A Brief Idea on Meiosis Cell Division

This PPT intends to explore the the different aspects experienced by the cell in course of meiosis cell division.

1. Welcome to Meiosis Cell Division
By
N.Sannigrahi, Associate Professor of Botany
Nistarini College, Purulia (W.B) India

2. MEIOSIS-MEMORY LANE
Meiosis-areductioncelldivisionoccurinthereproductivecells
ofplantsandanimals(Microsporemothercellsofanther&
MegasporemothercellsofCarpel)wherethedaughtercells
possesshalfofthechromosomenumberoftheparentscells
quantitativelyandbearsomesortsofvariationqualitatively.
MILESTONES:
1887:Weismann-Proposedthisatthetimeofgametogenesis
1887:V.Benedin-Reportedthereductionofchromosome
number
1888:E.A.Strasburger-Detailstudy
1900:Sutton&Winiwater-Detailanalysis
1905-Farmer&MoorecoinedthetermMeiosis

3. WHERE DOES MEIOSIS TAKE PLACE?

4. MEIOSIS AT A GLANCE
MEIOSISdividedintotwomainphases:
A.MeiosisIordivisionI(Reduction)consistingof4stages-
ProphaseI,MetaphaseI,AnaphaseI&TelophaseI
ProphaseI,thelongest&complexprocesssubdividedintofive
substages-Leptotene(Leptonema),Zygotene,Pachytene,
Diplotene&Diakinesis
MeiosisII(Equational)Comprisingof4stages-Prophase
II,MetaphaseII,AnaphaseII&TelophaseII
Beforemeiosis,anInterphaseistherewhichconsistsofG1,S
andG2wheretheDNAreplicationalongwiththesynthesisof
thedesiredproteinsandotherenzymessynthesistakeplaceas
usuallikethatofMitosis.Thustwodivisionsbutone
interphase-“Godneverplaysdice”.

5. MEIOSIS-WHAT HAPPENS?

6. MEIOSIS I

7. PROPHASE I SUB STAGES
(a)Leptotene:Thediploidnucleusenlargesinvolume.Thechromosomes
appearaslong,thinandsinglethreadswhichsoonbegintocoil.Several
small,bead-likegranules(chromomeres)appearineachthread-like
chromosome.
(b)Zygotene:
Thehomologouschromosomescometogether,getthemselvesarrangedside
byside,andformpairsorbivalents.Thispairingisalsocalledsynapsis.The
pairingchromosomessoonbegintoshortenandgetthickened,butthereis
noactualfusion.
(c)Pachytene:
Inthisstagethechromosomesbecomeshorter,thickerandget
splittedintochromatidslinkedatthecentromeres.Fromapair
ofeachhomologouschromosomesarethusproducedfour
chromatids.Identificationofthehomologouschromosomescan
bemadeinpachytene,whichisalongstageofprophaseI.

8. PROPHASE I SUB STAGES
d)Diplotene:
Centromeresofpairedchromosomesmoveawayfromeach
other.Thismovementisbecauseofthedevelopmentofsome
repulsiveforcebetweenthehomologouschromosomes.
However,thehomologouschromosomesremainconnectedat
oneormorepointscalledchiasmata.
Thephysicalexchangeofgeneticmaterialtakesplaceateach
chiasmaundertheprocesscalledcrossingover.Furthercoiling
andshorteningofchromosomesisalsoseeninlatestageof
diplotenewhichsoonchangesintodiakinesis.

9. PROPHASE I SUB STAGES
e)Diakinesis:
Inthislaststageofthefirstmeioticprophasethechromosomes
areshortestandthickest.Thenuclearmembranestarts
disintegrating.Thenucleolusalsodisintegratesanddisappears.
Thechromosomesbivalentsmovetowardstheperiphery,ofthe
nucleusandremainconnectedonlyatthepointsofchiasmata.
Thechromosomesarefinallyreleasedintothecytoplasm.
AfterthecompletionoftheprophaseI,thescheduledeventsof
themeioticIhappensthatcomprisesofMetaphaseI,Anaphase
IandTelophaseIandthecharactersofthestagesareasstated
below.

10. CROSSING OVER

11. MEIOTIC I SUB STAGES
2.MetaphaseI:
TwomajoreventsofmetaphaseIincludecomplete
disintegrationofnuclearmembraneandtheformationof
spindle.Allthechromosomes,eachalongwiththeirtwo
chromatids,movetotheequatorialregionofthenewlyformed
spindle.
Differingfromthemetaphasestageofmitosis,thecentromeres
ofchromosomepairsinmetaphasestageofmeiosisIbecome
attachedwiththespindlefibresneartheequatorialregion.The
centromeresremainclearlyapartfromeachotherandfacethe
oppositepoleswhilethearmsofthechromosomepairslie
towardstheequator.

12. MEIOTIC I SUB STAGES
3.AnaphaseI:
Thereisfirstarepulsionandthenmovementofthetwo
centromeresofthehomologouschromosomestowardsthe
oppositepolesofthespindleinanaphaseI.Acentromere
carrieseitherapaternaloramaternalchromosometoone
polebutnotboththechromosomes.Thisactuallyreduces
thechromosomenumberfromdiploid(2n)tohaploid(n),
whichisthemainfeatureofmeiosisofreductiondivision.
4.TelophaseI:
Anuclearmembranedevelopsaroundeachgroupof
homologouschromosomespresentonthetwoopposite
polesintheformofacompactgroupintelophaseI.The
nucleolusreappears.Boththesoformeddaughternuclei
containhaploidnumber(n)ofchromosomes,andeach
chromosomecontainsapairofchromatids

13. MEIOSIS II

14. MEIOTIC II STAGES
1.ProphaseII:
Thechromosomessplitintochromatidsinboththehaploid
nucleiandcellsformedaftermeiosisdivisionI.Thesplitted
chromatidsremainconnectedonlyatthecentromeres.The
chromosomesstartcoilingandbecomeshorterandthicker.The
nuclearmembraneandnucleolusstartdisintegratingandsome
spindlefibresalsostartappearing.
2.MetaphaseII:
Thechromosomesgetarrangedinanequatorialpositioninthe
newly-formedspindle.Verysoon,thechromosomepair
separates,ofwhicheachcontainsitsowncentromere.Thisisa
veryshortphaseofmeiosisdivisionII.

15. MEIOTIC II STAGES
3.AnaphaseII:
Inthisphase,thetwosisterchromosomesofeachpairstartto
movetowardstheoppositepolesofthespindle.Theyarebeing
drawntowardstheoppositepolesbytheircentromeres.
4.TelophaseII:
Eachpolargroupofchromosomesgetenvelopedbya
nuclearmembrane,andthereisthereappearanceof
nucleolus.Fourcellsareformedbycytokinesis,andthe
nucleusinallthesesoformedfouryoungcellscontain
haploidnumber(n)ofchromosomes.Inthisway,four
haploidcellsareresultedfromasinglediploidcellinthe
processofmeiosis.

16. MEIOSIS-OVERVIEW

17. OUTCOME OF MEIOSIS

18. IMPORTANT OUTCOME
Meiosis-themostimportantandcriticalbutsuccessfulavenue
forsexualreproductionofalldiploidorganisms.Itisthe
mechanismbywhichthediploidamountofgeneticinformation
isreducedtothehaploidamount.Inanimals,meiosisleadsto
theformationofgameteswhereasinplantshaploid
spores(microspores&megaspores)areproducedwhichinturn
leadtotheformationofgametes.Notonlythat,asapartofthe
gameofthecrossingover,thehaploidcellspotentiallycontain
eitherthepaternalorthematernalrepresentativeofevery
homologouspairofchromosomesandbytheprocess,
reshufflingoftheallelesbetweenthematernalandmaternal
membersresultthegreatamountofgeneticvariationinthe
producinggametessupposedtoundertakeintheprocessof
fertilizationtobringF1,Thus,itisnoveltechniqueinthe
domainofreproductivebiology.

19. SIGNIFICANCE
1.Constancyofspeciesspecificchromosomenumberfrom
generationtogeneration,
2.Crossingoverbringsnewcombinationoftraitstodevelop
variationthatacceleratestheorganicevolutionandcreationof
newspeciesinthepassageofevolution,
3.SegregationandIndependentassortmentofthenon-linked
genes,
4.Essentialforsexualreproductionthatenablesthefusionof
twohaploidgametestoformZygote(2n),
5.Anavenueforthealternationofgeneration,
6.ChromosomalvariationbyaneuploidyandPolyploidy&
chromosomalaberrationsduetonon-disjunctionduring
anaphaseIorIIandfailureofcytokinesis.

20. SURE, THE JOURNEY IS A TRESURE.THANKS

21. ACKNOWLEDGEMENT
1.Goggle
Concepts of Genetics-Klug, Cummings, Spencer and
Palladino
The different WebPages from open sources