Nucleus- Different Parts, structure and Function

This presentation offers a very brief idea of the cell nucleus along with their structural complexity and functional attributes. The content consists of the different ultra microscopic part of nucleus in general and nuclear pore, nucleolus. nucleolgenesis, nuclear pore along with the structural complexity of chromatin network.

1. Nucleus:Structure-Nuclear Envelope, Nuclear Pore
Complex, Nuclear Lamina, Molecular Organization of
Chromatin; Nucleolus, Endomembrane System
Dr. Nandadulal Sannigrahi,
Associate Professor,
Department of Botany,
Nistarini college, Purulia
D.B. Road, Purulia,
INDIA (W.B)

2. CELL-THE BASIC UNIT OF LIFE
Acellisthesmallestlivingthinginthehumanorganism,andallliving
structuresinthehumanbodyaremadeofcells.Therearehundredsof
differenttypesofcellsinthehumanbody,whichvaryinshape(e.g.
round,flat,longandthin,shortandthick)andsize(e.g.smallgranule
cellsofthecerebelluminthebrain(4micrometers),uptothehuge
oocytes(eggs)producedinthefemalereproductiveorgans(100
micrometers)andfunction.
However,allcellshavethreemainparts,theplasmamembrane,the
cytoplasmandthenucleus.Theplasmamembrane(oftencalledthe
cellmembrane)isathinflexiblebarrierthatseparatestheinsideofthe
cellfromtheenvironmentoutsidethecellandregulateswhatcanpass
inandoutofthecell.Internally,thecellisdividedintothecytoplasm
andthenucleus.Thecytoplasm(cyto-=cell;-plasm=“something
molded”)iswheremostfunctionsofthecellarecarriedout.
Itlooksabit-likemixedfruitjelly,wherethewateryjellyiscalledthe
cytosol;andthedifferentfruitsinitarecalledorganelles.Thecytosol
alsocontainsmanymoleculesandionsinvolvedincellfunctions.
Differentorganellesalsoperformdifferentcellfunctionsandmanyare
alsoseparatedfromthecytosolbymembranes.

3. CELL-THE BASIC UNIT OF LIFE
Thelargestorganelle,thenucleusisseparatedfromthecytoplasmbya
nuclearenvelope(membrane).ItcontainstheDNA(genes)thatcode
forproteinsnecessaryforthecelltofunction.
Generallyspeaking,theinsideenvironmentofacelliscalledthe
intracellularfluid(ICF),(intra-=within;referredtoallfluidcontained
incytosol,organellesandnucleus)whiletheenvironmentoutsidea
celliscalledtheextracellularfluid(ECF)(extra-=outsideof;referred
toallfluidoutsidecells).Plasma,thefluidpartofblood,istheonly
ECFcompartmentthatlinksallcellsinthebody.
Theintegrationofthecellismaintainedbythecytoskeletonandthe
cellwallconifersprotectionandcellmembranemaintainaselective
permeabilitytodesignthecellasanosmoticsystem.
Inadditiontothese,differentmembraneandnon-membranebound
organellesalongwithothersubstancesmakethecellasthestructural
&functionalunitoflife.

4. NUCLEUS
Cell is the basic unit of life,
Thecellnucleusisthemostnoticeableorganellewithin
theeukaryoticcell,andperhapsthemostimportantanddefining
featureoftheeukaryoticcells.Mostofthegeneticmaterial(DNA)is
containedinthenucleus,whileasmallamountofitisfoundin
mitochondria.
Themajorityofhumancellshaveasinglenucleus,althoughthereare
severalcelltypesthathavemultiplenuclei(e.g.osteoclasts)ordon't
haveanucleusatall(erythrocytes).
RobertBrown(1883)wasthediscovererofthenucleus.
J.Hammerling(1934),aGermanscientistdemonstratedthatnucleus
determinesthecharacterofcellaswellasthecharacterofindividual,
Itusuallyplacedcentrallybutinsomeplantcellsandepithelialcells
ofanimals,itisperipheralinposition,
Itmaybespherical,oval,C-shaped,pyriformormaybelobulated.

5. CELL STRUCTURE -PLANT

6. NUCLEUS
Thenucleusisnormallyaround5-10μmindiameterinmany
multicellularorganisms,andthelargestorganelleinthecell.The
smallestnucleiareapproximately1μmindiameterandarefoundin
yeastcells.
Mostlytheshapeofthenucleusissphericaloroblong.Usuallycells
haveonenucleusbutmanyattimestherearemultinucleatedcells.
Multi-nucleationincellsmaybeduetokaryokinesis(whencell
undergoesnucleardivision)orwhencellsfusetoformsyncytium,
likeinmaturemusclecells.
Thenucleushasveryimportantrolestoplay.Asitcontainsgenetic
material,itcoordinatescellactivitieslikeproteinsynthesisandcell
division.
Anatomicallythenucleusismadeupofseveralcomponents:nuclear
envelope,nuclearlamina,nucleolus,chromosomes,nucleoplasmare
someofthesecomponents.

7. NUCLEUS

8. NUCLEUS
Allofthesecomponentsworktogetherinorderforthenucleusto
accomplishallofitsfunctions.Namely,thesefunctionsare:
controlofthegeneticallyinformationofthecellandthustheheredity
characteristicsofanorganism,
controloftheproteinandenzymesynthesis
controlofcelldivisionandcellgrowth
storageofDNA,RNAandribosome
regulationofthetranscriptionofthemRNAtoprotein
productionofribosome
Whenacellishistologicalstained,thenucleusnormallyappearsasa
large,darkorganelle,mostlyatornearthecentreofacell.

9. NUCLEAR ENVELOPE
Asitsnamesuggests,thenuclearenvelopesurroundsthenucleus,
separatingitfromthecell'scytoplasm.Itisadoublemembrane.
Eachmembraneisaphospholipidsbilayerassociatedwithproteins,and
thetwomembranesaredividedby20to40nmofspace.
Thetwomembranesofthenuclearenvelopeareoftenreferredtoasthe
innerandouternuclearmembranes.
Theoutermembraneiscontinuouswiththecell’sendoplasmicreticulum,
andthereforethespacebetweentheinnerandouternuclearmembranes
linkstothelumenoftheendoplasmicreticulum.
Liketheendoplasmicreticulum,theouternuclearmembranehas
ribosomeattachedtoit.Contrastingly,theinnermembraneofthenuclear
envelopeisattachedtoproteinsthatarespecifictothenucleus,and
thereforefoundnowhereelse.
Thenuclearenvelopeisperforatedwithtinynuclearporeswithdiameters
ofaround100nm.Theinnerandoutermembranesoftheenvelopeare
continuousaroundthepores.Eachporeislinedwithastructureof50to
100differentproteinsknownasthenuclearporecomplex.

10. NUCLEAR ENVELOPE
Thenucleoporesarecircularoroctagonalinsurfaceview
Itisbelievedduetotheresultofdistortionduringprocessingandbecause
bythepresenceofeightsymmetricallyarrangedannularsubunits,
Theporesarearrangedinrowsinplantcellsorinclustersinlymphocytes
butusuallyarerandomlydistributed,
‘Basket-likestructure’hasbeenfoundtotheinnersideofnuclearpore
(Donglass.J),
Theporecomplexis125millionDaltoninsizewith130nmdiameter
and50mminthickness,
Consistsoffourseparateelements-scaffold,transporter,thickfilaments
andabasket,
Scaffoldcomposedofthreecloselyapposedrings-cytoplasmicring,
nucleoplasmicringandacentralringhavingeightfoldsymmetryineach;
thespokesofthecentralringareattachedtothetransporteroftheinner
sideandtothenucleocytoplasmicandcytoplasmicringsontheouter
side.
Thespikesareinterspacedby9mmwideaqueouschannelsthrough
whichproteinsandmetabolitescanbediffusedbetweenthenucleusand
cytoplasm,

11. NUCLEAR ENVELOPE

12. NUCLEAR ENVELOPE
Transportersare36-38mmwideprotenaceousringsconsistsoftwo
irisesofeightarmsineach,
Onthecytoplasmicsideofpore,thickfibersof3.3mmdiameterare
projectedintothecytoplasm,
FUNCTIONOFNUCLEARPORECOMPLEX
Theseporecomplexesregulatethemovementsofmacromolecules,
RNAsandproteinsintoandoutofthenucleusbythreemethods-
signalsfortransportacrossthepore,importofnuclearproteins,
exportofRNAfromthenucleusandExportandreimportofRNA
Thismovementofmoleculesisknownasnucleartransport.Small
moleculescanmovepassivelythroughthepores,butlarger
molecules,includingRNAsandmanyproteins,aretoolargeforthis
andmustmoveactively.
Duringthisactiveprocess,theyareselectivelyrecognizedand
transportedinonespecificdirection.
ThetrafficofRNAsandproteinsthroughthenuclearporecomplexis
particularlyimportant,astheyplayaroleingeneexpression.

13. NUCLEAR PORE COMPLEX

14. NUCLEAR PORE COMPLEX -FUNCTION
Signalfortransportacrossthemembrane-importofproteinsandRNPs,
somesignalsequencesasNuclearLocalizationSequences(NLS),
NLSusedforexportarenuclearexportsignal(NES)likeHIVRes
protein,PhosphokinaseAinhibitor(PKI)andTFIIIAusedfor
transcribing5SRNAandtRNAgenes;M9domainofhnRNAproteinin
theonlyknownsignalsequencethatdirectsbothexportandimport
processes,
Importofnuclearproteins-NLSproteinreceptorcomplexwiththehelp
ofhydrolysisofGTPandRanGTPaseplaysignificantroleinthisregard,
ExportofRNAfromthenucleus-Revresponseelement(RRE)andits
activationplayasignificantrolehere,
ExportandreimportofRNA-Exportfollowedbyreimportof5srRNA
andUsnRNAoccursthroughnuclearpore;TFIIIAandribosomal
protein1.5probablyactasmediatorsinexportof5SrRNA.Itseems
containRev-NESlikesequence,
Thus,theverycomplexbiochemicalphenomenonisassociatedherein
theimport,exportandreimportofdifferentmoleculesvianuclearpore
andthisporeactsasconnectingnetworkbetweencytoplasmandnucleus.

15. NUCLEAR LAMINA
Theinnernuclearmembraneisinternallylinedbyproteinfilaments
meshworkorganizedinanet-likefashion,callednuclearlamina.
Theproteinsthatmakeupthenuclearlaminaareknownaslamins,
whichareintermediatefilamentproteins.
Thesesupportthenuclearenvelope,ensuringthattheoverallshape
andstructureofthenucleusismaintained.
Inadditiontolaminsthereisanothersetofmembraneproteinscalled
laminaassociatedproteins,whichhelptomediatetheinteraction
betweenthelaminaandinnernuclearmembrane.
Thenuclearlamina,alongwithproteinfiberscalledthenuclear
matrix,isalsothoughttoaidintheorganizationofgeneticmaterial,
allowingittofunctionmoreefficiently.

16. NUCLEOLUS
Whenanucleusisnotdividing,astructurecalledanucleolusbecomes
visible.Infact,itisthemostprominentstructurewithinthenucleus.
Usuallythereisonlyasinglenucleoluspresent,butsomenucleihave
multiplenucleoli.Itisamassofgranulesandfibersattachedto
chromatin.Itisabsentinyeast,bacteria,algae,oneinmostcasesbut
twoinlivercells,multipleinpolyploidycells,
FirstdiscoveredbyFontanain1874locatedincloseassociationwith
nucleolarorganizerregion(NOR)oftwoormorechromosomes,
Nucleolioftwotypes-truenucleoliorplasmasomesstainedwith
acidicdyeanddisappearduringmitosisandfalsenucleoli,
karyosomesstainedwithbasicdye,actuallyflakesofchromatins,
Underlightmicroscope.Continuouscoiledfilamentcalled
nucleonemma,
Fourzones-amorphousmatrix,nuclearassociatedchromatinfibrils
andgranules,nuclearassociatedchromatinconsistsifDNAwith
perinuclearchromatin,
FibrilsaresmallwithRNA,
Thegranulescontainvesicleswithcentralcoreanddenseperipheral
structure

17. NUCLEOLUS

18. NUCLEOLOGENESIS
Theprocessoftheformationofnucleolusiscallednucleologenesis,
Conspicuousininterphasebutdisappearduringcelldivision,The
processinvolvesthefollowingsubstagesafterHadjiolov(1985)
Fromthepre-metaphasetotelophase,anumberofnon-ribosomal
nucleolarproteinssuchasB23,fibrillarin,nucleolin,,p52andU3
snoRNAarefoundintheperipheralregionsofchromosomes,
Thenumberofnucleolusderivedfoci(NDF)canreachupto100per
cellatlatemetaphase,
AsNDFdecreases,theperinuclearbodies(PNB)graduallyappears
andnucleolusformationstart,
ThePNBandNDFaretheprecursorsofnewlyformednucleoli,
ActinomycinDblockstheformationofnucleolifromPNB.
Thus,itisclearlysuggestedthatnucleolusformationrequiresactive
transcriptionoftRNAfromribosomalgenesbyRNApol-1.

19. FUNCTION OF NUCLEOLUS
ThenucleolusisimportantbecauseitisthesiteofribosomalRNA
(rRNA)production.Insidethenucleolus,rRNAmoleculesarecombined
withproteinstoformribosome.Thebiogenesisprocessi.e.initiation,
production,andmaturationofribosometakesplaceinthenucleolus
regionfromthecentretotheperipheryinthethreedistinctregions,
Inthefibrillarcentre(FC),therDNAsofNORarelocatedwhich
transcribesrRNA.TheprerRNAtranscriptionactuallyoccursattheedge
ofthetranscriptionzoneofFCandaccordingtoMiller,thestructureis
describedasChristmastree.
ThenucleolusisinvolvedinrRNAtranscription,pre-rRNAprocessing
andribosomesubunitassembly.
Thenucleolusisnotsurroundedbyamembrane,butithasaunique
density,separatingitfromthesurroundingnucleoplasm,andallowingitto
bevisualizedunderamicroscope.
Aswellasbeinginvolvedinribosomalbiogenesis,thenucleolusis
thoughttohaveotherroles,asitcontainsanumberofproteinsunrelated
torRNAandribosomesynthesis.Itisthoughtbeplayaroleinactivities
suchasDNAdamagerepair,cellcycleregulationandRNAediting.

20. FUNCTION OF NUCLEOLUS
snoRNAandrRNAprocessing:
SmallNucleolarRNAscanbegroupedintotwocategories-those
derivedfromboxC(5′RUAUGA3′)andboxD(5′GUCA3′)present
atthe5′and3′endrespectively,
ItisbelievedthatsnoRNAofboxC/Dclassareencodedwithinthe
intronsofothergenes,
U3snoRNAassociateswithprerRNAandpassesthroughdifferent
regionsofnucleolusandundergoespassingintomaturerRNAwhich
isthenexported,
TheU3snRNArecyclesfromGCtoDFCagainforassociationwith
nascentprerRNA.
SevensnoRNAsarenowknowntoberequiredforprocessingofpre
rRNAoftheseU3andU14areboundtoexternalterminal
spacer(ETS)intheprocessingofprerRNA.
Probably,thesesnoRNAsactaschaperonestohelpcorrectfoldingor
theyareinvolveddirectlyincleavagereaction.

21. CHROMOSOMES
TheDNAofacellisfoundwithinthenucleus.Itisorganizedintounits
knownaschromosomes,eachcontainingalongDNAmoleculewhichis
associatedwithvariousproteins.
TheDNAcoilsaroundproteincomplexescallednucleosomes,formedof
proteinscalledhistone,makingiteasierforthechromosometofitinside
thenucleus.
ThemassofDNAandproteinsinsideachromosomeisreferredtoas
chromatin.
Whenacellisnotdividing,itisdifficulttoseethechromosomeswithina
cell,evenwhenitisstained.However,whenDNApreparesandbeginsto
divide,thechromosomescanbevisualizedmoreclearly.
Duringthemetaphaseofmitosis,thechromosomesbecomevisibleas
theypreparetodividebyaligningwithoneanother.Thechromosomes
arecopied,formingsisterchromosomesknownachromatids.
Humancellnucleicontain46chromosomes,althoughgametenuclei
contain23.Thewholeofthenucleusisnotfilledbychromatinmaterial,
infact,therearechromatinfreeregionscalledinterchromosomal
domainscontainingpolyRNAs.

22. CHROMATIN
Intheinterphasestage,thenuclearsapcontainstwistedfilamentsof
chromatinwhichrepresentsthechromosome,
Chromatinfibersappearedasarraysofsphericalparticlesabout10nmin
diameter,connectedbyfilamentswhichareabout2nmindiameter.
Thesebodiesarecallednubodieswhichlateronconfirmedas
nucleosome,
Intheinterphasenucleus,thechromatinfilamentsmayshowboth
centromericandtelomericattachment,
Thechromatinfibersofinterphasenucleiareattachedtothethatmay
functionasinitiationpointofDNAsynthesis,
WhenchromatinbodiesarestainedwithacetocarmineorFeulgenat
prophase,alineardifferentiationintoregionshavingdensestainandthat
havelightstainsbecomeconspicuous,
Thedarkstainedregionsareheterochromatinandlightstainedregions
areeuchromatin,
Certainheterochromatinregionsparticularlyproximaltothecentromere
arecalledconstitutiveheterochromatinserveaschromosomemarkers
andotherheterochromaticregionsarecalledfacultatative
heterochromatin.

23. CHROMATIN

24. NUCLEOPLASM
Nucleoplasmissimilartothecytoplasmofacell,inthatitissemi-
liquid,andfillstheemptyspaceinthenucleus.
Itisaformofprotoplasmandsurroundsthechromosomesand
nucleoliinsidethenucleus.Italsohasvariousproteinsandenzymes
dissolvedwithinit.
Nuclearbodiescanbefoundinthenucleoplasm,andtheseinclude
structuressuchasCajalbodies,Geminibodies,andPolycombbodies.
Cajalbodiesarebetween0.3-1.0µmindiameter,andcanbefoundin
proliferatingcellssuchasembryonicandcancerouscells,aswellas
incellswhichhaveahighmetabolicrate,suchasneurons.
Sometimesreferredtoascoiledbodies,Cajalbodiesareboundto
nucleolibyspecializedproteinscalledcoilinproteins.
HavingtheseproteinsconcentratedwithinCajalbodiesimprovesthe
efficiencyofnuclearprocessessuchasthemodificationandassembly
ofUsnRNPs,whichcanbecomespliceosome.

25. NUCLEOPLASM

26. ENDOMEMBRANE SYSTEM
Theendomembranesystem(endo-=“within”)isagroupof
membranesandorganellesineukaryoticcellsthatworkstogetherto
modify,package,andtransportlipidsandproteins.Itincludesa
varietyoforganelles,suchasthenuclearenvelopeandlysosomes,
whichyoumayalreadyknow,andtheendoplasmicreticulumand
Golgiapparatus,whichwewillcovershortly.
Althoughit'snottechnicallyinsidethecell,theplasmamembraneis
alsopartoftheendomembranesystem.Aswe'llsee,theplasma
membraneinteractswiththeotherendo-membraneorganelles,andit's
thesitewheresecretedproteins(likethepancreaticenzymesinthe
intro)areexported.Importantnote:theendomembranesystemdoes
notincludemitochondria,chloroplasts,orperoxisomes.
Thenetworkamongthedifferentorganellesofthecellisimportantto
coordinatethedifferentdiversefunctionsofthecellasitneedsforthe
cellstructureandfunctionalongwiththeirintegrity,
TheERoftwotypes,ribosome,lysosomes,Golgibodiesetcplayan
extensivenetworkbythevirtueoftheendomembranesystem.

27. THANK YOU FOR YOUR VISIT
ACKNOWLEDGEMENTS:
a.Google for images,
b.Different websites for enriching the course content,
c.Science Direct pages,
d.A textbook of Botany-Vol III –Hait, Bhattacharya & Ghosh.
e.A Text Book of Cell and Molecular Biology-Ajay Paul,
f.Cell and Molecular Biology-Kar and Halder,
g.Concept of Genetics-Klug. Cummings, Spencer, Palladino,
DISCLAIMER:
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