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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.
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