25个考研英语阅读经典试题及答案(13)医学类.doc

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------最新25个考研英语阅读经典试题及答案(13)医学类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,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’sdecodedtodeterminejustwhattheirpr