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www.BioInteractive.org LESSON STUDENT HANDOUT The Making of the Fittest: Natural Selection and Adaptation Lesson The Making of the Fittest Natural Selection and Adaptation Student Handout Published February, 2017 Page 1 of 6 MOLECULAR GENETICS OF THE COLOR MUTATIONS IN ROCK POCKET MICE INTRODUCTION THE ROCK POCKET MOUSE The rock pocket mouse, Chaetodipus intermedius, is a small, nocturnal animal found in the deserts of the southwestern United States. Most rock pocket mice have a sandy, light-colored coat that enables them to blend in with the light color of the desert rocks and sand on which they live. However, populations of primarily dark- colored rock pocket mice have been found living in areas where the ground is covered in a dark rock called basalt caused by geologic lava flows thousands of years ago. Scientists have collected data from a population of primarily dark-colored mice living in an area of basalt called the Pinacate lava flow in Arizona, as well as from a nearby light-colored population. Researchers analyzed the data from these two populations in search of the genetic mutation responsible for the dark coat color. Their analyses led to the discovery of a mutation in the Mc1r gene that is involved in coat-color determination. THE MC1R GENE Two pigments primarily determine the coat color of rock pocket mice: eumelanin, which is dark-colored, and pheomelanin, which is light-colored. The products of several genes, including the Mc1r gene, control the synthesis of these pigments. This gene encodes a protein called the melanocortin 1 receptor (MC1R) and is found embedded in the cell membranes of melanocytes, specialized pigment-producing skin cells. The melanocytes of wild-type (nonmutant) mice produce more pheomelanin than eumelanin. The result is a sandy- colored mouse. The mutated version of the Mc1r gene, however, triggers melanocytes to increase the production of eumelanin, resulting in the dark coat-color phenotype. GENE MUTATION A gene mutation is any change in the DNA sequence of a gene. Gene mutations can change the structure of the resulting protein. A change in protein structure can change, negate, or have no effect on function. There are several types of mutations, and they affect the amino acid sequences of proteins in different ways. Types of mutations: • Substitution mutation: the replacement of one nucleotide of DNA for another. Mutations that affect a single nucleotide are called “point mutations.” • Insertion mutation: the addition of one or more nucleotide(s) to the DNA gene sequence. The insertion of nucleotide(s) can result in “frame-shift” mutations. • Deletion mutation: the loss of one or more nucleotide(s) from the DNA gene sequence. The deletion of nucleotide(s) can result in “frame-shift” mutations.

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Page 1: MOLECULAR GENETICS OF THE COLOR MUTATIONS IN ROCK … · 2019-05-01 · Mc1r gene that is involved in coat-color determination. THE MC1R GENE Two pigments primarily determine the

www.BioInteractive.org

LESSONSTUDENTHANDOUT

The Making of the Fittest: Natural Selection and Adaptation

Lesson The Making of the Fittest Natural Selection and Adaptation

Student Handout

PublishedFebruary,2017

Page1of6

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

Page2of6

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

PublishedFebruary,2017

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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

PublishedFebruary,2017

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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.


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