EVOLUTION OF EUKARYOTIC CELLS AND ENDOSYMBIOSIS.pdf

Evolution is the call of the time irrespective of its nature, whether living or non-living. The abiotic entities gave birth to living ones in the passage of evolution. Prokaryotes came after a lot of hurdles. From prokaryotes, Eukaryotes evolved. There are lot of theories to explore the route bit the most convenient and acceptable one is the endosymbiotic theory. This presentation offers the origin of eukaryotic cells from the prokaryotic one through the long passage of evolution in the context of geological time scale.

1. EVOLUTION OF EUKARYOTIC CELLS
PRESENTED BY
Dr. Nandadulal Sannigrahi,
Associate Professor,
Department of Botany,
Nistarini College, Purulia,
D.B. Road, Purulia (W.B)
723101, INDIA

2. INTRODUCTION
Evolutionisthecallofthetimewhetheritislivingornon-livingentities.Lifeis
notbeyonditsexception.
Non-livingentitiesarethemotheroflivingbodiesandfromancientlifeforms,
simplecellslikebacteriaappeared.
Cellsaredividedintotwomainclasses,initiallydefinedbywhetherthey
containaancientnucleus(bacteria)lackinganuclearenvelope;eukaryoticcells
haveanucleusinwhichthegeneticmaterialisseparatedfromthecytoplasm.
Prokaryoticcellsaregenerallysmallerandsimplerthaneukaryoticcellsin
additiontotheabsenceofanucleus,theirgenomesarelesscomplexandtheydo
notcontaincytoplasmicorganellesoracytoskeleton.
Theprokaryotesareverysimpleforlifestyleanddegreeofcomplexityof
structureandfunction,
Inspiteofthesedifferences,thesamebasicmolecularmechanismsgovernthe
livesofbothprokaryotesandeukaryotes,indicatingthatallpresent-daycellsare
descendedfromasingleprimordialancestor.Howdidthisfirstcelldevelop?
Andhowdidthecomplexityanddiversityexhibitedbypresent-daycells
evolve?Theanswerliesintheunderstandingofevolutionofeukaryotes.

3. PROKARYOTIC CELLS
Thepresentdayprokaryotesconsistsofvariousformsofbacteria,
Mostlydividedintotwogroups-ArchaebacteriaandEubacteria,which
divergedintheevolution,
TheArchaebacteriamostlytheancientonethatwithstandtheveryharsh
climateoftheprimitiveearthfutureslikethermo-acidophilesthatcan
toleratehightemperatureandverylowpHatabout2,
Eubacteriaarepresentdaybacteriawhichfoundalmostallthesurfacesof
theearthandmostofthemserveashumanpathogens,
Eubacteriawithdiverseshapewiththerigidcellwalleithergram+orgram-
withthediversity,
Plasmamembranewithphospholipidsbi-layersandassociatedproteins,
CircularDNAmoleculeswithyalmost30000ribosomeof70Stypes,
DNAcontains0.6millionto5millionnucleotidesthatalmostcodes5000
differentproteins,
Thelargestprokaryotesarecyanobacteriathatevolvethemechanismof
photosynthesis.

4. PROKARIOTIC AND EUKARYOTIC CELL

5. EUKARYOTIC CELL
Eukaryoticcellsarealsosurroundedbycellmembranealongwithouterrigid
cellwallofcellulose,pectinetc.(plantcell),
Morecomplexwithorganizednucleusandothercellorganelles,
Muchlargerthantheprokaryotesfrequentlyhavingcellvolumeatleasta
thousandfoldgreater,
Thecompartmentalizationprovidedbythecytoplasmicorganellestoperform
thecellefficientlyworking,
Mitochondriaandplastidsplayasignificantroleintheenergyutilizationalong
withthepivotalroleinthemetabolism.
Lissomeandperoxisomesprovidemetaboliccompartmentsforthedigestionof
macromoleculesandvariousoxidativereactions,
Thetwocytoplasmicorganelles-ERandGolgiapparatusproteins
transportationsandtothedestinations.
Eukaryoticcellshaveanothercytoskeletonthatprovidesthestructural
frameworkofthecellbygivingthecellshapeandtheorganizationofthe
cytoplasm,
Nowthequestion-howdoeseukaryoteoriginate?

6. POSSIBLE ROUTE OF EVOLUTION

7. UNIQUE TRAITS AND ADAPTATIONS FOR EUKARYOTES
Therearemanyuniquetraitsandadaptationsofeukaryotesthatallowus
todistinguishthemasamonophyleticgrouponthephylogenetictreeof
life.Basedontheprincipleofparsimony,thefollowinglistsasubsetof
traitsmusthavebeenpresentinthemostrecentcommonancestor
(MRCA)fromwhichalleukaryoticlifeemerged:
Cellswithnucleisurroundedbyanuclearenvelopewithnuclear
pores:Thisisthesingletraitthatisbothnecessaryandsufficientto
defineanorganismasaeukaryote.Allextanteukaryoteshavecellswith
nuclei.
Mitochondria:Someextanteukaryoteshaveveryreducedremnantsof
mitochondriaintheircells,whereasothermembersoftheirlineages
havemitochondria.
Mitosis:aprocessofnucleardivisionwhereinreplicatedchromosomes
aredividedandseparatedusingelementsofthecytoskeleton.Mitosisis
universallypresentineukaryotes.
Meiosisandsex:aprocessofgeneticrecombinationuniqueto
eukaryotesinwhichdiploidnucleiatonestageofthelifecycleundergo
meiosistoyieldhaploidnucleiandsubsequentkaryogamy,astage
wheretwohaploidnucleifusetogethertocreateadiploidzygote
nucleus.

8. ENDOSYMBIOSIS THEORY
Therearetwodifferentstoriestotellabouttheoriginofeukaryotes,oneforthe
originoforganellessuchasmitochondriaandchloroplasts,andanotherforthe
originoftheotherpartsofthecell.
1.OriginofMitochondriaandChloroplasts:theoryofEndosymbiosisOneof
themostintriguingspeculationsisthatMitochondriaandChloroplastsarosenot
byagradualevolutionaryprocessbutabruptlyinanunusuallystrikingmanner,
i.e.,Symbiosis.
Ascellbiologydevelopedinthetwentiethcentury,itbecameclearthat
mitochondriaweretheorganellesresponsibleforproducingATPusingaerobic
respiration.Inthe1960s,AmericanbiologistLynnMarguliesdeveloped
Endosymbiosistheory,whichstatesthateukaryotesmayhavebeenaproductof
onecellengulfinganother,onelivingwithinanother,andevolvingovertime
untiltheseparatecellswerenolongerrecognizableassuch.
Endosymbiosishypothesisoriginallyproposedthat-Mitochondriaaretheresult
ofendocytosisofaerobicbacteriachloroplastsaretheresultofendocytosisof
photosyntheticbacteriainbothcasesbylargeanaerobicbacteriawhowouldnot
otherwisebeabletoexistinanaerobicenvironment.
Thisarrangementbecameamutuallybeneficialrelationshipforbothcells
(symbiotic).So,theaerobicbacteria(thatrequireoxygen)wereingestedby
anaerobicbacteria(poisonedbyoxygen),andmayeachhavehadasurvival
advantageaslongastheycontinuedtheirpartnership.

9. ENDOSYMBIOSIS THEORY
TheEukaryotesdeveloped2.7billionyearsagofollowingsameto1-1.5
billionyearsofprokaryoticevolution,
DNAsequencessuggestthatarchaebacteriaandeubacteriaarequite
differenttoeachother,noneuspredecessorsorsuccessorstotheother,asis
eitherfromthepresentdayeukatiotes,
Evolutionenjoyedthethreedistinctdivergencelineofevolutionfroma
commonancestor,
Acriticalstageoftheevolutionofeukaryoticcellwastheacquisitionof
membraneenclosedsubcellularorganellesallowingthedevelopmentof
complexityasfarasthecharacteristicoftheeukaryoticcellsareconcerned.
Organellesarethoughttohavebeenevolvedfromtheprokaryoticcellswith
thenumberofdistinctsimilaritieswiththesamegroup,
Thetwotheorieshavebeenproposedtoexplaintheiroriginofeukaryotes-
theoryofimaginationandEndosymbiosistheory.

10. ENDOSYMBIOSIS THEORY
Thehypothesisthateukaryoticcellsevolvedfromasymbioticassociationof
prokaryotes—endosymbiosis—isparticularlywellsupportedbystudiesof
mitochondriaandchloroplasts,whicharethoughttohaveevolvedfrom
bacterialivinginlargecells.
Bothmitochondriaandchloroplastsaresimilartobacteriainsize,andlike
bacteria,theyreproducebydividingintwo.Mostimportant,both
mitochondriaandchloroplastscontaintheirownDNA,whichencodessome
oftheircomponents.
ThemitochondrialandchloroplastDNAsarereplicatedeachtimethe
organelledivides,andthegenestheyencodearetranscribedwithinthe
organelleandtranslatedonorganelleribosome.
Mitochondriaandchloroplaststhuscontaintheirowngeneticsystems,
whicharedistinctfromthenucleargenomeofthecell.
Furthermore,theribosomeandribosomalRNAsoftheseorganellesare
morecloselyrelatedtothoseofbacteriathantothoseencodedbythe
nucleargenomesofeukaryotes.

11. ENDOSYMBIOSIS THEORY

12. ENDOSYMBIOSIS THEORY
Anendosymbiosisoriginfortheseorganellesisnowgenerallyaccepted,
withmitochondriathoughttohaveevolvedfromaerobicbacteriaand
chloroplastsfromphotosyntheticbacteria,suchasthecyanobacteria.
Theacquisitionofaerobicbacteriawouldhaveprovidedananaerobiccell
withtheabilitytocarryoutoxidativemetabolism.
Theacquisitionofphotosyntheticbacteriawouldhaveprovidedthe
nutritionalindependenceaffordedbytheabilitytoperformphotosynthesis.
Thus,theseendosymbiosisassociationswerehighlyadvantageoustotheir
partnersandwereselectedforinthecourseofevolution.
Throughtime,mostofthegenesoriginallypresentinthesebacteria
apparentlybecameincorporatedintothenucleargenomeofthecell,soonly
afewcomponentsofmitochondriaandchloroplastsarestillencodedbythe
organellegenomes.
Thus,fromthepointoforiginisconcerned,theengulfmentofaerobic
bacteriatoprokaryotestoaddresstheenergyneedandcyanobacteriato
addressrespiratorysubstratesaretwoimportantmilestonesinthisregard.

13. ENDOSYMBIOSIS THEORY
EVIDENCESTOSUPPORTTHETHEORY
i.Newmitochondriaandplastidsareformedonlythroughprocesssimilar
tobinaryfissionthattakesplaceinbacteria,
ii.Transportproteinscalledporinsarefoundinmembraneofboth
mitochondriaandchloroplastanditiscloselysimilartothebacterialcell
membrane,
iii.Amembranelipid,cardiolipinisexclusivelyfoundintheinnermembrane
ofmitochondriaandbacterialcellmembrane,
iv.BothmitochondriaandplastidscontainsinglecircularDNAthatisquite
differentfromcellnucleusbutsimilartothatofbacteriumbothsizeand
structure,
v.Genomecomparisonindicatethecyanobacteriacontributedtothegenetic
originofplastids,
vi.Theribosomeofchloroplastsandmitochondriaarelikethosefondin
bacteriaif79Stypes,
vii.Severalenzymesandtransportsystemsofmitochondriasimilartothose
ofbacteria,

14. INVAGINATRION OF SURFACE MEMBRANE

15. INVAGINATRION OF SURFACE MEMBRANE
MembraneInfolding/InvaginationofsurfacemembraneTheinvasionsof
thehostprokaryotecellprobablyweresuccessfulbecausethehostcell
membraneinfoldedtosurroundbothinvadingprokaryotecellsandthereby
helptransportthemintothecell.
Themembranedidnotdissolvebutremainedintact,andtherebycreateda
secondmembranearoundtheprotomitochondriaandprotochloroplast.Itis
alsoknownthatinmodern-dayeukaryotestheinnermembraneofboththe
mitochondriaandchloroplastcontainstructuresmoresimilartoprokaryotes
thaneukaryotes,whereastheoutermembraneretainseukaryote
characteristics!
Itisalsosuggestedthatcontinuedmembraneinfoldingcreatedtheendo-
membranesystem.Itcanbesaidthatpossiblythefirsteukaryoticcelltype
wasmiraculouslybornfromprokaryotic,symbiotic,multicellinteractions.

16. AUTOGENOUS MODEL
AUTOGENOUS MODEL
Accordingtotheautogenousmodel,theeukaryotesarosedirectlyfroma
singleprokaryoteancestorbycompartmentalizationoffunctionsbrought
aboutbyinfoldingsoftheprokaryoteplasmamembrane.
Thismodelisusuallyacceptedfortheendoplasmicreticulum,Golgi,and
thenuclearmembrane,andoforganellesenclosedbyasinglemembrane
(suchaslysosomes).
Accordingtotheautogenoushypothesis,mitochondriaandchloroplasts
haveevolvedwithintheprotoeukaryotecellbycompartmentalizing
plasmids(vesiclesofDNA)withinapinchedoffinvaginationofthecell
membrane.Similaritiesbetweenmitochondriaorchloroplastsand
eubacteriacanbeaccountedforbymosaicevolutioninwhichthe
componentsinthecompartmentevolvemoreslowlythanotherpartsofthe
cell,andthusretainmanyeubacteriafeatures.Mitochondriaorchloroplasts
mayhaveacquiredtheirdouble-membranestatusbysecondaryinvagination
ormoreelaboratefoldingofmembrane

17. SYNAPOMORPHIES THEORY
SYNAPOMORPHIES (characteristicspresentinanancestralspeciesand
sharedexclusivelybyitsevolutionarydescendants)–
Thealternativetheoriesmakequitedifferentpredictionsaboutthe
similaritiesthatonemightexpecttofindamongeubacteria,nuclear,and
organellegenomes.
Theautogenousoriginhypothesispredictsthatplastidandmitochondrial
genomesshouldsharemoresynapomorphiestonucleargenomesthanto
prokaryotic(eubacteriaandarchaebacteria)genomesinbasicfeaturesof
structure,organizationandexpression.
Thisisbecauseorganelleandnucleargenomeswouldhaveshareda
commonancestormorerecentlythanorganelleandprokaryotegenomes.
Thexenogenousoriginhypothesispredictsjusttheoppositeandspecifically
predictsthatorganellegenomesshouldsharemoresynapomorphieswith
eubacteriathanwitheithernuclearorarchaebacteriagenomes.

18. THANKS FOR YOUR JOURNEY
ACKNOWLEDGEMENT
i.Google for images,
ii.Some WebPages for illustrations and information,
iii.Biomolecules and Cell Biology-Sahu.
iv.Microbiology of Microorganisms-Madigan, Martinko, Bender,
Buckley, Stahl,
v.Microbiology-Dubey and Maheswari.
vi.NIH Website,
vii.Science direct.
DISCLAIMER:
Thispresentationhasbeendevelopedtoaddresstheneedof
academicfraternity.Theauthordoesnotclaimanyfinancial
interest.Itisfreetouseanddownload.