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Astrobiology Lesson9 Discussion Biochemistry,moleculargene8csand metabolism LifeonEarthasamodelforwhattolookfor • MoleculesbuildinguplifeonEarthconsistofC boundtootherCatomswithcovalentbonds • Acovalentbondisachemicalbondconsis8ngof sharedelectronpairsbetweenatoms • BondsareformedbetweenC=CandwithH,OandN Biochemistry,moleculargene8csand metabolism Covalentbonds Waterisalsoauniquemolecule • ThebindingsbetweenHandOarestronglypolarized • TheanglesbetweenHandOformadipolèH2Ocanform weakchemicalbondswithchargeddissolvedmoleculesand inorganicionsçèwaterisaverygoodsolvent • ThebondsbetweenindividualH2Omoleculesgivewaterhigh mel8ngpoints,boilingpointsandsurfacetension • WatercanbeionizedintoH3O+andOH-(oxoniumand hydroxyl)andbehaveaseitheranacidorabaseandhave highelectricconduc8vity • LifeisprobablynotpossiblewithoutH2Obutalterna8ves suggestedinclude: – Ammonia,NH3 – Formamide,CH3NO èAcompletelydifferentbiochemistry LifeasweknowitisbasedonCarbon • CarbonhasthepropertythatwithH,O,Nitcanform anenormousamountofdifferentandbothsimple andcomplexmolecules–organicmolecules • Secondly,smallerorganicmolecules(nucleo8des, aminoacidsandmonosaccharides=simplesugars) havethepropertythattheycanformlongchains • Excep&onsareCO2andcarbonatemineralswhicharenot considertobeorganic • Mostimportantorganics:DeoxiriboNucleidAcid (DNA),RiboNucleidAcid(RNA),proteinsand carbohydrates LifeasweknowitisbasedonCarbon • Usualstar8ngpointisthatlifeintheUniversemust bebasedonthesamecomponentsasonEarth • GiventhesizeoforganicchemistryèAdifferent biochemistrybasedonCarbonispossible • E.gSicouldbethebaseforsimilarchemistrybutthe numberofpossibili8esissignificantlylower • Numberofpossibili8esisaproblemçèRandom mixofsimplebuildingblocks(aminoacidsandup) wouldtakealong8metoformallorganicmolecules neededatthesame8me(Hoyle,Morrisè10120 Years) LifeasweknowitisbasedonCarbon • Apossiblepathwayforlife4Gyragocouldhave beentheforma8onoftheaminoacids,nitrogeneous bases,monosaccharides,fa_yacids • Thesethenènucleicacids,proteins,carbohydrates andlipids • Thenextstepmusthavebeentheforma8onofacell membraneèfirstcellwhichcouldcopyitself • Thisisourancestor DNAandRNA • Itisnucleicacids,DNAandRNA,thatcarriesthe informa8onfromgenera8ontogenera8onbackto thefirstcell.... • Thesearechainformedmoleculeswithnobranches • Surprisinglysimpleindesign:Anucleo8deisa monosaccharideconsis8ngof5C,atleastone phosphategroupand4nitrogeneousbases • RNAisalongchainwiththenucleo8desconnected oneabertheotherandonedifferentnitrogeneous base(comparedtoDNA).Thesinglestrandcanturn backonitselfandcreateverycomplexstructures RNAvsDNA Ribosome making protein (blue) at active site (red) DNA AdeninebindstoThymineandGuaninetoCytosine(andviceversa) Thebasesare“mounted”onaphosphategroup“backbone” Chromosomes • InalllivingcellsonEarth,youfindDNAmoleculesin Chromosomes • Organismsw/ocellnucleus(Prokaryotes,bacteria) oneringformedchromosomedribingfreeinthecell • Eukaryotes(organismswithacellnucleus)have (many)chromosomesw/ithenucleus • EverychromosomecontainsaverylongDNA moleculewithgenes.Ageneisasec8onofthe chromosomeconsis8ngofDNAlikepearlsonastring • Eachgenecodesforaprotein(itisthetemplatefor it) TheChromosme DNAandRNA • Agenesnitrogeneousbasesequenceistranscripted toapieceofRNAwithacomplementarysequence • Thistakesplaceinthenucleusofthecellwherea pieceofmRNA(messengerRNA)with complementarysequenceiscreatedandtransferthe “instruc8on”outintothecytoplasmofthecell • TheRibosomesinthecytoplasmarethecell’sprotein factories.mRNAisthepa_ernforaTransla&on wherenitrogeneousbasearereadinTriplets.Each tripletiscalledaCodonandcodesforaspecial aminoacid DNAandRNA • Thelinkbetweennitrogeneousbaseandaminoacid isamoleculecalledtRNA(transferRNA) tRNA is typically 73 to 94 nucleotides in length Peptide is a short chain of amino acids and part of a protein One or more poly peptides form a protein Theac8oninaribosome Proteins • Pep8dechainsformproteinsoutofaminoacidsin long(twisted)chains.Differenthelixesarecommon • Therearemanyaminoacidsbutwithfewexcep8ons thesame20aminoacidsformallproteins • Aproteincanconsistofmorethan1000aminoacids • Allaminoacidshavetwodifferentformsthatare geometrical(quantummechanicallyequal)mirrorsof eachother:DandLforms. • AllproteinsonEarthconsistofL-forms • L-formsalsodominateinaminoacidsfoundin meteorites BuildingblocksoftheCell(1) • Proteinsarefoundinallcellsandcons8tuteup½of (dry)massofcell. • Enzymesareabsolutenecessaryproteinsforlifesincethey speedupprocessesthatotherwisewouldruntoslowly • Carbohydrates(sugars)aree.g.Monosacharidesand Polymers(largemoleculesconsis8ngofsmaller buildingblocks) • DNAandRNAcontainthemonosacharidesdeoxiriboseand riboserespec8vely • Photosyntheses(Plantsandsomebacteria)useenergyin theformoflighttobuildupcarbohydratesfromCO2with O2asawasteproduct.Themetabolismcanthenbreak thesedowntoproduceenergy,CO2,H2O BuildingblocksoftheCell(2) • Lipidsarefats,phospholipids,glycolipidsand steroids • Lipidsareusedbycellsasasourceofenergy(storage)and ascarriersofVitamins • Adenotrifosfate(ATP)acquire,storesandtransport energyinchemicalform • Itisusedastheenergyinputforthesyntheseof moleculeslikeDNAandRNA • Chlorophyllcatchesthelightandusetheenergyforthe produc8onofATP • Whenenergyisused,ATPisbrokendowntoADP again.Oxygenisawasteproduct TheCell • TheCellisconsideredtobethesmallestlivingen8ty • Virusaresmallerbutaregenerallynotconsideredto bealive • AllnowlivingcellsonEarthhavebeenformed throughgrowthanddivision(fission)ofolderliving cells • OnthecurrentEarth,asfarascanbediscerned,with ahighprobability,nonewcellsareformedfromfree moleculesinNature • Sofar“lifeinatesttube”hasbeenbuiltnotfrom scratchbutfromexis8ngbitsandpiecesofcells Let’stakealookatafirstpoten8allivingcell • “Ouroldestancestor” • AlllifeonEarthtodayappeartobetraceablebacktoa firstcellwitha(rela8vely)completebiochemistrywith alltheimportantcomponents–ifnotthesame structure • Othertypesoflifeappeartohavebeenlost.“Dead ends”havebeenfound,assoonasfossilsareavailable • Acentralissueishowapplicableallwewilllearnabout lifeonEarthwillbe,withrespecttotheissueof“Lifein theUniverse”? Criteriaforlife • Areplanetsofthesamesizeandcomposi8onasthe Earthnecessary? • Areatmosphereandanoceannecessary? • Oxygenintheatmosphereis–undersome circumstances–consideredtobeanindicatorofthe presenceoflife,but.... – WeknowitisnotnecessarybasedonearlystageoftheEarth • Whatshouldbenecessaryisanenvironmentallowing themetabolismofenergy • Whatcouldbethedifferentstepsleadinguptolife? Prebio8cmacromolecules • Thereareseveralpossiblewaysfortheproduc8onof organicsubstancesthatcanbethestar8ngpoint • Gasesintheatmospherewithenergyintheformoflightningor impactsofheavenlybodiescouldformaminoacidswhichwere depositedinacoldsea(longlife8me) • Iron-sulphurrichenvironmentalongthebordersofcon8nental plates(hotsiteswithareducingenvironment) • Externalarrival(meteors,comets) • WegetPolymers(largemoleculesconsis8ngof smallerbuildingblocks–aminoacids) Symmetryofaminoacids • Anaminoacidmoleculeanditsmirrorimagearenot iden8cal(excep8onisglycine,thesimplestform) • ThisiscausedbythestructureoftheacidlikeinAlanine Todays proteins built inside the complex of the cell are of the L, “levo” type Symmetryofaminoacids • Itisunknownwhythisisso – Actuallysomeproteinswouldhavebeeneasierto constructgeometricallyifbothDandLtypeaminoacids couldhavebeenused • Aguesswasthatoneformwasmoreabundantin theearliestphases(foranunknownreason) • Largeaminoacidsarebuiltfromsmallerformsand acquiredthesamegeometryasthebuildingblocks • TheaminoacidsintheMurchesonmeteoritehad moreL-typethanD-type(10%over) ThePep8deworld • Theforma8onofpep8dechainsmusthavebeena firstimportantstepèLifeonEarth • Weknowthatrela8velysmallpep8descanhavaa cataly8ceffect,i.e.Startorspeedupachemical reac8on • Wecanimagineaworldwherepep8desofdifferent typeswherebuiltupandthencataly8callyformed e.g.Aminoacids • SidneyFox(1912-1998)andco-workersprepared solu8onsofaminoacidsatT>100oCunderhigh pressure ThePep8deworld • Inthesesolu8onsaminoacidsgoesoutofsolu8on andformsmall,containedmicrosphereswhere pep8deswereformed • Foxthoughtthesemicrosphereswereproto-cells • Thepep8desinthesemicrospheres,called protenoidshaveindeedcataly8cproper8es • Whentheproteonidsgettobigtheydivideandstart theprocessagain • Maybeasteponthewaytolife ThePep8deworld • AlthoughtheSydneyFoxexperimentlooksclearly likethereissomethingthere,scien8stshavefound longagothatsomePep8descanbuildcopiesof themselveswithouthavingtogothroughthe “normal”proteinsyntheses • Pep8deworldmodelslookcompellingbuttheyare verylimited–thisisnotlife.Thereisnochange • Itisveryfarfromtheac8vitywithinthesimplest livingorganism • Anotherstepisneeded.... Whatislife? • Afactoryanaloguegiveussomethingthatcan discriminatebetweenthepep&deworldandtheDNA world • Ifwehaveafactorywheremachinesbuildnewiden&calmachines itleadstonothingnew.Ifsomethinggoeswrongitleadstoa uselessmachinethatpossiblycanmakerepairsçèPep&de world • Ifweinsteadhavemachinesrunbyasheetofinstruc&onswhich alsoiscopiedintothenewmachine.Thenwhensomethinggoes wronginthecopyingsomethingcompletelynewiscreated.Most ofthe&meitisuselessbutsome&mesthereisanenhancement thatproducesmoremachinesofthenewkindçèDNAworld Requirementforlife • Whatisrequiredforlifeisareplicator–amolecule withthepossibilityofcopyingitselfandcarry informa8onintheprocess • E.g.Separatedifferentunitsconnectedinlongchains whichcanbecopiedbynewunitsputtogetherinthe sameorder–thesequencecreatesinforma&on • Ifthechainiscopiedwithanerrorsomewherewe getadifferentpieceofinforma8ontransferred–but alsotherequirementforevolu8on • Ifsurvivable,morefunc&onal,theerroneousformis copied.... Requirementforlife • AkeystepisthediscoveryofRNAmoleculeswitha cataly8cfunc8on,Ribozymes,capableofcatalyzing specificbiochemicalreac8ons,similartotheac8on ofproteinenzymes • The1982discovery(AltmanandCech,Nobel,1989)ofribozymes demonstratedthatRNAcanbebothgene&cmaterial(likeDNA)anda biologicalcatalyst(likeproteinenzymes),andcontributedtotheRNA worldhypothesis,whichsuggeststhatRNAmayhavebeenimportantin theevolu&onofprebio&cself-replica&ng • SowehavetheAminoacidsèPep8deworldand thenwehaveapossibleRNAworldèfurtheron. • ButthestepbetweenthePep8desandtheRNAis missing Firstlife-summary Manyrandomandimprobableeventsmusthavehappened simultaneously: 1. ItwasnecessarythatRNA(orapre-cursor)èproteins witharelevantfunc8on 2. Itwasnecessarythatmanycomponentswouldwork together 3. ProteinsthatgovernedthereadingoftheRNA informa8onmusthaveformedatthesame8measthe actualinforma8oncarriersinamu8-tudeofseparatebut interac8ngprocesses 4. Feedbackprocessesmusthavebeenformedinorderto createstability Life at the Extreme • LifeonEarthcanadapttothestrangestenvironments Lifeinacid Life at bottom of frozen lake Life at the Extreme Deinochoccusradiodurans canwithstand500000rad (500radwillkillahuman) Itwasdiscoveredduring experimentstoseeif meatcouldberendered sterilethroughγ-ray radia8on How can we find out if life of any kind exist on an exo-planet? – Ultimately use spectroscopy to search for “Biomarkers” The goal is to find rocky planets with an atmosphere out of photo-chemical balance. (cf. Lederberg, 1965 and Lovelock, 1965) Working definition: Life ≡ O3 + H2O + CO2 Ideally, detect reduced molecules (CH4, N2O) as well Biomarkersorbiosignatures • Abiomarkerorbiosignatureisthedetec8onofanysubstance –suchasanelement,isotope,molecule,orphenomenon– thatprovidesscien8ficevidenceofpastorpresentlife. Measurablea_ributesoflifeincludeitscomplexphysicaland chemicalstructuresandalsoitsu8liza8onoffreeenergyand theproduc8onofbiomassandwastes.Duetoitsunique characteris8cs,abiosignaturecanbeinterpretedashaving beenproducedbylivingorganisms • Itis,however,importantthattheynotbeconsidered defini8vebecausethereisnowayofknowinginadvance whichonesareuniversaltolifeandwhichonesareuniqueto thepeculiarcircumstancesoflifeonEarth Methods to find target planets for spectroscopy A tentative roadmap for searching for life Analyze the reflected light from the planet to see if the planet has an atmosphere Look for oxygen Look for liquid water Look for other signs of biological activity (methane) and rule out all other explanations. 17