万物简史英文版_比尔·布莱森-第76章

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rty…six　little　bundles　of　plexity；　of　which　twenty…three　e　from　yourmother　and　twenty…three　from　your　father。　with　a　very　few　exceptions；　every　cell　in　yourbody鈥�99。999　percent　of　them；　say鈥攃arries　the　same　plement　of　chromosomes。　（theexceptions　are　red　blood　cells；　some　immune　system　cells；　and　egg　and　sperm　cells；　which　forvarious　organizational　reasons　don鈥檛　carry　the　full　genetic　package。）　chromosomes　constitutethe　plete　set　of　instructions　necessary　to　make　and　maintain　you　and　are　made　of　longstrands　of　the　little　wonder　chemical　called　deoxyribonucleic　acid　or　dna鈥斺�渢he　mostextraordinary　molecule　on　earth；鈥潯�as　it　has　been　called。

dna　exists　for　just　one　reason鈥攖o　create　more　dna鈥攁nd　you　have　a　lot　of　it　inside　you：

about　six　feet　of　it　squeezed　into　almost　every　cell。　each　length　of　dna　prises　some　3。2billion　letters　of　coding；　enough　to　provide　103；480；000；000possible　binations；　鈥済uaranteed　tobe　unique　against　all　conceivable　odds；鈥潯�in　the　words　of　christian　de　duve。　that鈥檚　a　lot　ofpossibility鈥攁　one　followed　by　more　than　three　billion　zeroes。　鈥渋t　would　take　more　than　fivethousand　average…size　books　just　to　print　that　figure；鈥潯�notes　de　duve。　look　at　yourself　in　themirror　and　reflect　upon　the　fact　that　you　are　beholding　ten　thousand　trillion　cells；　and　thatalmost　every　one　of　them　holds　two　yards　of　densely　pacted　dna；　and　you　begin　toappreciate　just　how　much　of　this　stuff　you　carry　around　with　you。　if　all　your　dna　werewoven　into　a　single　fine　strand；　there　would　be　enough　of　it　to　stretch　from　the　earth　to　themoon　and　back　not　once　or　twice　but　again　and　again。　altogether；　according　to　onecalculation；　you　may　have　as　much　as　twenty　million　kilometers　of　dna　bundled　up　insideyou。

your　body；　in　short；　loves　to　make　dna　and　without　it　you　couldn鈥檛　live。　yet　dna　is　notitself　alive。　no　molecule　is；　but　dna　is；　as　it　were；　especially　unalive。　it　is　鈥渁mong　the　mostnonreactive；　chemically　inert　molecules　in　the　living　world；鈥潯�in　the　words　of　the　geneticistrichard　lewontin。　that　is　why　it　can　be　recovered　from　patches　of　long…dried　blood　or　semenin　murder　investigations　and　coaxed　from　the　bones　of　ancient　neandertals。　it　also　explainswhy　it　took　scientists　so　long　to　work　out　how　a　substance　so　mystifyingly　low　key鈥攕o；　in　aword；　lifeless鈥攃ould　be　at　the　very　heart　of　life　itself。

as　a　known　entity；　dna　has　been　around　longer　than　you　might　think。　it　was　discoveredas　far　back　as　1869　by　johann　friedrich　miescher；　a　swiss　scientist　working　at　the　universityof　t眉bingen　in　germany。　while　delving　microscopically　through　the　pus　in　surgicalbandages；　miescher　found　a　substance　he　didn鈥檛　recognize　and　called　it　nuclein　（because　itresided　in　the　nuclei　of　cells）。　at　the　time；　miescher　did　little　more　than　note　its　existence；　butnuclein　clearly　remained　on　his　mind；　for　twenty…three　years　later　in　a　letter　to　his　uncle　heraised　the　possibility　that　such　molecules　could　be　the　agents　behind　heredity。　this　was　anextraordinary　insight；　but　one　so　far　in　advance　of　the　day鈥檚　scientific　requirements　that　itattracted　no　attention　at　all。

for　most　of　the　next　half　century　the　mon　assumption　was　that　the　material鈥攏owcalled　deoxyribonucleic　acid；　or　dna鈥攈ad　at　most　a　subsidiary　role　in　matters　of　heredity。　itwas　too　simple。　it　had　just　four　basic　ponents；　called　nucleotides；　which　was　like　havingan　alphabet　of　just　four　letters。　how　could　you　possibly　write　the　story　of　life　with　such　arudimentary　alphabet？　（the　answer　is　that　you　do　it　in　much　the　way　that　you　create　plexmessages　with　the　simple　dots　and　dashes　of　morse　code鈥攂y　bining　them。）　dna　didn鈥檛do　anything　at　all；　as　far　as　anyone　could　tell。　it　just　sat　there　in　the　nucleus；　possibly　bindingthe　chromosome　in　some　way　or　adding　a　splash　of　acidity　on　mand　or　fulfilling　someother　trivial　task　that　no　one　had　yet　thought　of。　the　necessary　plexity；　it　was　thought；had　to　exist　in　proteins　in　the　nucleus。

there　were；　however；　two　problems　with　dismissing　dna。　first；　there　was　so　much　of　it：

two　yards　in　nearly　every　nucleus；　so　clearly　the　cells　esteemed　it　in　some　important　way。　ontop　of　this；　it　kept　turning　up；　like　the　suspect　in　a　murder　mystery；　in　experiments。　in　twostudies　in　particular；　one　involving　the　pneumonococcus　bacterium　and　another　involvingbacteriophages　（viruses　that　infect　bacteria）；　dna　betrayed　an　importance　that　could　only　beexplained　if　its　role　were　more　central　than　prevailing　thought　allowed。　the　evidencesuggested　that　dna　was　somehow　involved　in　the　making　of　proteins；　a　process　vital　to　life；yet　it　was　also　clear　that　proteins　were　being　made　outside　the　nucleus；　well　away　from　thedna　that　was　supposedly　directing　their　assembly。

no　one　could　understand　how　dna　could　possibly　be　getting　messages　to　the　proteins。　theanswer；　we　now　know；　was　rna；　or　ribonucleic　acid；　which　acts　as　an　interpreter　betweenthe　two。　it　is　a　notable　oddity　of　biology　that　dna　and　proteins　don鈥檛　speak　the　samelanguage。　for　almost　four　billion　years　they　have　been　the　living　world鈥檚　great　double　act；　andyet　they　answer　to　mutually　inpatible　codes；　as　if　one　spoke　spanish　and　the　other　hindi。

to　municate　they　need　a　mediator　in　the　form　of　rna。　working　with　a　kind　of　chemicalclerk　called　a　ribosome；　rna　translates　information　from　a　cell鈥檚　dna　into　terms　proteinscan　understand　and　act　upon。

however；　by　the　early　1900s；　where　we　resume　our　story；　we　were　still　a　very　long　wayfrom　understanding　that；　or　indeed　almost　anything　else　to　do　with　the　confused　business　ofheredity。

clearly　there　was　a　need　for　some　inspired　and　clever　experimentation；　and　happily　the　ageproduced　a　young　person　with　the　diligence　and　aptitude　to　undertake　it。　his　name　wasthomas　hunt　morgan；　and　in　1904；　just　four　years　after　the　timely　rediscovery　of　mendel鈥檚experiments　with　pea　plants　and　still　almost　a　decade　before　gene　would　even　bee　a　word；he　began　to　do　remarkably　dedicated　things　with　chromosomes。

chromosomes　had　been　discovered　by　chance　in　1888　and　were　so　called　because　theyreadily　absorbed　dye　and　thus　were　easy　to　see　under　the　microscope。　by　the　turn　of　thetwentieth　century　it　was　strongly　suspected　that　they　were　involved　in　the　passing　on　of　traits；but　no　one　knew　how；　or　even　really　whether；　they　did　this。

morgan　chose　as　his　subject　of　study　a　tiny；　delicate　fly　formally　called　drosophilamelanogaster；　but　more　monly　known　as　the　fruit　fly　（or　vinegar　fly；　banana　fly；　orgarbage　fly）。　drosophila　is　familiar　to　most　of　us　as　that　frail；　colorless　insect　that　seems　tohave　a　pulsive　urge　to　drown　in　our　drinks。　as　laboratory　specimens　fruit　flies　had　certainvery　attractive　advantages：　they　cost　almost　nothing　to　house　and　feed；　could　be　bred　by　themillions　in　milk　bottles；　went　from　egg　to　productive　parenthood　in　ten　days　or　less；　and　hadjust　four　chromosomes；　which　kept　things　conveniently　simple。

working　out　of　a　small　lab　（which　became　known　inevitably　as　the　fly　room）　inschermerhorn　hall　at　columbia　university　in　new　york；　morgan　and　his　team　embarked　ona　program　of　meticulous　breeding　and　crossbreeding　involving　millions　of　flies　（onebiographer　says　billions；　though　that　is　probably　an　exaggeration）；　each　of　which　had　to　becaptured　with　tweezers　and　examined　under　a　jeweler鈥檚　glass　for　any　tiny　variations　ininheritance。　for　six　years　they　tried　to　produce　mutations　by　any　means　they　could　think　of鈥攝apping　the　flies　with　radiation　and　x…rays；　rearing　them　in　bright　light　and　darkness；　bakingthem　gently　in　ovens；　spinning　them　crazily　in　centrifuges鈥攂ut　nothing　worked。　morgan　wason　the　brink　of　giving　up　when　there　occurred　a　sudden　and　repeatable　mutation鈥攁　fly　thathad　white　eyes　rather　than　the　usual　red　ones。　with　this　breakthrough；　morgan　and　hisassistants　were　able　to　generate　useful　deformities；　allowing　them　to　track　a　trait　throughsuccessive　generations。　by　such　means　they　could　work　out　the　correlations　betweenparticular　characteristics　and　individual　chromosomes；　eventually　proving　to　more　or　lesseveryone鈥檚　satisfaction　that　chromosomes　were　at　the　heart　of　inheritance。

the　problem；　however；　remained　the　next　level　of　biological　intricacy：　the　enigmatic　genesand　the　dna　that　posed　them。　these　were　much　trickier　to　isolate　and　understand。　aslate　as　1933；　when　morgan　was　awarded　a　nobel　prize　for　his　work；　many　researchers　stillweren鈥檛　convinced　that　genes　even　existed。　as　morgan　noted　at　the　time；　there　was　noconsensus　鈥渁s　to　what　the　genes　are鈥攚hether　they　are　real　or　purely　fictitious。鈥潯�it　may　seemsurprising　that　scientists　could　struggle　to　accept　the　physical　reality　of　something　sofundamental　to　cellular　activity；　but　as　wallace；　king；　and　sanders　point　out　in　biology：　thescience　of　life　（that　rarest　thing：　a　readable　college　text）；　we　are　in　much　the　same　positiontoday　with　mental　processes　such　as　thought　and　memory。　we　know　that　we　have　them；　ofcourse；　but　we　don鈥檛　know　what；　if　any；　physical　form　they　take。　so　it　was　for　the　longest　timewith　genes。　the　idea　that　you　could　pluck　one　from　your　body　and　take　it　away　for　study　wasas　absurd　to　many　of　morgan鈥檚　peers　as　the　idea　that　scientists　today　might　capture　a　straythought　and　examine　it　under　a　microscope。

what　was　certainly　true　was　that　something　associated　with　chromosomes　was　directingcell　replication。　finally；　in　1944；　after　fifteen　years　of　effort；　a　team　at　the　rockefellerinstitute　in　manhattan；　led　by　a　brilliant　but　diffident　canadian　named　oswald　avery；succeeded　with　an　exceedingly　tricky　experiment　in　which　an　innocuous　strain　of　bacteria　wasmade　permanently　infectious　by　crossing　it　with　alien　dna；　proving　that　dna　was　far　morethan　a　passive　molecule　and　almost　certainly　was　the　active　agent　in　heredity。　the　austrian…born　biochemist　erwin　chargaff　later　suggested　quite　seriously　that　avery鈥檚　discovery　wasworth　two　nobel　prizes。

unfortunately；　avery　was　opposed　by　one　of　his　own　colleagues　at　the　institute；　a　strong…willed　and　disagreeable　protein　enthusiast　named　alfred　mirsky；　who　did　everything　in　hispower　to　discredit　avery鈥檚　work鈥攊ncluding；　it　has　been　said；　lobbying　the　authorities　at　thekarolinska　institute　in　stockholm　not　to　give　avery　a　nobel　prize。　avery　by　this　time　wassixty…six　years　old　and　tired。　unable　to　deal　with　the　stress　and　controversy；　he　resigned　hisposition　and　never　went　near　a　lab　again。　but　other　experiments　elsewhere　overwhelminglysupported　his　conclusions；　and　soon　the　race　was　on　to　find　the　structure　of　dna。

had　you　been　a　betting　person　in　the　early　1950s；　your　money　would　almost　certainly　havebeen　on　linus　pauling　of　caltech；　america鈥檚　leading　chemist；　to　crack　the　structure　of　dna。

pauling　was　unrivaled　in　determining　the　architecture　of　molecules　and　had　been　a　pioneer　inthe　field　of　x…ray　crystallography；　a　technique　that　would　prove　crucial　to　peering　into　theheart　of　dna。　in　an　exceedingly　distinguished　career；　he　would　win　two　nobel　prizes　（forchemistry　in　1954　and　peace　in　1962）；　but　with　dna　he　became　convinced　that　the　structurewas　a　triple　helix；　not　a　double　one；　and　never　quite　got　on　the　right　track。　instead；　victory　fellto　an　unlikely　quartet　of　scientists　in　england　who　didn鈥檛　work　as　a　team；　often　weren鈥檛　onspeaking　terms；　and　were　for　the　most　p