《A Short History of Nearly Everything》第134章

n’t　actually　seem　to　do　anything。
ninety…seven　percent　of　your　dna　consists　of　nothing　but　long　stretches　of　meaninglessgarble—“junk；”　or　“non…coding　dna；”　as　biochemists　prefer　to　put　it。　only　here　and　therealong　each　strand　do　you　find　sections　that　control　and　organize　vital　functions。　these　are　thecurious　and　long…elusive　genes。
genes　are　nothing　more　（nor　less）　than　instructions　to　make　proteins。　this　they　do　with　acertain　dull　fidelity。　in　this　sense；　they　are　rather　like　the　keys　of　a　piano；　each　playing　asingle　note　and　nothing　else；　which　is　obviously　a　trifle　monotonous。　but　bine　the　genes；as　you　would　bine　piano　keys；　and　you　can　create　chords　and　melodies　of　infinite　variety。
put　all　these　genes　together；　and　you　have　（to　continue　the　metaphor）　the　great　symphony　ofexistence　known　as　the　human　genome。
an　alternative　and　more　mon　way　to　regard　the　genome　is　as　a　kind　of　instructionmanual　for　the　body。　viewed　this　way；　the　chromosomes　can　be　imagined　as　the　book’schapters　and　the　genes　as　individual　instructions　for　making　proteins。　the　words　in　which　theinstructions　are　written　are　called　codons；　and　the　letters　are　known　as　bases。　the　bases—theletters　of　the　genetic　alphabet—consist　of　the　four　nucleotides　mentioned　a　page　or　two　back：
adenine；　thiamine；　guanine；　and　cytosine。　despite　the　importance　of　what　they　do；　thesesubstances　are　not　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—hence　the　name“deoxyribonucleic　acid。”　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—inonly　a　matter　of　seconds；　which　is　quite　a　feat。
most　of　the　time　our　dna　replicates　with　dutiful　accuracy；　but　just　occasionally—aboutone　time　in　a　million—a　letter　gets　into　the　wrong　place。　this　is　known　as　a　single　nucleotidepolymorphism；　or　snp；　familiarly　known　to　biochemists　as　a　“snip。”　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—more　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’t　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　“it’s　like　pumping　oil；”　in　the　words　of　temple　university　anthropologist　charles　weitz。
that’s　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’t　let　you　bee　too　different—not　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’s；　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　“the”　human　genome；but　in　a　sense　we　don’t　even　have　“a”　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；　“you　could　sayall　humans　share　nothing；　and　that　would　be　correct；　too。”
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’s　words；　“exist　for　the　pure　and　simple　reason　that　they　are　good　at　gettingthemselves　duplicated。”
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—when　it　codes　for　them；　asscientists　put　it