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BIOL221

Ch12CellCycleandMitosis

TheKeyRolesofCellDivision

• Celldivision

• Cellularreproduction

• Anabilityoforganismsthatbestdistinguisheslivingthingsfromnonlivingmatter

• Unicellularorganisms

• Cellulardivisionreproducestheentireorganism

• Multicellularorganisms

• dependoncelldivisionfor:

• Developmentfromafertilizedcell

• Growth

• Repair

• Cellcycle

• Lifeofacellfromformationtoitsowndivision

• Celldivisionisanintegralpart

CellDivision

100 µm 200 µm 20 µm

(a) Reproduction (b) Growth anddevelopment

(c) Tissue renewal

2

CellularOrganizationoftheGeneticMaterial• Genome

• AlltheDNAinacell

• MaybeasingleDNAmolecule

• prokaryoticcells

or…

• ManyDNAmolecules

• eukaryoticcells

• Chromosomes

• PackagedDNAmoleculesinacell

• OnemoleculeofDNAequalsonechromosome

• Humanrange51million- 245millionbasepairsperchromosome

• Eacheukaryoticspecies

• Hasacharacteristicnumberofchromosomesineachcellnucleus

• Somaticcells

• Non-reproductivecells

• havetwoofeachtypeofchromosome

• Diploid- 46

• Germcells

• MakeGametes

• Reproductivecells:spermandeggs

• Onlyoneofeachchromosome

• Haploid- 23

CellularOrganizationoftheGeneticMaterial

• Chromatin

• ComplexofDNAandproteinthatcondensesduringcelldivision

• InEukaryoticchromosomes

• TypicalDNAformwhennotactivelydividing

CellularOrganizationoftheGeneticMaterial

3

ChromosomeDistributionDuringEukaryoticCellDivision• Preparationforcelldivision

• DNAisreplicatedandthe

chromosomescondense

• Sisterchromatids

• Duplicatedchromosome

• Twocopieswhichseparate

duringcelldivision

• Centromere

• Junctionpointoftheduplicated

chromosome,wherethetwo

chromatidsareattached

Fig.12-4

0.5 µm Chromosomes

Chromosomeduplication(including DNAsynthesis)

Chromo-some arm

Centromere

Sisterchromatids

DNA molecules

Separation ofsister chromatids

Centromere

Sister chromatids

• Eukaryoticcelldivisionconsistsof:

• Mitosis

• Divisionoftheduplicatedgenome

• Cytokinesis

• Divisionofthecytoplasm

• Meiosis

• Avariationofcelldivisionformakinggametes

• Yieldsnon-identicaldaughtercells

• Onlyonesetofchromosomes,halfasmanyastheparentcell

• Haploid

TypesofEukaryoticCellDivision

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PhasesoftheCellCycle

• Thecellcycleconsistsof2phases

• Mitotic(M)phase

• mitosisandcytokinesis

• Interphase

• cellgrowth

• copyingofchromosomesin

preparationforcelldivision

• Interphase

• about90%ofthecellcycle

• canbedividedintosubphases:

• G1 phase

• firstgap

• Sphase

• Synthesisofduplicate

DNA

• G2 phase

• secondgap

PhasesoftheCellCycle- Interphase

S(DNA synthesis)G1

G2

• Mitosisisconventionallydividedintofive

phases:

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

• Cytokinesisiswellunderwaybylate

telophase

PhasesoftheCellCycle- MitoticPhase

PLAY

5

Fig.12-6

G2 of Interphase

Centrosomes(with centriolepairs)

Chromatin(duplicated)

Nucleolus Nuclearenvelope

Plasmamembrane

Early mitoticspindle

Aster Centromere

Chromosome, consistingof two sister chromatids

Prophase Prometaphase

Fragmentsof nuclearenvelope

Nonkinetochoremicrotubules

Kinetochore Kinetochoremicrotubule

Metaphase

Metaphaseplate

Spindle Centrosome atone spindle pole

Anaphase

Daughterchromosomes

Telophase and Cytokinesis

Cleavagefurrow

Nucleolusforming

Nuclearenvelopeforming

Prophase PrometaphaseG2 of Interphase

PrometaphaseProphaseG2 of InterphaseNonkinetochore

microtubulesFragmentsof nuclearenvelope

Aster CentromereEarly mitoticspindle




Plasmamembrane



• Interphase(G2)

• Diffusechromatin

• duplicatedstate

• Chromosomesnotvisible

underlightmicroscope

• Nucleolusstillpresent








Plasmamembrane



• Prophase

• Chromatinbegins

tocondense

• Centrosomes

begintomigrate

• Spindlebeginsto

grow

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Plasmamembrane



• Prometaphase

• Chromsomes fullycondensed

• Nuclearmembranebrokendown

• Centrosomesestablishpoles

• Spindlefibersattachtokinetochores

Fig.12-6c

Metaphase Anaphase Telophase and Cytokinesis


Cleavagefurrow

Nucleolusforming

Metaphaseplate

Centrosome atone spindle pole

SpindleDaughterchromosomes


• Metaphase

• Chromosomesline

upatmetaphase

plate

• Singlefile

• Guidedby

spindlefibers

Fig.12-6c



Cleavagefurrow

Nucleolusforming

Metaphaseplate




• Anaphase

• Sisterchromatids

separate

• Now

daughter

chromosomes

• Migratetopoles

• Spindlefibers

shorten

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Fig.12-6c



Cleavagefurrow

Nucleolusforming

Metaphaseplate




• Telophase

• Daughter

chromosomes

reachpoles

• Beginto

decondense

• Nuclear

membranesform

• Cytokinesis begins

• Cleavagefurrow

forms

TheMitoticSpindle:ACloserLook

• Mitoticspindle

• Apparatusofmicrotubulesthatcontrolschromosomemovementduringmitosis

• Centrosome

• Microtubuleorganizingcenter

• Consistsoftwocentrioles

• Duringprophaseassemblesspindlemicrotubules

• Thecentrosomereplicates

• formingtwocentrosomes

• migratetooppositeendsofthecell

• spindlemicrotubulesgrowoutfromthem

• Aster

• aradialarrayofshortmicrotubules

• extendsfromeachcentrosome

• anchorscentrosometorestofcytoskeleton

• Thespindleincludesthecentrosomes,thespindlemicrotubules,andtheasters

TheMitoticSpindle:ACloserLook

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• Kinetochore

• Attachmentpointfor

microtubulesofspindle

• Proteinstructurebound

toDNAatcentromere

TheMitoticSpindle

Fig.12-7

Microtubules Chromosomes

Sisterchromatids

Aster

Metaphaseplate

Centrosome

Kineto-chores

Kinetochoremicrotubules

Overlappingnonkinetochoremicrotubules

Centrosome 1 µm

0.5 µm

• Inanaphase

• sisterchromatidsseparateandmovealongthekinetochore

microtubulestowardoppositeendsofthecell

• Spindlemicrotubules

• shortenbydepolymerizingattheirkinetochoreends

TheMitoticSpindle

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Fig.12-8

EXPERIMENT

Kinetochore

RESULTS

CONCLUSION

Spindlepole

Mark

Chromosomemovement

Kinetochore

Microtubule Motorprotein

Chromosome

Tubulinsubunits

• Non-kinetochoremicrotubules

• fromoppositepolesoverlapandpushagainsteachotherelongatingthe

cell

• Intelophase

• geneticallyidenticaldaughternucleiformatoppositeendsofthecell

TheMitoticSpindle

Cytokinesis:ACloserLook• Animalcells

• cytokinesisoccursbycleavage,formingacleavagefurrow

• Plantcells

• acellplateformsduringcytokinesis

Cleavage furrow100 µm

Contractile ring ofmicrofilaments

Daughter cells

(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM)

Vesiclesformingcell plate

Wall ofparent cell

Cell plate

Daughter cells

New cell wall

1 µm

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Sea Urchin (Time Lapse)

Animal Mitosis

PLAY

PLAY

BinaryFission

• Prokaryotes(Eubacteriaandarchaebacteria)

• Reproducebyatypeofcelldivisioncalledbinaryfission

• Singlechromosomereplicates(beginningattheoriginof

replication)

• twodaughterchromosomesactivelymoveapart

Fig.12-11-1

Origin ofreplication

Two copiesof origin

E. coli cell Bacterialchromosome

Plasmamembrane

Cell wall

11

Fig.12-11-2


Two copiesof origin


Plasmamembrane

Cell wall

Origin Origin

Fig.12-11-3


Two copiesof origin


Plasmamembrane

Cell wall

Origin Origin

Fig.12-11-4


Two copiesof origin


Plasmamembrane

Cell wall

Origin Origin

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TheEvolutionofMitosis

• Binaryfissionpredatesmitosis

• Sinceprokaryotesevolvedbeforeeukaryotes,mitosisprobablyevolved

frombinaryfission

• Someprotists areintermediatebetweenbinaryfissionandmitosis

• Dinoflagellates

• Diatoms/Yeasts

Bacterialchromosome

Chromosomes

Microtubules

(a) Bacteria

(b) Dinoflagellates

Intact nuclearenvelope

Kinetochoremicrotubule

(c) Diatoms and yeasts

Kinetochoremicrotubule

(d) Most eukaryotesFragments of nuclear envelope

Intact nuclearenvelope

RegulationoftheCellCycle

• Thefrequencyofcelldivisionvarieswiththe

typeofcell

• Musclecells

• Neurons

• Hepatocytes

• Thesecellcycledifferencesresultfrom

regulationatthemolecularlevel

• Signalingmolecules

TheCellCycleControlSystem

• Cellcyclecontrolsystem

• Regulatedbybothinternalandexternalcontrols

• Hasspecificcheckpointswherethecellcyclestopsuntilago-ahead

signalisreceived

SG1

M checkpoint

G2M

Controlsystem

G1 checkpoint

G2 checkpoint

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• G1 checkpoint

• Appearsmostimportant

• cellreceivesago-aheadsignalattheG1 checkpoint,itwillusuallycompletetheS,G2,andMphasesanddivide

• Nogo-aheadsignal

• Cellsexitscycle,switchingintoG0 phase- anon-dividingstate

TheCellCycleControlSystem

G1

G0

G1 checkpoint

(a) Cell receives a go-aheadsignal

G1

(b) Cell does not receive ago-ahead signal

Cyclins&Cyclin-DependentKinases• Twotypesofregulatoryproteins

involvedincellcyclecontrol

• Cyclins

• Cyclin-dependentkinases

(Cdks)

• Activityfluctuatesduring

thecellcycle

• MPF (maturation-promotingfactor)

• Cyclin-Cdkcomplexthat

triggersacell’spassagepasttheG2 checkpointintotheM

phase

Stop/GoSigns:SignalsattheCheckpoints• Internalsignal

• Mcheckpoint

• kinetochoresnotattachedtospindlemicrotubules

• sendamolecularsignalthatdelaysanaphase

• Externalsignals

• Growthfactors

• proteinsreleasedbycertaincellsthatstimulateothercellstodivide

• platelet-derivedgrowthfactor(PDGF)stimulatesthedivisionof

humanfibroblastcellsinculture

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• Otherexternalsignals

• density-dependentinhibition

• crowdedcellsstopdividing

• anchoragedependence

• Mostanimalcellsmustbeattachedtoasubstratuminordertodivide

Stop/GoSigns:SignalsattheCheckpoints

Anchorage dependence

Density-dependent inhibition

Density-dependent inhibition

(a) Normal mammalian cells (b) Cancer cells25 µm25 µm

Tumor

A tumor growsfrom a singlecancer cell.

Glandulartissue

LymphvesselBloodvessel

Metastatictumor

Cancercell

Cancer cellsinvade neigh-boring tissue.

Cancer cells spreadto other parts ofthe body.

Cancer cells maysurvive andestablish a newtumor in anotherpart of the body.

1 2 3 4

• Transformation

• Anormalcellisconvertedtoacancerouscellwhichformstumors

• massesofabnormalcellswithinotherwisenormaltissue

• Benigntumor

• Abnormalcellsremainattheoriginalsite

• Malignanttumors

• Invadesurroundingtissues

• Canmetastasizeexportingcancercellstootherpartsofthebody,possiblyformsecondarytumors

LossofCellCycleControlsinCancerCells

Youshouldnowbeableto:

1. Describethestructuralorganizationoftheprokaryoticgenomeandtheeukaryoticgenome

2. Listthephasesofthecellcycle;describethesequenceofeventsduringeachphase

3. Listthephasesofmitosisanddescribetheeventscharacteristicofeachphase

4. Drawordescribethemitoticspindle,includingcentrosomes,kinetochoremicrotubules,nonkinetochore microtubules,andasters

5. Comparecytokinesisinanimalsandplants

6. Describetheprocessofbinaryfissioninbacteriaandexplainhoweukaryoticmitosismayhaveevolvedfrombinaryfission

7. Explainhowtheabnormalcelldivisionofcancerouscellsescapesnormalcellcyclecontrols

8. Distinguishbetweenbenign,malignant,andmetastatictumors

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