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SAMPLE13

[医学类]

Untilaboutfiveyearsago,theveryideathatpeptidehormonesmightbemadeanywhereinthebrainbesidesthehypothalamuswasastounding.Peptidehormones,scientiststhought,weremadebyendocrineglandsandthehypothalamuswasthoughttobethebrains’onlyendocrinegland.Whatismore,becausepeptidehormonescannotcrosstheblood-brainbarrier,researchersbelievedthattheynevergottoanypartofthebrainotherthanthehypothalamus,wheretheyweresimplyproducedandthenreleasedintothebloodstream.

Butthesebeliefsaboutpeptidehormoneswerequestionedaslaboratoryafterlaboratoryfoundthatantiserumstopeptidehormones,wheninjectedintothebrain,bindinplacesotherthanthehypothalamus,indicatingthateitherthehormonesorsubstancesthatcross-reactwiththeantiserumsarepresent.Theimmunologicalmethodofdetectingpeptidehormonesbymeansofantiserums,however,isimprecise.Cross-reactionsarepossibleandthismethodcannotdeterminewhetherthesubstancesdetectedbytheantiserumsreallyarethehormones,ormerelycloserelatives.Furthermore,thismethodcannotbeusedtodeterminethelocationinthebodywherethedetectedsubstancesareactuallyproduced.

Newtechniquesofmolecularbiology,however,provideawaytoanswerthesequestions.ItispossibletomakespecificcomplementaryDNA’s(cDNA’s)thatcanserveasmolecularprobesseekoutthemessengerRNA’s(mRNA’s)ofthepeptidehormones.Ifbraincellsaremakingthehormones,thecellswillcontainthesemRNA’s.Iftheproductsthebraincellsmakeresemblethehormonesbutarenotidenticaltothem,thenthecDNA’sshouldstillbindtothesemRNA’s,butshouldnotbindastightlyastheywouldtomRNA’sforthetruehormones.ThecellscontainingthesemRNA’scanthenbeisolatedandtheirmRNA’sdecodedtodeterminejustwhattheirproteinproductsareandhowcloselytheproductsresemblethetruepeptidehormones.

ThemolecularapproachtodetectingpeptidehormonesusingcDNAprobesshouldalsobemuchfasterthantheimmunologicalmethodbecauseitcantakeyearsoftediouspurificationstoisolatepeptidehormonesandthendevelopantiserumstothem.Roberts,expressingthesentimentofmanyresearchers,states:“Iwastrainedasanendocrinologist.Butitbecamecleartomethatthefieldofendocrinologyneededmolecularbiologyinput.Theprocessofgrindingoutproteinpurificationsisjusttooslow.”

If,astheinitialtestswithcDNAprobessuggest,peptidehormonesreallyaremadeinbraininareasotherthanthehypothalamus,atheorymustbedevelopedthatexplainstheirfunctioninthebrain.Somehavesuggestedthatthehormonesareallgrowthregulators,butRosen’sworkonratbrainsindicatesthatthiscannotbetrue.Anumberofotherresearchersproposethattheymightbeusedforintercellularcommunicationinthebrain.

1.Whichofthefollowingtitlesbestsummarizesthetext?

[A]IsMolecularBiologytheKeytoUnderstandingIntercellularCommunicationintheBrain?

[B]MolecularBiology:CanResearchersExploitItsTechniquestoSynthesizePeptideHormones?

[C]TheAdvantagesandDisadvantagesoftheImmunologicalApproachtoDetectingPeptideHormones.

[D]PeptideHormones:HowScientistsAreAttemptingtoSolveProblemsofTheirDetectionandtoUnderstandTheirFunction?

2.Thetextsuggeststhatasubstancedetectedinthebrainbyuseofantiserumstopeptidehormonesmay

[A]havebeenstoredinthebrainforalongperiodoftime.

[B]playnoroleinthefunctioningofthebrain.

[C]havebeenproducedinsomepartofthebodyotherthanthebrain.

[D]haveescapeddetectionbymolecularmethods.

3.Accordingtothetext,confirmationofthebeliefthatpeptidehormonesarecreatedinthebraininareasotherthanthehypothalamuswouldforcescientiststo

[A]rejectthetheorythatpeptidehormonesaremadebyendocrineglands.

[B]revisetheirbeliefsabouttheabilityofantiserumstodetectpeptidehormones.

[C]inventtechniquesthatwouldallowthemtolocateaccuratelybraincellsthatproducepeptidehormones.

[D]developatheorythataccountfortheroleplayedbypeptidehormonesinthebrain.

4.Whichofthefollowingismentionedinthetextasadrawbackoftheimmunologicalmethodofdetectingpeptidehormones?

[A]Itcannotbeusedtodetectthepresenceofgrowthregulatorsinthebrain.

[B]Itcannotdistinguishbetweenthepeptidehormonesandsubstancesthatareverysimilartothem.

[C]Itusesantiserumsthatareunabletocrosstheblood-brainbarrier.

[D]Itinvolvesapurificationprocessthatrequiresextensivetraininginendocrinology.

5.TheideathatthefieldofendocrinologycangainfromdevelopmentsinmolecularbiologyisregardedbyRobertswith

[A]incredulity.

[B]derision.

[C]indifference.

[D]enthusiasm.