Quasi_species_Error_Threshold_and_Hypercycles.pptx
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Quasi_species_Error_Threshold_and_Hypercycles.pptx
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Quasi-species,ErrorThresholdandHypercycles,AModelforMolecularEvolution,MorDolevJanIhmels,Introduction,Darwinianevolution:
diversityofspeciesStartingpoint:
cellCell:
molecularmechanisms,complexmachinery,highlyconservedOriginoflife:
processesthatleadtoemergenceofthecellEigen+Schuster:
molecularevolutionDoweneedlivingthingsforevolution?
ConditionsforNaturalSelection,Metabolism:
Spontaneous,independentformationanddegradationfarfromequilibriumSelf-reproduction:
Abilitytoinstructownsynthesis(autocatalysis),conservationofinformationMutability:
Limitedfidelityofself-reproductiveprocess,RibonucleicAcid,SatisfiesrequirementsfornaturalselectionEigen&Schusterscandidatefortheoriginoflife,DynamicsofSelection,Simplestmodel:
Reproducibleglobalconditions:
DynamicsofSelection,Systematconstantoverallconcentration:
where,Quasi-species,Singlespeciesarecoupled.Competition,butco-operationbetweenrelatedspecies.Reorganize:
NspeciesNquasi-species.,Transformedequation:
Quasi-species,Steadystate:
Naturalselection:
onequasi-speciessurvivesTargetofselection:
quasi-species(distribution),notsinglemolecularspeciesCharacterizedbymastersequence,Example:
RNAClones,Wildtype:
averagesequenceSpeciesisadistribution,ModelSummary,NaturalselectionofinanimateobjectsOpenquestions:
Darwiniandiversityvs.prebioticunityCanwehaveintegrationofseveralfunctions?
Errorthreshold,Theerrorthreshold,-therelativeconcentrationofthemastersequence.-therelativeconcentrationofalltheothers.,Wecansimplifyourequations:
Theerrorthreshold,Wewanttohaveasteadystate,atwhich,Solvingtheequationsweget:
Demanding,weget:
Theerrorthreshold,Wheresisthesuperiorityofthemastersequence.Letqbethereplicationfidelityperdigit.Then:
Themaximallengthofamoleculeislimitedbytheerrorrate,1-q.,Themaximallengthofagenomeisinverselyproportionaltotheerrorrate:
Theerrorthreshold:
Isthereacorrelationbetweenerrorrateandgenomesize?
Theerrorcatastrophe,InRNAreplicationwithoutareplicasetheerrorrateisaround0.01.Thisallowsamaximallengthofnotmorethan100nucleotides.Toincreasethefidelityofreplication,anenzymeisneeded.Eigensparadox:
noenzymewithoutalargegenome,andnolargegenomewithoutenzymes.,Howcanweincreasetheamountofinformationmaintainedinthesystem,withoutanincreaseintheerrorrate?
Asinglelongmoleculevs.twoshortones:
Ifwecouldhaveastablecoexistenceofmorethanonequasi-speciesinthesystem,thissystemwouldkeepmoreinformation.Theproblem:
competitionbetweenquasi-speciespreventsastablecoexistence.,TheHypercycle,Organizedarrangementofcatalyticmolecules,Catalyst:
Catalyticcycle:
Autocatalyst,Hypercycle,Nexthigherorder:
CatalytichypercycleCycliclinkageofSelf-replicativeunitsIntermediatesInareCatalyticcycles,Properties,Allowscollectivegrowthofcooperativesystem(Eigensparadox).Competitionbetweenmemberspeciesisswitchedoff.Threebasictypesofgrowthrates:
Constant:
coexistence.Linear:
exponentialgrowth.Higherthanlinear:
onceforalltime-selection.,Properties,Entireensemblecapableofexpansionandoptimization.Replacementisdifficult.Geneticcode,mechanisms,chirality.,Hypercyclesexhibithyperbolicgrowth:
Anexampleforahypercycle:
RNA-phageinfection:
Hypercyclesareallaroundus,ProblemsWithHypercycles,Mutantsinhypercycles,A,A”,Selfish:
BetterTarget,Altruistic:
BetterReplicase,Large(4)cyclesfluctuategreatly:
instabilityShortcutscandestroycyclesParasitescandestroyhypercycle,Compartments,EigensuggestscompartmentsFunctionaladvantagescanbeselectedforFeedbackfromphenotypetogenotype,Compartments,Improvedreplicasemutationscanbeselectedfor,Dowestillneedhypercycles?
Summary,Quasispecies:
SimplemolecularevolutionErrorCatastrophe:
EigensparadoxHypercyclesasasolution:
Cooperationbetweenquasi-speciesCanovercomeerrorthresholdOnceandforall-selectionProblemsofthehypercycle:
noselectionfavoring“altruistic”mutantsneedforcompartments,References,M.Eigen,P.Schuster,TheHypercycle,Springer-Verlag,1979M.Eigen,“Stepstowardslife:
APerspectiveonEvolution”,OxfordUnivPress,1996J.Hofbauer,K.Sigmund,“Thetheoryofevolutionanddynamicalsystem”,CambrigeUnivPress,1988J.M.Smith,Hypercyclesandtheoriginoflife,Nature20:
445-6,1979J.M.Smith,E.Szathmary“Themajortransitionsinevolution”,W.H.Freeman,1995,
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