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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