asexuallyHydra by budding 46. 5. Gamete production in plantsantherMeiosis producesSPORESMitosis producesGAMETES[Only MALE spores are shown here]. 47. Centromeres, centrioles andcentrosomesCentromereis a point of constriction onthe chromosome containingcertain repeated DNAsequences that bind specificproteinsthese proteins make up a disc-like structurecalled the kinetochore 48. Kinetochoreis a protein/DNA complex which forms at thecentromereserves as an attachment site for microtubulesnecessary to separate the sister chromatids the two sisterchromatids have a singlecentromere due to theirbeing attached bycohesin proteins at thecentromere site 49. Centriolesare organelles situated in the cytoplasmclose to the nuclear envelope in cells ofeukaryotic organisms which have cilia orflagella at some stage in their life cycleare absent in higher plants 50. Centriolesdo not possess their own DNA, howeverthey are able to self replicate 51. Centriolesoccur in pairs and lie at right angles to eachother in a region called the centrosomemay be involved in orienting the spindlewhich arises from the centrosomesKinetochoreKinetochore microtubuleMitotic centre(centrosome)Mitotic centre(centrosome)CentriolePolarmicrotubule 52. Centrosomes are organelles located near the nucleus are not membrane-bound and serve as amitotic centre in animals absent in plants divide and migrate to opposite poles of thecell during mitosis 53. Centrosomes centrosomes and NOT centrioles areresponsible to assemble the spindle fibresSpindle fibres form during mitosis and meiosisare composed of microtubules and contain theprotein tubulin 54. Cytoplasmictubulin poolCentrosomaltubulinTubulin recruited toform spindle fibres 55. The spindle serves as a:1. railroad track along which thechromosomes will move2. framework keeping the two polesapart 56. SEP 2013Give TWO functions to illustrate the biologicalimportance of tubulin. (2)1. Tubulin subunits can be assembledtogether to form microtubules.2. They can be disassembled quickly, as duringspindle breakdown.SEP 2013Distinguish between the following pairs ofterms:kinetochore and centromere; (2) 57. NNOOThe spindle fibresoriginate from othermitotic centres 58. Spindle fibres run from:pole to polepole to kinetochore 59. Interphase: A stage prior to mitosisAnimal CellPlant Cell 60. Interphase prior to mitosisvariable duration depending on functionof the celljust before nuclear division the DNA ofeach chromosome replicates 61. Interphase prior to mitosiseach chromosome now exists as a pair ofchromatids joined together by a centromere 62. Interphase prior to mitosisat this stage each cell is 4n (four copies ofeach DNA molecule, two in eachchromosome of a homologous pair)A homologous pair Four copies of eachDNA molecule 63. AAmmoouunntt ooff DDNNAA dduurriinngg cceellll ccyyccllee 64. Interphase prior to mitosisChromosomes: are long & thin dispersed in nucleus cannot be seen individuallyCentrioles have replicatedDiffusedchromosomes 65. MITOSIS 66. G1StelophaseanaphasemetaphaseprophaseMitosis G2interphaseStages ofmitosis in thecell cycle 67. Mitosis is divided into FOURphases:PROPHASE METAPHASEANAPHASE TELOPHASE 68. PROPHASEusually the longest phase of divisionthe chromosomes shorten and thicken bycoiling: condensationindividual chromosomes can be seen 69. Different states of chromosomes throughoutthe life of a cellCondensed chromosomes asin metaphase,anaphaseDiffused chromosomes asin interphase 70. only occurs in eukaryotesMitosis:Mitotic division results in geneticallyidentical eukaryotic cells (a clone) 71. Bacteria divideby binary fission,NOT mitosisNonucleus 72. PROPHASEin animal cells: the centrioles move to opposite poles of the cell asters form Asters: short microtubulesradiating from the centriolesFunction of asters:help to manipulate chromosomesduring nuclear division to ensurethat each daughter cell has theappropriate complement ofchromosomes 73. PROPHASE the nucleoli disappearthe nuclear envelopebreaks up a spindle is formed 74. PROPHASEAnimal Cell Plant CellSpindle fibers 75. What is 2n for this cell? 2n = 8 76. Spindle Apparatusone of the condensedchromosomesspindle equatormicrotubules organisedas a spindle apparatusone spindle poleone spindle pole 77. The spindle fibres form during mitosis and meiosisare composed of microtubules &contain the protein tubulinmicrotubules move chromosomestowards opposite polesTubulin sub-units 78. imaginary planethrough thecenter of the cellwhere thechromosomesalign 79. METAPHASEChromosomes line up around the equator of thespindle, attached by their centromeres to the spindlefibres.AAnniimmaall CCeellllPPllaanntt CCeellll 80. ANAPHASE this stage is very rapid the centromeres split into two 81. ANAPHASE the spindle fibres pull thedaughter centromeres toopposite poles the separated chromatids are pulled behindthe centromeres 82. Why are the chromosomes V shaped?As the chromosome is pulled towards the poleat the centromere / kinetochore. 83. Chromatid attachment and separation 84. ANAPHASEAnimal Cell Plant Cell 85. TELOPHASE the chromosomes: reach the poles of the cell uncoil and lengthen spindle fibres disintegrate nuclear envelope re-forms nucleoli reappear mitosis is complete at theend of telophase 86. TELOPHASE can be viewed as a reversalof prophase, bringing the cell back to thestate of interphaseAnimal Cell Plant Cell 87. InterphaseProphaseMetaphase Anaphase Telophase 88. Cell division is still not complete at theend of mitosis: as the division of thecytoplasm has not yet beguncytokinesis:the division of the cytoplasmcytokinesis normally: follows telophase leads into the G1 phaseof interphase 89. Cytokinesis occurs differently inanimal and plant cells 90. Cytokinesis in animal cellsMicrofilaments drawin the cell surfacemembrane to form acleavage furrow 91. Cytokinesis in animal cells 92. Cytokinesis in plant cells 93. Cytokinesis: Plant CellsCell Plate FormationVesicles from theGolgi apparatusappear at theequatorial plate.Contents ofvesicles form thecell plate thebeginning of thenew cell wall.These fuse to formnew plasmamembrane. 94. Cytokinesis in plant cells 95. Mitosis: plant vs animal cellsPlant cell Animal CellCentrioles,Centrosomes& AsterAbsent PresentCytokinesis Cell plateformationCleavagefurrow 96. Question No. 2 pg 46The drawing shows a stage of mitosisin an animal cell.a)Name this stage of mitosis.Anaphaseb) Describe and explain what happensduring this stage which ensures that twogenetically identical cells are produced.The centromere splits and the sisterchromatids carrying the same geneticinformation move to opposite poles. 97. The table shows the average duration of each stage ofthe cell cycle in the cells of a mammalian embryo.Give one piece of evidence from the table whichindicates that these cells are multiplying rapidly.Stage Mean duration /minutesInterphase 12Prophase 50Metaphase 15Anaphase 10Telophase 42Interphase is short. 98. Question No. 5:A student examined the cells in the garlic roottip under the microscope, and obtained thefollowing data.What do you conclude?Stage Number of cellsProphase 74Metaphase 18Anaphase 8Telophase 10Prophase is theLONGEST phaseand anaphasethe SHORTEST. 99. Question No. 8:The diagram belowshows the life cycleof a simple plant.a) If each spore contained 8 chromosomes, how manychromosomes would there be in:i) a male gamete,ii) the spore-producing stage?881616 100. b) Suggest a reason for the fact thatcommercially produced microscopepreparations used to show meiosis aremade from male rather than femalereproductive organs.Sex cells are more numerous in a male sexorgan and so easier to view. 101. Question: [SEP, 2011]A scientist intends to observe mitosis in cells of thecommon onion (Allium cepa).1. Which part of the onion plant should thescientist use for observing cells undergoingmitosis? (1)Root tip.2. Why is this part of the plant particularlysuitable for this purpose? (2)The root tip is a growing point in plants. Itcontains a meristematic region where cells arecontinuously dividing. 102. 3.Describe how the scientist would processthe specimen (the plant structure referred toin Question 1) in order to be able to observemitosis through a light microscope. Youranswer should refer the steps required toprepare the specimen, to the stainingprocedure and to the temporary mounting ofthe specimen on a microscope slide. (9) 103. The tips have beenstained red withFeulgen stain.AAcceettiicc oorrcceeiinn ssttaaiinnccaann bbee uusseedd ttoooo.. 104. QQuueessttiioonn:: [[SSEEPP,, 22001111]]4. The scientist draws some of the cells thathave been observed. In which stage of thecell cycle are the cells shown below? (4)Cell A: metaphaseCell B: interphaseCell C: anaphaseCell D: prophase 105. CCoollcchhiicciinnee Is made from autumncrocus (Colchicumautumnale) - poisonousAdded to actively dividingcells to inhibit spindleformation 106. Chromatid pairs remain in their metaphasepositionsThis technique enables the number andstructure of chromosomes to be examinedunder the microscope 107. QuestionDividing cells were treated with colchicine. Thisstopped them dividing. After a few hours, thecolchicine was removed and the cells began todivide again. The diagram shows thechromosomes from one of the treated cells afterthe cell began dividing again.What has happened to the chromosome number?Chromosome number has doubled. 108. Question: [MAY, 2006 Paper 3]In an experiment, the researcher applies Colchicine, an alkaloidobtained from Autumn Crocus (Colchicum autumnale) to therelevant parts of the onion bulb. Colchicine inhibits spindleformation in cells during mitosis and is used to halt mitosis atmetaphase.Suggest a possible advantage of the application of Colchicine tothe onion cells.Metaphase is a very rapid phase so in normalpreparations this stage is hard to find and observe.By arresting cells at metaphase more cells can befound in this stage.This allows the researcher to study the number andstructure of chromosomes and take photos. 109. QQuueessttiioonn:: [SEP, 2009]Suggest explanations for each the followingobservation.Colchicine inhibits microtubule formation and istherefore a mitotic poison. (2)Spindle fibres made of microtubules are neededduring mitosis to separate the sister chromatidsduring anaphase.Colchicine inhibits microtubule formation as it doesnot allow tubulin subunits to come together .Thus colchicine is a poison as it interfers with mitosis. 110. Interphase & Mitosis in Plant cellsInterphase Early Prophase Late ProphaseMetaphaseEarly AnaphaseTelophaseLate AnaphaseDaughter cells 111. Question: [MAY, 2012]The graph below illustrates how the quantity ofDNA, within a mammalian somatic cell, variesduring different phases of the cells cycle.Additionally the graph shows the two growthphases, G1 and G2, which are separated by anintermediate phase S.Explain thesignificance ofmitosis in livingorganisms. (3) 112. MMEEIIOOSSIISS 113. Where does meiosisoccur in a male anda female human?Inside testes ina male andovaries in afemale. 114. Overall functions of meiosis: Reduce chromosome number fromdiploid to haploid. Promote genetic diversity among theproducts.Unique phenotypes maygive a reproductiveadvantage to someorganisms. 115. QQuueessttiioonn:: [[MMAAYY,, 22001111]]Name and describe TWO main advantages ofmeiosis relative to mitosis. (4)1) Creates new combinations of geneswhich may result in the offspring being able tosurvive in a changing environment.2) Halves the number of chromosomes sothat after fertilisation, the diploid number isrestored. 116. Features of MeiosisMeiosis includes two rounds of division meiosis I and meiosis II.First meioticdivisionSecond meioticdivisionProphase I Prophase IIMetaphase I Metaphase IIAnaphase I Anaphase IITelophase I Telophase IIShort interphase Haploid cells 117. How is the HAPLOIDcondition achieved?Chromosomes replicateONCE but divide TWICEDuring meiosis I,homologous chromosomes(homologues) separate.2n nDuring meiosis II,sister chromatids separate.n n[just like mitosis] 118. PPrroopphhaassee IIProphase I may last a long time.Human males: about 1 week for prophase I 1 month for entire meiotic cycleHuman females: prophase I begins before birth, andends up to decades later during themonthly ovarian cycle 119. PPrroopphhaassee II Meiosis I is preceded by an S phase during whichDNA is replicated Lasts longer and is more complex than prophase inmitosisInterphase Prophase I(Bivalents) 120. PPrroopphhaassee II chromosomes begin to condensehomologues, each consisting of two sisterchromatids, pair up: synapsisPairing[synapsis] 121. Synapsis during prophase I:process by which bivalents/tetrad formHomologous chromosomessister chromatids Bivalents/Tetrad sister chromatids 122. Crossing over: genetic recombinationbetween non-sister chromatidsplural(chiasmata)- sites of crossing over- hold homologues together 123. Crossing Over 124. The longer the chromosome, the morechiasmata that can form 125. MMeettaapphhaassee II AAnnaapphhaassee II 126. TTeelloopphhaassee II 127. Meiosis I: chromosome numberhas been cut in half end result: 2 haploidcellsMeiosis II:resembles a mitoticdivisionDNA does notreplicate 128. PPrroopphhaassee IIII MMeettaapphhaassee IIII 129. AAnnaapphhaassee IIII TTeelloopphhaassee IIII 130. Anaphase I is thereduction division.Centromere remainsintact 131. Meiosis II NoNointerphase II(or very short -no more DNAreplication)interphase II(or very short -no more DNAreplication)Remember:Meiosis II issimilar to mitosisRemember:Meiosis II issimilar to mitosishttp://highered.mcgraw-hill.com/sites/0072495855/ 132. GGeenneettiicc vvaarriiaattiioonn iiss aallmmoosstt iinnffiinniittee aass aarreessuulltt ooff mmeeiioossiiss 133. Genetic Variation in meiosis occurs inthree ways:1. Crossing over: prophase I2. Random orientation of bivalents:metaphase I3. Random orientation of sisterchromatids : metaphase II 134. 1. Crossing OverCrossing over does not createnew alleles, it simplyrecombines existing ones in newways.Meiosis IMeiosis IINEW COMBINATIONS 135. 2. Random orientation ofbivalentsThe pairs ofchromosomescould orientate indifferent ways atmetaphase Imetaphase Imetaphase Imetaphase IImetaphase II 136. Thus different characteristics arelater expressed by individualsBrown eyesBlack hairBrown eyesRed hairBlue eyesRed hairBlue eyesBlack hair 137. 2. Random orientation of bivalentsNo. of different combinations: 2n2n = 4n = 2No. ofcombinations:22 = 4 138. How many possible combinations?2n2n = 6 139. How many possible combinations?2n = 6n = 3No. ofcombinations:23 = 8 140. QQUUEESSTTIIOONNIn terms of random orientation - how manydifferent combinations of sperm could ahuman male produce? Formula: 2n Human chromosomes: 2n = 46n = 23 223 = ~8 million combinations 141. Genetic Variation in meiosisoccurs in three ways:1. Crossing over: prophase I2. Random orientation of bivalents: metaphase I3.Random orientation ofsister chromatids :metaphase IIhttp://highered.mcgraw-hill.com/sites/007337797x/student_view0/chapter10/animation_ 142. Random orientation of sisterchromatids 143. Independent assortment:in a dihybrid cross, describes the random separationof alleles for each of the genes. For genes ondifferent chromosomes this results from the randomorientations of different homologous pairs duringmetaphase I of meiosis 144. Question: [MAY, 2012]Use your knowledge of biology to explain theevolutionary importance of the following:chiasmata formation and independentassortment during meiosis. (5)Genetic variation is created by each process.Individuals of a population show differentphenotypes. If environment changes, someindividuals are able to adapt to the new conditions,thus the species survives. 145. Random Fertilisation= even more variation A human ovum plus a human sperm- 1 of 8 million combinations possible foreach ovum and sperm- 223 x 223 = over 70 trillion combinationsUUNNIIQQUUEE!!!!Any 2 parents will produce azygote with over 70 trillionpossible combinations withoutconsidering crossing over 146. A comparison of mitosis and meiosis 147. Property Mitosis MeiosisDNA replication DuringinterphaseDuringinterphaseDivisions One TwoSynapsis andcrossing overDo not occur Form bivalentsin prophase IDaughter cells,geneticcompositionTwo diploid,identical toparent cellFour haploid,different fromparent cell andeach otherRole in animalbodyProduces cellsfor growth andtissue repairProducesgametes 148. 1)No synapsis occurs2)No bivalents form3)No crossing over occurs4)Two cells produced no halving ofchromosome number 149. The DNA content of the cell as shown belowis 10 units.What will the DNA content be at the end ofmitosis? 150. DNA content and mitosis 151. DNA ContentDNA duplicationduring interphaseMitosis20 units40 units20 units 20 units 152. Question No. 3 pg. 47Diagrams A, B and C below show the same stagein mitosis, meiosis I and meiosis II in a plant cell.a)Identify the stage shown giving yourreason.Metaphase chromosomes aligned at equator. 153. b) The cell in diagram Ahas 20 units of DNA.How many units of DNAwould there be in a cellfrom this plant at theend of:Cell A has 8 chromatids =20 units.At end of mitosis, 4chromosomes present.So = 10 unitsCell A has 8 chromatids =20 units.At end of mitosis, 4chromosomes present.So = 10 unitsii) meiosis?2n for Cell A is 4chromosomes = 10 units.At end of meiosis, 2chromosomes present.So = 5 units2n for Cell A is 4chromosomes = 10 units.At end of meiosis, 2chromosomes present.So = 5 unitsii) mitosis? 154. Question No. 5 pg. 47The simplified diagram below shows a nucleusfrom a cell of an insect.a) Draw diagrams to show the two nuclei which would beformed if the cell would divide by mitosis. (2) 155. b) Draw diagrams to show three differentnuclei which could be formed if thecell divided by meiosis. (Assume thatno crossing over has taken place). (3)Any 3 from: 156. The next few slides are about DownSyndrome which actually comes in anotherunit, but repeating does no harm!!The following information is in Evolutionsection. 157. Down Syndrome: A meiotic errorCause: Nondisjunction during either:aannaapphhaassee IIanaphase II 158. aannaapphhaassee II anaphase IIHHoommoollooggoouusscchhrroommoossoommeess ffaaiill ttoosseeppaarraatteeSister chromatidsfail to separate 159. Down Syndrome: trisomy 21 In humans, if both chromosome 21 homologuesgo to the same pole, and the resulting egg isfertilised, it will be trisomic for chromosome 21.EggcellSpermcelln + 1n (normal)Zygote2n + 1 160. DDOOWWNN SSYYNNDDRROOMMEEKKAARRYYOOTTYYPPEE 161. The chance of having a Down syndrome childgoes up with maternal age 162. Egg is in center of pictureMany cumulus cells fromthe ovary are seen aroundthe egg. This is a lowquality, oocyte from awoman 41 years old.Egg is irregularly shapedand dark.A "good" egg from a32 year old woman.Photos from Advanced Fertility Clinic in Chicago 163. Question: [SEP, 1999]The diagram below represents the karyotype ofa human.a) Why does thestructure shownin Figure 2 havean X-shapedform? (2)Two sister chromatidsare joined at thecentromere. 164. b) Does the karyotype in Figure 1 belong to amale or to a female? Give ONE reason foryour answer. (2)Male.Sex chromosomes shown in set 23 are notequal in length. 165. c) This karyotype indicates a disorder that isgenetic in origin. What condition is this?Give ONE reason for your answer. (2)Down syndrome.Three copies of chromosome 21 are present. 166. d) How does the condition referred to inquestion (c) arise? (4)Nondisjunction of either homologouschromosomes in anaphase I or ofsister chromatids in anaphase II.Sex cell with an extra chromosome isfertilised resulting in zygote having 47chromosomes. 167. Nondisjunction can also produce gametes withextra or missing sex chromosomes 168. Nondisjunction of the XChromosome 169. Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical TraitsXXY, XXYY, XXXY male Klinefeltersyndromesterility, smalltesticles, breastenlargementXYY male XYY syndrome normal male traitsXO female Turner syndromesex organs do notmature atadolescence,sterility, shortstatureXXX female Trisomy Xtall stature,learningdisabilities,limited fertility 170. QQuueessttiioonn:: [SEP, 2009]Suggest explanations for each of thefollowing observations.1.Meiosis generates biological diversityin eukaryotes. (2)New combinations of alleles arisethrough crossing-over and randomorientation of both bivalents and sisterchromatids. 171. Question: [SEP, 2009]Suggest explanations for each of the followingobservations.2. Klinefelters Syndrome, where individuals wouldbe characterised by a XXY combination of sexchromosomes, is caused by an anomalous eventduring meiosis. (2)Due to non-disjunction of sex chromosomes,one cell would result in having two copies of the Xchromosome rather than one. This can happen ateither anaphase I or II. 172. Question No. 1Read the passage below and insert the appropriateterms from the table below in the blank spacesprovided. Note that each term in the table may be usedonce, more than once or not at all.Fertilisation Two Crossing-over Four Forty-sixGenetic variation Haploid Homologous Chromatids EightGametes Diploid Twenty-three Chromosomes NucleusMeiosis is an important biological process that reducesthe chromosome number by half during the formation of gametesin animals. During the first meiotic"reduction division," the chromosomal pairs arepartitioned so that each gamete contains one of eachchromosomal pair that is it becomes .haploid 173. fertilisationWhen haploid gametes unite during theresulting zygote is diploidhaving received onechromosome of each pair from each parent.Meiosis involves twosuccessive nuclear divisionsthat produce fourhaploid daughter cells. The firstdivision (meiosis I) is the reduction division; thesecond division (meiosis II) separates the.. chromatids.Meiosis increases genetic variationin thepopulation. Each diploid cell undergoing meiosis canproduce 2n different chromosomal combinations, wheren is the haploid number. 174. twenty threeIn humans the haploid number is .. whichtherefore provides more than eight million differentcombinations. In addition, the variation increasesdramatically because during meiosis I, each pair of. homologouschromosomes comes together in aprocess known as synapsis. Chromatids ofhomologous chromosomesmay exchange parts ina process called crossing over. 175. 4. Cell A in the diagram opposite has two pairsof chromosomes. Cell B is one of four cellsthat were produced from cell A by nucleardivision.a) State the type of nuclear division which hasoccurred to produce cell B, and give areason for your answer. (2)Meiosis.Number of chromosomesis halved / crossingover has occurred. 176. b) Explain what happened during thisnuclear division which resulted in thedifferent appearance of chromosomeP. (3)Crossing over has occurred between thenon sister chromatids. Genes areexchanged at points calledchiasmata.c) Give the exact location in a humanmale where this type of nucleardivision occurs. (1)Seminiferous tubules in the testis. 177. 9. This question is concerned with celldivision.Give a brief description of the Cell Cycle.(2)When a somatic cell reaches itsmaximum size, its nucleus divides bymitosis, then the cytoplasm divides.Each daughter cell enters a growthphase called interphase.[May 2006]