molecular genetics of the color mutations in rock … · 2019-05-01 · mc1r gene that is involved...
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LESSONSTUDENTHANDOUT
The Making of the Fittest: Natural Selection and Adaptation
Lesson The Making of the Fittest Natural Selection and Adaptation
Student Handout
PublishedFebruary,2017
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MOLECULARGENETICSOFTHECOLORMUTATIONSINROCKPOCKETMICE
INTRODUCTIONTHEROCKPOCKETMOUSE
Therockpocketmouse,Chaetodipusintermedius,isasmall,nocturnalanimalfoundinthedesertsofthesouthwesternUnitedStates.Mostrockpocketmicehaveasandy,light-coloredcoatthatenablesthemtoblendinwiththelightcolorofthedesertrocksandsandonwhichtheylive.However,populationsofprimarilydark-coloredrockpocketmicehavebeenfoundlivinginareaswherethegroundiscoveredinadarkrockcalledbasaltcausedbygeologiclavaflowsthousandsofyearsago.Scientistshavecollecteddatafromapopulationofprimarilydark-coloredmicelivinginanareaofbasaltcalledthePinacatelavaflowinArizona,aswellasfromanearbylight-coloredpopulation.Researchersanalyzedthedatafromthesetwopopulationsinsearchofthegeneticmutationresponsibleforthedarkcoatcolor.TheiranalysesledtothediscoveryofamutationintheMc1rgenethatisinvolvedincoat-colordetermination.
THEMC1RGENE
Twopigmentsprimarilydeterminethecoatcolorofrockpocketmice:eumelanin,whichisdark-colored,andpheomelanin,whichislight-colored.Theproductsofseveralgenes,includingtheMc1rgene,controlthesynthesisofthesepigments.Thisgeneencodesaproteincalledthemelanocortin1receptor(MC1R)andisfoundembeddedinthecellmembranesofmelanocytes,specializedpigment-producingskincells.Themelanocytesofwild-type(nonmutant)miceproducemorepheomelaninthaneumelanin.Theresultisasandy-coloredmouse.ThemutatedversionoftheMc1rgene,however,triggersmelanocytestoincreasetheproductionofeumelanin,resultinginthedarkcoat-colorphenotype.
GENEMUTATION
AgenemutationisanychangeintheDNAsequenceofagene.Genemutationscanchangethestructureoftheresultingprotein.Achangeinproteinstructurecanchange,negate,orhavenoeffectonfunction.Thereareseveraltypesofmutations,andtheyaffecttheaminoacidsequencesofproteinsindifferentways.
Typesofmutations:
• Substitutionmutation:thereplacementofonenucleotideofDNAforanother.Mutationsthataffectasinglenucleotidearecalled“pointmutations.”
• Insertionmutation:theadditionofoneormorenucleotide(s)totheDNAgenesequence.Theinsertionofnucleotide(s)canresultin“frame-shift”mutations.
• Deletionmutation:thelossofoneormorenucleotide(s)fromtheDNAgenesequence.Thedeletionofnucleotide(s)canresultin“frame-shift”mutations.
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Lesson The Making of the Fittest Natural Selection and Adaptation Student Handout
PublishedFebruary,2017
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Potentialeffectsagenemutationhasonaprotein:
• Silentmutation:Thismutationdoesnotcauseachangeintheaminoacidsequenceoftheprotein;therefore,thereisNOchangeintheresultingprotein.
• Missensemutation:Thismutationcausesanaminoacidinthesequencetobechangedtoanotheraminoacid.Thistypeofmutationcausesachangeintheprimarystructureoftheprotein(thelinearsequenceofaminoacids),whichcanresultinachangeinthethree-dimensionalconformationoftheprotein.
• Nonsensemutation:Thismutationcausestheproteintobetruncated(cutshort)duetotheincorporationofa“stop”signalintotheDNAsequence.Thisresultsintranslationbeingstoppedbeforetheaminoacidsequenceoftheproteiniscompleted.
MATERIALS• geneticcodechart(seepage6ofthishandoutorabiologytextbook)• blue,red,andgreencoloredpencils
PROCEDURE1.WatchtheHowardHughesMedicalInstitute’s10-minutefilmTheMakingoftheFittest:NaturalSelectionandAdaptation.
2.UsingtheDNAnucleotidesequencesforthewild-typeandmutantgenesinthefollowingtables(page3),determinethecomplementarymRNAsequenceforthefiveportionsoftheMc1rgeneprovided.(Note:YouareonlytranscribingshortportionsoftheDNAsequenceforthisprotein.Theactualgenecontains954basepairs.)
3.UsingthemRNAsequencecompletedinStep2,determinetheresultingaminoacidsequenceoftheMC1Rprotein.(Note:Youaretranslatingonlyaportionofprotein.Thefullproteinis317aminoacidslong.Thenumbersabovethecolumnsinthetablesindicateaminoacidpositionsintheproteinsequence.)Youmayusethegeneticcodechartprovidedinyourtextbookortheoneonpage6ofthishandout.
4.Therearefivesubstitutionmutationsinthedark-coloredmutantMc1rgene.ComparetheDNAsequenceofthelight-coloredwild-typeMc1rgenewiththeDNAsequenceofthedark-coloredmutantMc1rgene.IndicatethelocationsofthefivemutationsbycirclingthefivesingleDNAnucleotidesthataremutatedinthemutantMc1rgenetable.
5.Usingtheinformationintheintroduction,determinewhethereachofthesemutationsisasilent,missense,ornonsensemutation.
a.UsingthemutantMc1rgenedata,shadeinthecolumns(includingDNA,mRNA,andaminoacid)ingenetable2thatcontainasilentmutationusingaBLUEcoloredpencil.
b.Likewise,shadeinthecolumnsthatcontainamissensemutationusingaREDcoloredpencil.
c.ShadeanycolumnsthatcontainnonsensemutationsusingaGREENcoloredpencil.
6.Answerthequestionsfollowingthegenetables.
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Lesson The Making of the Fittest Natural Selection and Adaptation Student Handout
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GENETABLE1:WILD-TYPEMC1RGENE(LIGHTCOAT-COLORPHENOTYPE) 015 022
DNA TTG AGG TGG GCG TGT CCG CAA GGAmRNA
AminoAcid
105 112DNA CGG GAC CGG TGG GCC CAC TGA CACmRNA
AminoAcid
154 161DNA TCA TAA CAC TGT GAC GGG GCC CGAmRNA
AminoAcid
209 212DNA GTG TAC GAA CGTmRNA
AminoAcid
230 237DNA GAA CAG GTG GTT CCA AAG GCT GAGmRNA
AminoAcid
GENETABLE2:MUTANTMC1RGENE(DARKCOAT-COLORPHENOTYPE) 015 022
DNA TTG AGG TGG ACG TGT CCG CAA GGAmRNA
AminoAcid
105 112DNA CGG GAC CGG TGG ACC CAC TGA CACmRNA
AminoAcid
154 161DNA TCA TAA CAC TGT GAC GGG ACC CGAmRNA
AminoAcid
209 212DNA GTG TAC GAG CGTmRNA
AminoAcid
230 237DNA GAA CAG GTG GTG CCA AAG GCT GAGmRNA
AminoAcid
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Lesson The Making of the Fittest Natural Selection and Adaptation Student Handout
PublishedFebruary,2017
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QUESTIONS1.Ingeneexpression,whichenzymeisresponsiblefortranscribingtheDNAsequenceintomRNAbyaddingcomplementaryRNAnucleotides?
2.Inaeukaryoticcell,wheredoestranscriptionoccur?
3.Describetheprocessoftranslation.
4.Inaeukaryoticcell,whichmainorganelleisinvolvedintranslation?
5.ExplaintherelationshipbetweenDNAsequence,aminoacidsequence,andproteinstructureandfunction.
6.TheMc1rgenecodesforthe protein.
7.IftheMC1Rproteinis317aminoacidslong,whyarethere954basepairsinthecodingregionofthegene?
8.OfthefivemutationsyouidentifiedinthemutantMc1rgene,howmanyare:
substitutions insertions deletions(Enteranumberoneachline.)
9.OfthefivemutationsyouidentifiedintheMc1rgene,howmanyare:
silent missense nonsense(Enteranumberoneachline.)
10.Micewiththewild-type(nonmutant)Mc1rgenehavelight-coloredfur.Whichpigmentisresponsibleforthiscoloration?
11.Usingtheinformationintheintroductiononmutationsandyourknowledgeofproteins,developahypothesistoexplainhowthechangesintheMC1Rprotein’saminoacidsequencemightaffectitsfunction.
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Lesson The Making of the Fittest Natural Selection and Adaptation Student Handout
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12.Explainhowsilentmutationsaffectthestructureandfunctionoftheprotein
13.Usingtheinformationprovidedintheintroduction,explainhowthemutantMC1Rproteindirectlyaffectsarockpocketmouse’scoatcolor.
14.Mutationsareasourceofgeneticvariation.Inthefilm,Dr.Carrollsaysthatmutationsoccurrandomly.Whatdoesthatmean?
15.Itisacommonmisconceptionthat“allmutationsarebad.”Usetheexampleofrockpocketmicetoexplainwhythisisnottrue.Inyouranswer,explainhowthedarkcoatcolormutationcanbeanadvantagetosomemiceandadisadvantagetoothers.
16.Useyourunderstandingofevolutionandtheinformationinthefilmtoexplainhowthedark-coloredmutationcametobesocommoninsomepopulationsofrockpocketmice.Bespecific.
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Lesson The Making of the Fittest Natural Selection and Adaptation Student Handout
PublishedFebruary,2017
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GENETICCODECHART
Belowisthestandardgeneticcode:mRNAàaminoacid.Theinnercirclerepresentsthefirstletterofthecodonfollowedbythesecondandthirdring.Findthecorrespondingaminoacidintheoutercircle.