Cell Cycle and Cell Division.pptx

Cell Cycle and Cell Division
© 2022, Aakash BYJU'S. All rights reserved

Cell cycle
Cell and celldivision
Interphase
Phases of cell cycle
G1phase
S phase
G2 phase
G0 phase
Cell cycle checkpoints:
Interphase
G1/S Checkpoint
G2/
M Checkpoint
Key takeaways
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2
3
4
5

Key takeaways
Mitosisphase
Stages of karyokinesis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Cell furrow formation
Cellplate formation
Regulation of cell cycle
Significance ofmitosis
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8
9
1
0
6

Key takeaways
Meiosis-I
Meiosis-II
Prophase-II
Metaphase-II
Anaphase-II
Telophase-II
Meiosis- IIvs Mitosis
Prophase-I
Metaphase-I
Anaphase-I
T
elophase-I
Significance ofmeiosis
Summary
1
2
1
3
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1
1

⚫ Cell is the basic structuralandfunctionalunitof life.
⚫ All living organisms aremade of cells.
Cell
Cellwall
Vacuole
Endoplasmic
reticulum
Cytoplasm
Nucleolus
Nucleus
Mitochondria
Nuclear
membrane
Golgi
complex
Cell
membrane
Ribosomes
Plant cell
Animal cell
Lysosomes
Centrioles
Ribosomes
Chloroplast

⚫ Cell division is a process by whicha parentcelldivides intotwodaughtercells.
⚫ Celldivisionis responsible forthefollowing:
o Woundhealing
o Regeneration
o Growth and development
Cell Division
Parent Cell
Daughter
cell
Daughter cell
Celldivision

⚫ The sequence of events bywhich a cell
duplicates its genome,synthesisesthe
otherconstituents ofthe cell and
eventuallydivides intotwo daughtercells
is termed cell cycle.
⚫ Duringthe growthof a cell,the cell
organelles duplicate, and DNA
replication takes place.
⚫ Cellgrowthresults in disturbingthe ratio
betweenthe nucleus andthe cytoplasm.
⚫ Therefore,it becomes essential forthe
cellto divide and restorethe nucleo-
cytoplasmic ratio.
Cell Cycle

⚫ Cell cycle consists oftwo basic phases
o Interphase
o M Phase (Mitosis phase)
⚫ I
nterphase is the phase betweentwo
successiveM phases,wherethe cell
prepares itself forcelldivision.
⚫ M phase is thephasewhere actualcell
division occurs.
⚫ Periodof cell cyclevaries fromorganismto
organism.
⚫ A human cell divides approximatelyevery
24 hours.
⚫ A yeast cell divides every 90 minutes.
Phases of Cell Cycle
Interphase
M Phase

Interphase
CELL
CYCLE
M Phase
G1
G2
S
⚫ Interphase is the most active
phase ofthe cell cycle.
⚫ Duringthis phase,cell growth and
DNA replication takes place.
⚫ It lasts formore than 95%ofthe
duration ofthe cell cycle.
⚫ It is also called as the resting phase
as there is no apparent activity related
to cell division.
⚫ Interphase is further divided into 3
stages:
o G1phase
o S phase
o G2 phase

⚫ G1 phase or Gap 1phase is the longest
phase of interphase.
⚫ It is present betweenmitosis and initiation
of DNA replication.
⚫ Inthisphase,thecellgrows in size.
⚫ Also,active synthesis of RNA and
proteinstakesplace inthisphase.
⚫ The cell organelles duplicate during
this phase.
G1Phase
CELL
CYCLE
M Phase
G2
S
G1phase
G1

S Phase
⚫ Thegeneticmaterial, inmostoftheorganisms, is
presentas DNA in the nucleus.
⚫ The DNA contains alltheinstructions requiredto
build and run a celldowntothe veryminute detail.
⚫ In S phase the DNAmolecules are duplicated.
This occurs through the process of DNA
replication.
⚫ DNA replication is theprocess of copyinga
DNAmoleculetoproducetwo identical DNA
molecules.
Parentalmolecule
Daughter
DNA
molecules
CELL
CYCLE
M
Phase
G
2
S
G1

⚫ Eventhoughthe DNA contentgets doubledduringthe S phase, the chromosome
number remains the same since the 2 copies of the DNA strand are still
attachedtoeachother
.
⚫ Inanimalcells, centrioles alsoduplicateinthisphase.
⚫ The centrioles helpindistributingtheduplicatedgeneticmaterial equally.
S Phase
Before S
phase
(G1
Phase)
After S
Phase
M Phase
CELL
CYCLE
G1
G2
S

⚫ G2 phase orgap2 phase is presentin betweenS phase andM phase.
⚫ Mitochondria, chloroplast andGolgi bodies duplicate inthis phase.
⚫ DNA synthesis stops atthisphase.
⚫ There is productionofproteins requiredforthe actualdividingphase.
⚫ By theendofthe G2 phase,thecell is nowready forcelldivision.
⚫ In animal cells,themitotic division
takes place onlyin the diploid
somatic cells.
⚫ In plantcells,mitotic division is
seen both in diploid and
haploid cells.
G2 Phase
CELL
CYCLE
M Phase
G1
G2
S

⚫ G0 phase is also called as quiescent stage.
⚫ Cells thatdonotdivide, exitfromcell cyclein G1
phase and enter into inactive G0 phase.
⚫ Somecells enterG0 phase permanentlyand
neverdivide again.
o Examples:Heart cells, nerve cells.
⚫ Cells thattemporarilyenterG0 phase can enter
G1 phase and undergo division.
o Example:Cambial cells thatundergo
division and help in secondary growth in
plants.
G0 Phase
CELL
CYCLE
M Phase
G1
G2
S
G0
Phase

Cell Cycle Checkpoints: Interphase
G1
/Scheckpoint
⚫ Itis the main checkpoint fora celltoprogress orhaltcell cycle.
⚫ Itchecks fornutrients,growthfactors, DNA damage
⚫ Ifconditions arenotfavourable,thecell exits G1 phase andenters G0
phase.
G2/Mcheckpoint
⚫ Cell checks for DNA damageandensuresthatDNA replicationis donewithouterrors.
⚫ G2 checkpoint, beforeM phase, ensures that cell division proceeds andhealthy
daughter cells are formed.
⚫ Iferrorshave occurredduringDNA replication,thencell pauses allowingthecelltoundergo
repair
.
⚫ Iferrors arenotrectified,thencellundergoes programmedcelldeath,wherethecell’s lysosomes
releasetheirhydrolyticenzymestodestroy itself.
G2
M
S
DNA
synthesis
G1
G0

Mitotic Phase
Followingthe
karyokinesis,the
cytoplasm divides andthis
results in the formation of
twodaughtercells.
(Cytos- Cell/Hollow,
Kinesis - Movement)
The replicated chromosomes separate
and two nuclei are formed.
Followingkaryokinesis,the cytoplasm
divides andthis results in the formation of
twodaughtercells.
(Karyon- Nucleus,Kinesis -
Movement)
⚫ Themitosisphase ofthecelldivisionphase includestwosteps:
Mitosis phase
Karyokinesis Cytokinesis

Mitosis Phase
⚫ Mitosiswas first observed by Strasburger in plant cell and
by Walter Flemming in animal cell.
⚫ The term mitosiswas given by Walter Flemming.
⚫ A typeof celldivisionthatproduces:
○ T
wo similardaughtercells
○ Havingthe samenumberof chromosomes as parentcell.
Mitosis
Prophase Metaphase Anaphase Telophase
Karyokinesis Cytokinesis

⚫ Prophase is the first phase ofkaryokinesis.
⚫ Itis thelongestphase interms ofthetimetakenforcompletion.
⚫ The chromatinfibres startcondensingduringtheearlyprophase andforma condensed
mass.
⚫ Since, itresembles a condensedball ofwool, earlyprophaseis alsoknownas the
spireme stage (tangle or coil of filament).
⚫ By the lateprophase,theyfurthercondensetoformthechromosomes.
Stages of Karyokinesis: Prophase

⚫ Duringprophasethe nuclear membrane degenerates
and the nucleolus disappears.
⚫ If nuclearmembranedisappears duringthemitosis,it is called
eumitosis and ifthe nuclearmembrane remains intact it is
calledpremitosis.
⚫ Disintegration of endoplasmic reticulum and Golgi
apparatus also takes place.
⚫ The centrosomeswithreplicatedcentrioles startmoving
towards the oppositepoles.
⚫ Eachcentrosomeradiatesmicrotubulesknownas asters.
Aster rays help the centrioles to hold their place in the
cytoplasm.
⚫ Inanimalcells,mitosis is calledamphiastral. Inplantcells, it
is called anastral.
⚫ The centrioles form spindle fibres.
Stages of Karyokinesis: Prophase
Nuclear
membrane
Centrioles

⚫ The completedegradationofthenuclearmembranemarks
the startofmetaphase.
⚫ The chromosomes come to lie at the equatorial plate
(equidistantfromthetwopoles). This process is knownas
congression.
⚫ Congressionoccurswiththeassembly ofthemitoticspindle
that mediates the microtubule-chromosome interactions
requiredforthemovementof chromosomes.
⚫ The centromere is surrounded by a smalldisc shaped
structurecalledkinetochore. Thekinetochoreformthe
site of attachment of microtubules.
⚫ Chromosomes areobservedto be the thickest andthe
shortest atthis stage.
Stages of Karyokinesis: Metaphase

⚫ The centromere splits.
⚫ The sister chromatids separate intotwoidenticaland
independentchromosomes.
⚫ Each chromatid now has its own centromere.
⚫ The spindlefibres pullthechromatids alongwiththe
centromeretowards theirrespectivepoles.
⚫ The chromatids move to opposite poles.
⚫ Half ofthemreachonepole andtheotherhalfreachthe
other
.
⚫ Duringmigration, the centromere of chromosomes
face towards the poles.The chromatids or arms of
chromosomes trail behind.
Stages of Karyokinesis: Anaphase

⚫ Chromosomes clusterat oppositepoles.
⚫ They start decondensing into chromatin
fibres and their individuality is lost as discrete
elements.
⚫ The nucleolus, ER, and Golgi apparatus
reappear.
⚫ The nuclear envelope develops around the
chromatin at each pole,formingtwo daughter
nuclei.
Formation of two daughter
nuclei during telophase
Stages of Karyokinesis: Telophase

⚫ In animal cells,cytokinesis is achieved by
the formation of a furrow.
⚫ Furrow appears intheplasmamembrane
and deepenstowards the centre in a
centripetal fashion.
⚫ Furrows fromboththe sides joinatthe
centre,dividingthe cytoplasm intotwo.
⚫ The formation of cell furrow is aided by
microfilaments andmicrotubules.
Cytokinesis

Cell plate formation
⚫ Inplant cells,wallformationstarts atthecentreofthecell
and grows outwards.
⚫ The formationofthenewcellwall begins withtheformation
of a cell plate.
⚫ Fragments from the Golgi complex
(phragmoplast),which are known as vesicles,fuse
togetherto form cell plates.
⚫ The cellplate is laid in a centrifugal manner.
⚫ The cellplaterepresentsthemiddlelamella betweenthe
walls oftwo adjacentcells.
⚫ Mitochondriaandplastidsgetdistributedbetweenthetwo
daughtercells.
Cytokinesis

Significance of Mitosis
⚫ Growth: Mitosis causes growthanddevelopmentinmulticellularorganisms.
⚫ Plants cangrowfroma tinyzygotetohugeorganisms duetomitosis.
⚫ Helps inmaintenanceofpropersurface area to volume ratio of a cell
⚫ Repair: The oldandworn-outcells are replacedby newcells.
⚫ Reproduction: Unicellularorganisms reproduce (multiply)throughmitosis.
⚫ Inunicellularorganisms, replicationof cells is synonymouswithgrowth.
⚫ Regeneration: Mitosis causes cellgrowththatcauses therevivalofthe lostbodyparts
inanimals suchas starfish,planaria,thetailof a lizard, etc.

Regulation of Cell Cycle
G2
M
S
DNA
synthesis
G1
G0
Metaphase checkpoint
⚫ Checks forchromosome spindle attachment.
Regulation of cell cycle
⚫ Cyclins are proteinsthat bindto and activatethe
cyclin-dependent kinases (CDK’s).
⚫ Cyclin-CDK complexes controltheprogressionof a
cell from one phase to the next phase of the cell
cycle.
⚫ A stage-specific cyclin binds to a CDK andtakes
thecellthrougha checkpoint.
⚫ T
o move tothe next phase,the previous cyclin is
degraded and a new cyclin specific forthe next
stagebinds to CDK, andthecellprogresses into
the nextphase.

Regulation of Cell Cycle
Degradation of
G2/M-phase cyclin
CDK
G1/S-phase cyclin
G1/S-phase CDK
CDK
CDK
S-phase
CDK
G2/M-phase cyclin
S-phase cyclin
Degradation of S
- phase cyclin
Degradation of G1/S-
phase cyclin
S
G1
M
G2
Start of M phase
Start of S phase
S-phase progression

Meiosis
⚫ The termmeiosis was coined by Farmer and Moore in 1905.
⚫ Meiosis is the kindof celldivisionwherethe chromosomenumber is reducedtohalf inthedaughtercells.
⚫ Itinvolves two sequential cycles of nuclear division butonly one cycle of DNA replication.
⚫ Itis reductional division thatoccurs indiploidgermcells. The single cells dividetwicetoproduce fourcells.
Eachdaughtercell contains halfthe amountofgenetic information.
Diploidmother cell
(2n)
Haploid daughter
cells (n)
MEIOSIS
46
23 23 23 23

Meiosis
⚫ Germ cells undergomeiosis togive risetohaploid
gametes.
⚫ Gamete formation happens by meiosis instead
ofmitosis.
⚫ It ensures thatgametes are haploid.
o Parent cells in humans have two pairs of
chromosomes, i.e., 23 +23 chromosomes. They
arediploid.
o Aftermeiotic cell division, the fourdaughtercells
formed have onlyhalfthe number of
chromosomes, i.e., 23 chromosomes.
o Such cells withonlyhalfthe setof chromosomes
are known as haploid cells.
o Haploidcells aredenoted by 2n/2 =n
⚫ Afterfertilisation, thediploidphase is restored.
Diploid
gamete
mother cell
23
23
Haploid
gametes
46
Diploid
zygote
46
46

Steps of Meiosis
Meiosis I Meiosis I
I
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase I
I
Metaphase I
I
Anaphase I
I
Telophase I
I
⚫ Meiosis involvestwo sequential cycles:
Meiosis

Meiosis I
⚫ Meiosis -I is initiatedaftertheparental chromosomeshave replicatedtoproduce
identicalsisterchromatidsinthe S phase.
⚫ Itis longerandmorecomplexwhencomparedtotheprophaseofmitosis.
⚫ Unlikemitosis,meioticprophase Ihas five substages based onthechromosomal
changes inthenucleus.
Prophase I
Leptotene Zygotene Pachytene Diplotene Diakinesis

Prophase-I
Leptotene (Bouquet stage)
⚫ Leptotene is the ‘thin thread’ stage.
⚫ Chromatin fibres start condensing.
⚫ Chromosomes become gradually visible under light
microscope.
Zygotene
⚫ Zygotene is the paired thread stage.
⚫ The two chromosomeswhich are similar in form,size and
structure are called homologous chromosomes.
⚫ Homologouspairs cometogetherto forma synaptonemal
complex.
⚫ Thehomologous chromosomes cometo lie side by side in
pairs and this pairing is known as synapsis.
⚫ The pairof synapsed chromosomes is known as the
bivalent or tetrad.
Bivalent
Tetrad
Homologous
chromosomes

Pachytene
⚫ Pachyteneis the ‘thick thread’ stage, as the synapse
chromosomes appearthick.
⚫ Bivalent chromosomes are clearlyvisible.They
appear as tetrads.
⚫ By theendofpachytene,therecombinationbetweenthe
homologous chromosomes is complete andthe two
chromatids are linked at the site of crossing over.
⚫ Recombination nodules appear on the non-sister
chromatids of homologous chromosomes.
⚫ The exchangeofgeneticmaterial betweenthenon-sister
chromatids ofhomologous chromosomestakes place,
which is also known as crossing over.
⚫ Recombination is catalysed by enzyme recombinase.
Prophase-I
Chiasma
Recombination
nodule
Chromatids

Diplotene
⚫ Diplotene is the ‘twin thread’ stage.
⚫ Inthis stage, dissolution of the synaptonemal
complex occurs.
⚫ The homologous chromosomes start separating i.e.
desynapsis occurs.
⚫ Recombinedhomologous chromosomes separate from
eachotheratall sites exceptatthe siteof crossover
.
⚫ X-shaped chiasmata is observed.
⚫ Inoocytes of somevertebrates,diplotenelasts formonth
or years. It is called dictyotene stage.
⚫ T
erminalisationof chiasmata starts in this stage.
Prophase-I
X-shaped
chiasmata

Diakinesis
⚫ This is the last stage ofprophase I.
⚫ The chromosomes are fullycondensed.
⚫ Termination of chiasmata is observed.
⚫ Spindle apparatus assembles.
⚫ The nuclearmembrane breaks down.
⚫ Nucleolus also disappears.
Prophase-I

⚫ As the earlymetaphase-I starts,the microtubules arisefromthe
oppositespindlepoles ofthe spindleapparatus.
⚫ Microtubules aremadeupoftubulinprotein.
⚫ Theyprovidebothmechanical supportandcellmovement.
⚫ As itenters latemetaphase-I, bivalentchromosomes alignonthe
equatorial plate.
⚫ Microtubules fromoppositepoles attachtothehomologous
chromosomes.
⚫ Thesemicrotubules attachtothe kinetochores ofthepairof
homologous chromosomes.
Metaphase- I
Early
metaphase I
Late
metaphase I

⚫ Inthis phase,the homologous chromosomes
are separated.
⚫ The spindlemicrotubulespullthehomologous
chromosomestowards the oppositepoles,
respectively.
⚫ The sister chromatids of the homologous
chromosomes are associatedwitheachotheratthe
centromere.
⚫ Reduction in the number of chromosomes
occurs during anaphase- I
Anaphase- I
Anaphase I

⚫ Telophase- Iis the final stepofmeiosis -I.
⚫ The chromosomes reach the poles.
⚫ The spindle fibres completely disappear.
⚫ Atthis stage,the nuclear membrane and the
nucleolus reappear afterthe homologous
chromosomeshave separated.
⚫ Itproducestwodaughternuclei eachcontaininghalfthe
number of chromosomes butdouble the amountof
nuclear DNA.
Telophase- I
Telophase I

⚫ Telophase Iis followedby cytokinesis.
⚫ Cytokinesis is theprocesswherethecytoplasmis dividedequallyintodaughtercells.
⚫ Thedaughtercells formedatthe endofmeiosishave bivalentchromosomes, andthis chromosome
is alsoknownas a dyad (onepairof chromosomesfromthetetrad).
Interkinesis
⚫ Itis a short-livedstage betweenmeiosis Iandmeiosis II.
⚫ Duringthis phase,thechromosomes are elongatedbutdonotformchromatinfibres.
⚫ This stage has no DNA replication.
⚫ The RNA andproteinrequiredduringmeiosis -IIare synthesizedduringthis phase.
Cytokinesis

Meiosis- I
I
Meiosis
Meiosis I Meiosis I
I
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase I
I
Metaphase I
I
Anaphase I
I
Telophase I
I

⚫ This phase is initiated aftercytokinesis Iand is
simplerthan prophase Iofmeiosis I.
⚫ Inearlyprophase II,the nuclear membrane
starts to disintegrate.
⚫ Chromatin fibres begin to condense to form
chromosomes.
⚫ As thecell enters lateprophase II,thenuclear
membrane disintegrates,and chromosomes
become compact.
⚫ The centrioles also move towards the
opposite ends.
Prophase I
I
Early prophase I
I
Late prophase I
I

Metaphase -I
I
⚫ Condensed chromosomes align at
the equatorial plate.
⚫ Themicrotubules ofthe spindle
apparatus get attached tothe sister
chromatids at kinetochore.
Anaphase-II
⚫ The microtubules of the spindle pull
the sister chromatidsto the opposite
poles.
⚫ The centromere of the sister
chromatids splits.
Metaphase I
I
, Anaphase I
I
Metaphase I
I
Anaphase I
I

⚫ Itmarks the end ofmeiosis II.
⚫ The nuclear membrane and the
nucleolus reappear.
⚫ The chromosomesdecondense
into chromatin.
⚫ The spindle fibersdegenerate.
Telophase I
I
Telophase I
I

⚫ Telophase IIis followedby cytokinesis.
⚫ The cytoplasm is divided into daughtercells.
⚫ The endof cytokinesis ismarkedby thetetradofthehaploidcells.
⚫ Thetwocellsgiveriseto four cells or a tetradof cells.
Cytokinesis
Cytokinesis
4 haploid cells

⚫ Meiosis produces gametes for sexual reproduction.
⚫ Itconserves the specific chromosome number of each species inthe
categoryof sexuallyreproducing organisms.
⚫ Itincreasesthe genetic variability fromonegenerationtothenext.
⚫ Genetic variations contributeto evolution.
Significance of Meiosis

⚫ Bothare equatorial divisions.
⚫ The sisterchromatids are separatedduringthe anaphaseto becomethe
chromosomes ofthe daughtercells.
⚫ Microtubules attachfromtheoppositedirections tothecentromereof each sister
chromatidpair
.
⚫ Chromosomes decondense duringtelophase.
Meiosis- I
Ivs Mitosis
Similarities:
Mitosis Meiosis -II
Mitosis occurs indiploid somatic cells. Meiosis -II always occurs inhaploidgermcells.
Mitosis is always followed by DNA replication. Meiosis-II is notfollowedby DNA replication.
Aftermitosis, thedaughtercells are exactlysimilar
to one another andthe parentcell.
Thedaughtercells formedareneither similarto
eachother nor similar tothe parentcell.
Differences:

Summary
Cell cycle
Cell division
Interphase M phase
G1phase:
● Cellgrow in size
● Protein production
● Nutrients synthesis
S phase:
● DNA replication
● Centriole duplication
G2 phase:
● Cellgrowth
● Protein production
Karyokinesis:
● Chromosomes separate
● Two nuclei are formed
Cytokinesis:
● Cytoplasm divides
● Two daughter
cells formed
Checkpoints in cell division
G1/S checkpoint:
i)Check for
nutrients
ii)Growth factors
iii)DNA
damage
G2/M checkpoint:
i)Check forcell size
ii)DNA replication
Metaphase
checkpoint:
i)Checks for
chromosome
spindle attachment.

Summary
Karyokinesis
Karyon =Nucleus;Kinesis =Movement
It is the division of the nucleus.
Prophase
- Condensation of chromatin fibres
- Nuclearmembrane degenerates
Anaphase
- Centromere splits andchromatids separate.
- Chromatidsmove to opposite poles.
Metaphase
- Chromosomes are attachedto spindle fibres.
-Chromosomes are arrangedinthe
equatorial plane.
Telophase
- Chromosome reach the poles
- Disappearance of spindle fibres
- Decondensationof chromosomes
Cytokinesis
Cytos =Cell,Kinesis =Movement
Itis thedivisionofthe cytoplasm.
Observed in animals
Cell plate formation
Observed in plants
Mitosis

Summary
Reduction
division
Equational
division
Centromere
intact
DNA replication
in Interphase
No DNA
replication
Meiosis I Meiosis II
Centromere
splits
Dyad of cells T
etrad of cells
Meiosis

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