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.Our view is different: the Earth is just oneinstance, though variations of it may be replicated many times in theGalaxy.We don t say that other systems are impossible, only that thesystem we have on Earth is not, so far as we can understand it, difficultto achieve, so it is probably an entirely ordinary example.Weconsider its basic elements to be parochial instances of universals, butnot universals in their own right, and this view enables us to extend theargument about terrestrial evolutionary innovations to hypotheticalalien environments.We think that the default may be something likewe ve got on Earth now, if we re-ran Earth another ordinary planetwith a great variety of lifeforms.But possible differences begin to giveus a geography of the imaginative phase space outside Life on Earth.First, let s think about the possibilities for different hereditary material.but not very different.Laboratory experiments have shown that it ispossible to ring the changes on virtually any feature of ourDNA/protein system, even if most of that system is left unchanged.That is, although the roles that these molecules play may be universal,the actual molecules that we use here are very parochial.Recall that DNA is a double helix, composed of two complementarystrands, each a sequence of bases chosen from a standard set of four.The way the strands assemble leads to a backbone of sugar moleculesrunning along the core of the helix.Each triple of bases encodes one oftwenty amino acids, or a stop instruction.Proteins are linear chains ofamino acids that fold into complex three-dimensional shapes, and theseshapes do all the work.Alternatives to DNA definitely exist.To date, only two have beenstudied in any depth, but it is clear that these are the tip of an iceberg.P.E.Nielson and colleagues have synthesised nucleic acids whosebackbones are related to those of proteins.A.Eschenmoser s group hasstudied RNA analogues in which the sugar (ribose) forming thebackbone is replaced by a different sugar, such as hexose.The resulting pyranosyl RNAs (pRNAs) spontaneously pair off to form doublehelices, and could therefore, probably, carry genetic information andreplicate.Strands of pRNA do not bind to RNA, so pRNA is not aplausible precursor to RNA on Earth.But pRNA could conceivably be218 NOT AS WE KNOW ITthe molecule of life for aliens.In 2000, Eschenmoser s group modified pRNA to match thegeometry of conventional RNA, by synthesising a variety of so-called L-±-threofuranosyl oligonucleotides, or TNAs.These form stableCrick Watson helices, and can also form hybrid helices in which onestrand is TNA and the other is DNA or RNA.Eschenmoser has statedthat an enormous variety of RNA-like molecules can be conceived, anyof which might be a plausible RNA precursor; and, though he didn tsay that, a plausible starting-point for alien genetic chemistry.It is also possible to extend the range of bases in DNA to add new letters to the code.The resulting artificial DNA could generate novelproteins.In 2000, Floyd Romesberg s group made twenty new,distinct, bases that combine with sugars just like the four natural bases. The biggest surprise, said Romesberg, was that there s nothing specialabout a natural base. These new bases can be inserted into DNA,extending its alphabet from four letters to twenty-four.Some, at least,of the new bases can be replicated by the same polymerase moleculesthat replicate conventional DNA.On the level of protein manufacture, new amino acids can bebrought into play.The standard twenty used by DNA to make proteinsare not the only amino acids in the molecular repertoire.In 1986 it wasdiscovered that one of the stop triples, UGA, could sometimesproduce a twenty-first amino acid, selenocysteine.(Here U stands for uracil , a molecule that replaces thymine in RNA.) In 2000 themechanism behind this change was unravelled, first for E.coli and thenfor mammalian cells.It is related not just to the DNA sequence, butalso to the geometry into which the molecule is folded, namely theoccurrence of a hairpin-like stem-loop structure.In E.coli this sectionof DNA is adjacent to the UGA triple concerned.Remarkably, inmammals it is some distance away along the DNA strand, and is morecomplicated.The only other known natural interloper is formyl-methionine, which also substitutes for a stop triple.In 2001 two groups, one headed by Lei Wang and the other byVolker Döring, persuaded E.coli to operate with a new repertoire ofamino acids.There are plenty of amino acids to try, such as L-ornithineand L-citrulline.These exist in all cells, but do not normally gain accessto the genetic code.In order for them to do so, various of the cell s219 WHAT DOES A MARTIAN LOOK LIKE?protection mechanisms must be overridden.The unnatural amino acidmust penetrate the cell membrane and bypass the usual checkingsystem that ensures accurate protein synthesis.Then the ribosome, thecell s DNA protein translation machine, must be able to use them asbuilding blocks.Wang s group achieved this using an amino acidrejoicing in the name of O-methyl-L-tyrosine; Döring s group usedaminobutyrate.Clearly the specific constituents of our ubiquitous DNA/proteinchemistry can be changed at almost any level of the process, withoutinterfering with its ability to carry out the fundamental information-carrying and replicatory processes of life.There is nothing sacrosanctabout DNA and the genetic code
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