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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鈥檚chapters 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鈥攖heletters of the genetic alphabet鈥攃onsist 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鈥攈ence the name鈥渄eoxyribonucleic acid。鈥潯he 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enerally 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n 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uman genome；but in a sense we don鈥檛 even have 鈥渁鈥潯uman 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rom 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 o