万物简史英文版_比尔·布莱森-第78章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


　made　of　anything　exotic。　guanine；　for　instance；　is　the　same　stuff　thatabounds　in；　and　gives　its　name　to；　guano。

the　shape　of　a　dna　molecule；　as　everyone　knows；　is　rather　like　a　spiral　staircase　ortwisted　rope　ladder：　the　famous　double　helix。　the　uprights　of　this　structure　are　made　of　a　typeof　sugar　called　deoxyribose；　and　the　whole　of　the　helix　is　a　nucleic　acid鈥攈ence　the　name鈥渄eoxyribonucleic　acid。鈥潯�the　rungs　（or　steps）　are　formed　by　two　bases　joining　across　thespace　between；　and　they　can　bine　in　only　two　ways：　guanine　is　always　paired　withcytosine　and　thiamine　always　with　adenine。　the　order　in　which　these　letters　appear　as　youmove　up　or　down　the　ladder　constitutes　the　dna　code；　logging　it　has　been　the　job　of　thehuman　genome　project。

now　the　particular　brilliance　of　dna　lies　in　its　manner　of　replication。　when　it　is　time　toproduce　a　new　dna　molecule；　the　two　strands　part　down　the　middle；　like　the　zipper　on　ajacket；　and　each　half　goes　off　to　form　a　new　partnership。　because　each　nucleotide　along　astrand　pairs　up　with　a　specific　other　nucleotide；　each　strand　serves　as　a　template　for　thecreation　of　a　new　matching　strand。　if　you　possessed　just　one　strand　of　your　own　dna；　youcould　easily　enough　reconstruct　the　matching　side　by　working　out　the　necessary　partnerships：

if　the　topmost　rung　on　one　strand　was　made　of　guanine；　then　you　would　know　that　thetopmost　rung　on　the　matching　strand　must　be　cytosine。　work　your　way　down　the　ladderthrough　all　the　nucleotide　pairings；　and　eventually　you　would　have　the　code　for　a　newmolecule。　that　is　just　what　happens　in　nature；　except　that　nature　does　it　really　quickly鈥攊nonly　a　matter　of　seconds；　which　is　quite　a　feat。

most　of　the　time　our　dna　replicates　with　dutiful　accuracy；　but　just　occasionally鈥攁boutone　time　in　a　million鈥攁　letter　gets　into　the　wrong　place。　this　is　known　as　a　single　nucleotidepolymorphism；　or　snp；　familiarly　known　to　biochemists　as　a　鈥渟nip。鈥潯�generally　these　snipsare　buried　in　stretches　of　noncoding　dna　and　have　no　detectable　consequence　for　the　body。

but　occasionally　they　make　a　difference。　they　might　leave　you　predisposed　to　some　disease；but　equally　they　might　confer　some　slight　advantage鈥攎ore　protective　pigmentation；　forinstance；　or　increased　production　of　red　blood　cells　for　someone　living　at　altitude。　over　time；these　slight　modifications　accumulate　in　both　individuals　and　in　populations；　contributing　tothe　distinctiveness　of　both。

the　balance　between　accuracy　and　errors　in　replication　is　a　fine　one。　too　many　errors　andthe　organism　can鈥檛　function；　but　too　few　and　it　sacrifices　adaptability。　a　similar　balance　mustexist　between　stability　in　an　organism　and　innovation。　an　increase　in　red　blood　cells　can　helpa　person　or　group　living　at　high　elevations　to　move　and　breathe　more　easily　because　more　redcells　can　carry　more　oxygen。　but　additional　red　cells　also　thicken　the　blood。　add　too　many；and　鈥渋t鈥檚　like　pumping　oil；鈥潯�in　the　words　of　temple　university　anthropologist　charles　weitz。

that鈥檚　hard　on　the　heart。　thus　those　designed　to　live　at　high　altitude　get　increased　breathingefficiency；　but　pay　for　it　with　higher…risk　hearts。　by　such　means　does　darwinian　naturalselection　look　after　us。　it　also　helps　to　explain　why　we　are　all　so　similar。　evolution　simplywon鈥檛　let　you　bee　too　different鈥攏ot　without　being　a　new　species　anyway。

the　0。1　percent　difference　between　your　genes　and　mine　is　accounted　for　by　our　snips。

now　if　you　pared　your　dna　with　a　third　person鈥檚；　there　would　also　be　99。9　percentcorrespondence；　but　the　snips　would；　for　the　most　part；　be　in　different　places。　add　morepeople　to　the　parison　and　you　will　get　yet　more　snips　in　yet　more　places。　for　every　one　ofyour　3。2　billion　bases；　somewhere　on　the　planet　there　will　be　a　person；　or　group　of　persons；with　different　coding　in　that　position。　so　not　only　is　it　wrong　to　refer　to　鈥渢he鈥潯�human　genome；but　in　a　sense　we　don鈥檛　even　have　鈥渁鈥潯�human　genome。　we　have　six　billion　of　them。　we　are　all99。9　percent　the　same；　but　equally；　in　the　words　of　the　biochemist　david　cox；　鈥測ou　could　sayall　humans　share　nothing；　and　that　would　be　correct；　too。鈥�

but　we　have　still　to　explain　why　so　little　of　that　dna　has　any　discernible　purpose。　it　startsto　get　a　little　unnerving；　but　it　does　really　seem　that　the　purpose　of　life　is　to　perpetuate　dna。

the　97　percent　of　our　dna　monly　called　junk　is　largely　made　up　of　clumps　of　lettersthat；　in　ridley鈥檚　words；　鈥渆xist　for　the　pure　and　simple　reason　that　they　are　good　at　gettingthemselves　duplicated。鈥�

2most　of　your　dna；　in　other　words；　is　not　devoted　to　you　but　toitself：　you　are　a　machine　for　reproducing　it；　not　it　for　you。　life；　you　will　recall；　just　wants　tobe；　and　dna　is　what　makes　it　so。

even　when　dna　includes　instructions　for　making　genes鈥攚hen　it　codes　for　them；　asscientists　put　it鈥攊t　is　not　necessarily　with　the　smooth　functioning　of　the　organism　in　mind。

one　of　the　monest　genes　we　have　is　for　a　protein　called　reverse　transcriptase；　which　hasno　known　beneficial　function　in　human　beings　at　all。　the　one　thing　itdoes　do　is　make　itpossible　for　retroviruses；　such　as　the　aids　virus；　to　slip　unnoticed　into　the　human　system。

in　other　words；　our　bodies　devote　considerable　energies　to　producing　a　protein　that　doesnothing　that　is　beneficial　and　sometimes　clobbers　us。　our　bodies　have　no　choice　but　to　do　sobecause　the　genes　order　it。　we　are　vessels　for　their　whims。　altogether；　almost　half　of　humangenes鈥攖he　largest　proportion　yet　found　in　any　organism鈥攄on鈥檛　do　anything　at　all；　as　far　aswe　can　tell；　except　reproduce　themselves。

all　organisms　are　in　some　sense　slaves　to　their　genes。　that鈥檚　why　salmon　and　spiders　andother　types　of　creatures　more　or　less　beyond　counting　are　prepared　to　die　in　the　process　ofmating。　the　desire　to　breed；　to　disperse　one鈥檚　genes；　is　the　most　powerful　impulse　in　nature。

as　sherwin　b。　nuland　has　put　it：　鈥渆mpires　fall；　ids　explode；　great　symphonies　are　written；and　behind　all　of　it　is　a　single　instinct　that　demands　satisfaction。鈥潯�from　an　evolutionary　pointof　view；　sex　is　really　just　a　reward　mechanism　to　encourage　us　to　pass　on　our　genetic　material。

scientists　had　only　barely　absorbed　the　surprising　news　that　most　of　our　dna　doesn鈥檛　doanything　when　even　more　unexpected　findings　began　to　turn　up。　first　in　germany　and　then　inswitzerland　researchers　performed　some　rather　bizarre　experiments　that　produced　curiouslyunbizarre　outes。　in　one　they　took　the　gene　that　controlled　the　development　of　a　mouse鈥檚eye　and　inserted　it　into　the　larva　of　a　fruit　fly。　the　thought　was　that　it　might　producesomething　interestingly　grotesque。　in　fact；　the　mouse…eye　gene　not　only　made　a　viable　eye　inthe　fruit　fly；　it　made　a　fly鈥檚　eye。　here　were　two　creatures　that　hadn鈥檛　shared　a　monancestor　for　500　million　years；　yet　could　swap　genetic　material　as　if　they　were　sisters。

the　story　was　the　same　wherever　researchers　looked。　they　found　that　they　could　inserthuman　dna　into　certain　cells　of　flies；　and　the　flies　would　accept　it　as　if　it　were　their　own。

2junk　dna　does　have　a　use。　it　is　the　portion　employed　in　dna　fingerprinting。　its　practicality　for　this　purposewas　discovered　accidentally　by　alec　jeffreys；　a　scientist　at　the　university　of　leicester　in　england。　in　1986jeffreys　was　studying　dna　sequences　for　genetic　markers　associated　with　heritable　diseases　when　he　wasapproached　by　the　police　and　asked　if　he　could　help　connect　a　suspect　to　two　murders。　he　realized　his　techniqueought　to　work　perfectly　for　solving　criminal　cases…and　so　it　proved。　a　young　baker　with　the　improbable　name　ofcolin　pitchfork　was　sentenced　to　two　life　terms　in　prison　for　the　murders。

over　60　percent　of　human　genes；　it　turns　out；　are　fundamentally　the　same　as　those　found　infruit　flies。　at　least　90　percent　correlate　at　some　level　to　those　found　in　mice。　（we　even　havethe　same　genes　for　making　a　tail；　if　only　they　would　switch　on。）　in　field　after　field；researchers　found　that　whatever　organism　they　were　working　on鈥攚hether　nematode　wormsor　human　beings鈥攖hey　were　often　studying　essentially　the　same　genes。　life；　it　appeared；　wasdrawn　up　from　a　single　set　of　blueprints。

further　probings　revealed　the　existence　of　a　clutch　of　master　control　genes；　each　directingthe　development　of　a　section　of　the　body；　which　were　dubbed　homeotic　（from　a　greek　wordmeaning　鈥渟imilar鈥潱�　or　hox　genes。　hox　genes　answered　the　long…bewildering　question　of　howbillions　of　embryonic　cells；　all　arising　from　a　single　fertilized　egg　and　carrying　identicaldna；　know　where　to　go　and　what　to　do鈥攖hat　this　one　should　bee　a　liver　cell；　this　one　astretchy　neuron；　this　one　a　bubble　of　blood；　this　one　part　of　the　shimmer　on　a　beating　wing。　itis　the　hox　genes　that　instruct　them；　and　they　do　it　for　all　organisms　in　much　the　same　way。

interestingly；　the　amount　of　genetic　material　and　how　it　is　organized　doesn鈥檛　necessarily；　oreven　generally；　reflect　the　level　of　sophistication　of　the　creature　that　contains　it。　we　haveforty…six　chromosomes；　but　some　ferns　have　more　than　six　hundred。　the　lungfish；　one　of　theleast　evolved　of　all　plex　animals；　has　forty　times　as　much　dna　as　we　have。　even　themon　newt　is　more　genetically　splendorous　than　we　are；　by　a　factor　of　five。

clearly　it　is　not　the　number　of　genes　you　have；　but　what　you　do　with　them。　this　is　a　verygood　thing　because　the　number　of　genes　in　humans　has　taken　a　big　hit　lately。　until　recently　itwas　thought　that　humans　had　at　least　100；000　genes；　possibly　a　good　many　more；　but　thatnumber　was　drastically　reduced　by　the　first　results　of　the　human　genome　project；　whichsuggested　a　figure　more　like　35；000　or　40；000　genes鈥攁bout　the　same　number　as　are　found　ingrass。　that　came　as　both　a　surprise　and　a　disappointment。

it　won鈥檛　have　escaped　your　attention　that　genes　have　been　monly　implicated　in　anynumber　of　human　frailties。　exultant　scientists　have　at　various　times　declared　themselves　tohave　found　the　genes　responsible　for　obesity；　schizophrenia；　homosexuality；　criminality；violence；　alcoholism；　even　shoplifting　and　homelessness。　perhaps　the　apogee　（or　nadir）　of　thisfaith　in　biodeterminism　was　a　study　published　in　the　journal　science　in　1980　contending　thatwomen　are　genetically　inferior　at　mathematics。　in　fact；　we　now　know；　almost　nothing　aboutyou　is　so　acmodatingly　simple。

this　is　clearly　a　pity　in　one　important　sense；　for　if　you　had　individual　genes　that　determinedheight　or　propensity　to　diabetes　or　to　baldness　or　any　other　distinguishing　trait；　then　it　wouldbe　easy鈥攑aratively　easy　anyway鈥攖o　isolate　and　tinker　with　them。　unfortunately；　thirty…five　thousand　genes　functioning　independently　is　not　nearly　enough　to　produce　the　kind　ofphysical　plexity　that　makes　a　satisfactory　human　being。　genes　clearly　therefore　mustcooperate。　a　few　disorders鈥攈emophilia；　parkinson鈥檚　disease；　huntington鈥檚　disease；　andcystic　fibrosis；　for　example鈥攁re　caused　by　lone　dysfunctional　genes；　but　as　a　rule　disruptivegenes　are　weeded　out　by　natural　selection　long　before　they　can　bee　permanentlytroublesome　to　a　species　or　population。　for　the　most　part　our　fate　and　fort鈥攁nd　even　oureye　color鈥攁re　determined　not　by　individual　genes　but　by　plexes　of　genes　working　inalliance。　that鈥檚　why　it　is　so　hard　to　work　out　how　it　all　fits　together　and　why　we　won鈥檛　bep