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4.1TheNatureofHeredity Heredityitselfistheprocessoftraitsbeingpassedfromparentstooffspring.Thereare severalimportanttermstounderstandbeforewespeakofasexualandsexual reproduction. Geneticsisthestudyofheredity.Geneticmaterialisatermthatdescribesallthe materialinanorganismthatcontainsthegeneticinformation.Inachromosomethat materialisreferredtoasDNA. AgeneisthesegmentofaDNAmoleculethatcontainscodesforaspecifictrait,whichis afoundatapreciselocationinthechromosome.Thatlocationiscalledthelocus. AsexualReproduction:theformofreproductioninwhichanewindividualisproduced fromasingleparentbycelldivision.Thismeansthatthecelldoesnotrequireapartner toreproduce.Thedaughtercellisanexactgeneticreplicaoftheparentcell. Theadvantagestoasexualreproductioninclude:theparentcelldoesnothavetoseeka mate,itdoesnothavetoformspecializedmatingtechniquesorhavespecial reproductiveorgans. SexualReproduction:Theproductionofoffspringfromthefusionoftwocells.Thesex cellsusuallycomefromtwodifferentorganisms.Theoffspringproducedfromtwo parentsisnotgeneticallyidenticaltoeither.Theycontainequalamountsofgenetic informationfromeitherparent. Advantagestosexualreproduction:ensuresdiversitywhichcanthenmaketheanimal adapttoachangingenvironment,orimmunetodiseases. Disadvantages:producingmatingcallsorhavingbrightlycolouredfeatherscanattracta matebutalsocanattractpredators.Sexisbiologicallycostlyandpotentiallyrisky. TheCellCycle Atalltimes,cellsaredoing oneoftwothings: 1.Growing 2.Dividing Thecellcycleisthispatternof growthanddivision. Interphase:GrowthconsistsoftheG1,SandG2phase.TheG1isthepreparationfor copyingDNA(rapidgrowth),Sisthesynthesis,oractualcopying,ofDNA.TheDNAat

GENETICS

thispointisintheformofchromatinandtheG2phaseisthepreparationforcell division. Mitoticphaseisthedivisionofthecell(eitherasexually[mitosis]orsexually[meiosis]). Thegeneticmaterialdivides. Cyokinesis:Divisionofthecytoplasmandorganellesintotwoseparatecells(daughter cells) CellDivision:Terms(Refertopg143) Chromatin:uncondensedformofDNA,longstrands(thisstageofDNAexistsin Interphase) Chromosomes:thecondensedformofDNAformsduringMitosis/Meiosis SisterChromatids:twoidenticalsisterchromatids(duplicatedduringInterphase)are attachedbythecentromeretoformachromosomeduringProphase Centromere:theattachingdevice Centrosome:Thestructurethatwillformspindlefibres Diploid:23Chromosomesdoubled=46.(2n) Haploid:23Chromosomessingle=23(n) Somatic:allcellsthatgrow(excludinggametes) Gametes:cellsusedspecificallyforfusionwithanothergamete(sperm+ovum=zygote) Humanshave23Chromosomes(doubled)=46.Theyhaveasetof23fromthemother, andasetof23fromthefather. 4.2CellDivisioninAsexualCells MitosisandCytokinesisformtheprocessofasexualcelldivision. Thegeneticmaterialinthenucleusofaeukaryoticcellisdividedequallyintotwonew nuclei.TherestofthecelldividesduringCytokinesis. ThepurposeofMitosisistoduplicatethe46singlestrandedchromosomes(into92)and thendividesoeachdaughtercellsisstill2n,or46chromosomes(diploid).Allsomatic cellsundergomitosis. Mitosis=PMAT(Prohpase,Metaphase,Anaphase,Telophase) 1. Prophase:thefirstphaseofmitosis.Thechromosomesreallystarttoform,the nuclearmembranedisappearsandspindlefibresareformedbythecentrioles. 2. Metaphase:Thespindlefibresalignthechromosomesinthemiddle. 3. Anaphase:thecentromeresdivideandthesisterchromatids(referredtoas chromosomesagain)separateandmovetooppositepolesofthecell.Thesame numberofchromosomesshouldbeateachside. 4. Telophase:thechromosomesbegintounwindbackintochromatin(strands). Thespindlefibresdissolveandthenuclearmembranereappears.Theresultis twodaughternuclei.

Cytokinesis:Thecytoplasmoftheeukaryoticcelldividestoformtwonewcells.Cellthen reentersInterphase,andpreparestogothroughthisallagain!

4.3CellDivisioninSexualCells Theprocessofsexualcelldivisionismeiosis+Cytokinesis. Purposeofsexualreproduction:Theresultingdaughtercellshavehalfthenumberof chromosomesastheparentcells.Parentcellsarediploid(2n)anddaughtercellsare haploid(n).Thesehaploidcellswillundergoasimilarprocesstomitosistoproduce identicaldaughtercells.Thesearecalledgametesorsexcells.Afemalegamete (ovum)willfusewithamalegamete(sperm)toformazygotethathas(2n) Termstoknow: Homologouschromosomes:matchingpairsofchromosomes,similarinsizeandcarrying informationforthesamegenes.However,onechromosomemighthaveadifferent versionofthegene(egblueeyes)whereasitshomologouspairmighthaveanother version(eggreeneyes).Thesearecalledalleles. Autosomes:Chromosomes122inhumans SexChromosome:Chromosome23inHumansisthesexchromosome.Infemalesitis XX,andinmalesitisXY. Meiosis

Meiosisismadeupoftwophases,eachwith8stages,separatedbyCytokinesis. MeiosisI:ProphaseI,MetaphaseI,AnaphaseI,TelophaseI MeiosisII:ProphaseII,MetaphaseII,AnaphaseII,TelophaseII MeiosisI ProphaseI:Dnacondenseintochromosomesfromchromatin. Homologouschromosomes(correspondingchromosomesfrommomand dad)synapseandcrossover,exchangeDNAfordiversity.Thelinedup chromosomesformatetrad.Thenuclearmembranebreaksdown,meiotic spindleformsandcentrosomesappearatthepolesofthesell.Thechromosomesattach tothespindlefibresatthecentromeres. MetaphaseI:Homologouschromosomes(eg.Ch1frommomandCh1from dad)lineupbesideeachotheralongthemetaphaseplate(centreofthe cell) AnaphaseI:Homologouschromosomesmovetooppositesideofthecell. TelophaseI:Cellbeginstopinchoff(cleavage)andmeioticspindlebreaksdown.The chromosomesdonotextend(differentthanmitosis),andthenuclearmembranedoes notreform. Cytokinesis:Organellesandcytoplasmaredividedamongdaughtercells,andthecell membranepinchesoff.Thesetwonewdaughtercellsarehaploid(n),becausetheyonly haveonesetofchromosomes. MeiosisII MeiosisIIisverysimilartoMitosisinthatittakesthedaughtercellsfromMeiosis1, whicharehaploid,butduplicatesof23chromosomes,anddividethemasexuallysothat eachnewcellhas23singlechromosomes,andishaploid. ProphaseII:Meioticspindlereformsandchromosomesreturntospindle MetaphaseII:Chromosomeslineupbeneathoneanotheralongthemetaphasicplate. AnaphaseII:Sisterchromatidsseparateandmovetopolesofthecell TelophaseII:Cellbeginstopinchoff,meioticspindlefibrebreaksdownand chromosomesextendbackintochromatin.Thenuclearmembranealsoreforms.

Cytokinesis:Organellesandcytoplasmaredividedamongthedaughtercells.Thecell membranepinchesoff. BacktoInterphase:thecellsrestarttheirnormalactivities. TheDifferenceBetweenMitosis&Meiosis

Section4.4AbnormalMeiosisand/orFertilization Sometimeserrorscanoccurduringthedivisionprocess.Oneoftheseerrorsiscalled nondisjunction.NondisjunctioncanoccurinAnaphaseI,orAnaphaseII. NonDisjunctionoccurswhenhomologouschromosomesfailtomovetooppositepoles ofthecellduringmeiosis.Thisresultsinanabnormalnumberofchromosomesina daughtercell.Lookatfigure1onpage161.

Section5.1MendelianInheritance Termstoknow: Reviewchromosome,locus,andallele Gene:AsegmentofDNAthatprovidestheinstructionsforthebuildingandregulation ofafunctionalmoleculeinthecell. Homozygous/Heterozygous: Recallthatwespokeofhomologouschromosomes(1M1D).Homologous chromosomeshavethesamecharacteristicsandshape.Theyhaveacopyofeverygene inthatchromosome.Inonechromosomethegenemaybethesameordifferenttothe samegeneinanotherchromosome.Theversionofthegenesiscalledtheallele.Ifthe allelesarethesametheindividualissaidtobehomozygous,iftheyaredifferentthey aresaidtobeheterozygous. Genotype&Phenotype: Genotype:Thecombinationofallelesapersonhasatalocus.Forexample:Bb(B= browneyes,b=blueeyes) Phenotype:thephysicalcharacteristicsproducedbythegenotype(egBrowneyes) MendelsCrosses Mendel:amonkwhocrossedpeaplantsbecausetheyhavemanytraitswithtwo specificforms.Mendelcouldseehowtheyappearedordisappearedinprogeny.First, Mendelhadtoproducedpeaplantspurebredforspecifictraitsforconsistency. Mendelcrossedapurebredwithanotherpurebredthathadadifferentcharacteristic. Thesetwoplantswerecalledhisparentalgeneration,orPgeneration.Thehybrid offspringofthePgenerationcrosswerethefilialgeneration,orF1generation.The offspringonlydifferedfromeachotherthroughonecharacteristic,makingthem monohybrid. TheresultsofMendelsfirstcrossrevealedthatoneofthetwotraitscompletely disappeared.WhenMendelallowedtheF1generationofplantstocross,andproduce F2,henoticedthatthedisappearingtraitreappeared.HemeasuredtheratiooftheF2 crosstobe3:1(3beingthetraitthatappearedinF1,1beingthedisappearingtrait). MendelsconclusionledtothefirstlawofMendelianInheritance LawofSegregation:Foreachcharacteristic(suchasflowercolour)anorganismcarries twofactors(genes),onefromeachparent.Parentorganismsonlydonateonecopyof eachgeneintheirgametes.Duringmeiosis,thetwocopiesofeachgeneseparate(or segregate,hencelawofsegregation)

Thequestionis,inheterozygousindividuals(recallaboveapairofhomologous chromosomeswithdifferentalleles)whichalleleisexpressed? Theanswerisbasedondominance&rececessivenessofalleles: DominantAllele:Theallelethat,ifpresent,isalwaysexpressed. RecessiveAllele:Theallelethatisexpressedonlyifitisnotinthepresenceofthe dominantallele.Theindividualmustbehomozygousfortherecessiveallele. Inthegenotype,thedominantalleleisrepresentedbyanuppercaseletter(egbrown eyes=B)andtherecessivealleleisrepresentedbyalowercaseletter(egblueeyes=b) E.g.anindividualcouldbe: 1. Heterozygous(Bb) 2. HomozygousforDominantAllele(BB) 3. HomozygousforRecessiveallele(bb) Inoptions1&2,theindividualwillhavebrowneyes,becausetheDominantAlleleis present.Inoption3theindividualwillhaveblueeyes,becausetherecessivealleleisnot maskedbyanydominantone. ThePunnettSquare ThePunnettSquareisadiagramthatsummarizeseverypossible combinationofeachallelefromeachparent.Itisthetooltodetermine theprobabilityofasingleoffspringhavingaparticulargenotype.For example,inthediagramtotheleft,theratiowouldbe:1GG:2Gg:1gg HereisanexamplePunnettSquareproblem:(Pg.189) Twoheterozygousyellowseedplants(Yy)arecrossed.Determinethegenotypeand phenotyperatiosoftheF2generation. 1. Defineyouralleles.Y=yellowseed,y=greenseed. 2. Determinethegenotypesoftheparents(YyxYy) 3. Determinethepotentialgameteseachplantcouldproduce:Pl