25个考研英语阅读经典试题及答案(13)医学类.pdf

25个考研英语阅读经典试题及答案(13)

SAMPLE13

[医学类]

题目序号题型归类

第1题中心主旨题型

第2题审题定位题型

第3题细节推导题型

第4题句间关系题型

第5题细节推导题型

Untilaboutfiveyearsago,theveryideathatpeptidehormonesmightbemadeanywherein
thebrainbesidesthehypothalamuswasastounding.Peptidehormones,scientiststhought,were
madebyendocrineglandsandthehypothalamuswasthoughttobethebrains’onlyendocrine
gland.Whatismore,becausepeptidehormonescannotcrosstheblood-brainbarrier,researchers
believedthattheynevergottoanypartofthebrainotherthanthehypothalamus,wheretheywere
simplyproducedandthenreleasedintothebloodstream.

Butthesebeliefsaboutpeptidehormoneswerequestionedaslaboratoryafterlaboratory
foundthatantiserumstopeptidehormones,wheninjectedintothebrain,bindinplacesotherthan
thehypothalamus,indicatingthateitherthehormonesorsubstancesthatcross-reactwiththe
antiserumsarepresent.Theimmunologicalmethodofdetectingpeptidehormonesbymeansof
antiserums,however,isimprecise.Cross-reactionsarepossibleandthismethodcannotdetermine
whetherthesubstancesdetectedbytheantiserumsreallyarethehormones,ormerelyclose
relatives.Furthermore,thismethodcannotbeusedtodeterminethelocationinthebodywherethe
detectedsubstancesareactuallyproduced.

Newtechniquesofmolecularbiology,however,provideawaytoanswerthesequestions.It
ispossibletomakespecificcomplementaryDNA’s(cDNA’s)thatcanserveasmolecularprobes

seekoutthemessengerRNA’s(mRNA’s)ofthepeptidehormones.Ifbraincellsaremakingthe
hormones,thecellswillcontainthesemRNA’s.Iftheproductsthebraincellsmakeresemblethe
hormonesbutarenotidenticaltothem,thenthecDNA’sshouldstillbindtothesemRNA’s,but
shouldnotbindastightlyastheywouldtomRNA’sforthetruehormones.Thecellscontaining
thesemRNA’scanthenbeisolatedandtheirmRNA’sdecodedtodeterminejustwhattheir
proteinproductsareandhowcloselytheproductsresemblethetruepeptidehormones.

ThemolecularapproachtodetectingpeptidehormonesusingcDNAprobesshouldalsobe
muchfasterthantheimmunologicalmethodbecauseitcantakeyearsoftediouspurificationsto
isolatepeptidehormonesandthendevelopantiserumstoth