15 Molecular event in fertilization

This presentation discusses various events leading to fertilization.

1. Animal cell science and Technology
15. Molecular events in fertilization
ShailendraSingh Shera, Ph.D

2. Outlines
1.Molecular events in fertilization
2.Role of Maternal genes in early development of embryo
3.Practice questions
15. Molecular events in fertilization

3. Definition of fertilization
•Fertilizationisthefusionofthemaleandfemalegamete.Itissasequenceofcoordinated
moleculareventsinvolvingthemergingofthespermwiththeegg,thefusionofthepronucleiand
theinterminglingofthematernalandpaternalchromosomes.
•Itistheformationofadiploidzygotefromahaploideggandsperm
3-StagesofFertilization
a.Spermpreparation:Capacitationandacrosomereaction
b.sperm-eggrecognitionandbinding,
c.Sperm-eggbindingandfusion

4. •Akeysignalresultingfromthebindingofaspermtoitsreceptorontheplasmamembraneofthe
eggisanincreaseinthelevelofCa
2+
intheeggcytoplasm,probablyasaconsequenceof
stimulationofthehydrolysisofphosphatidylinositol4,5-bisphosphate(PIP
2).
•OneeffectofthiselevationinintracellularCa2+istheinductionofsurfacealterationsthat
preventadditionalspermfromenteringtheegg.Becauseeggsareusuallyexposedtolarge
numbersofspermatonetime,thisisacriticaleventinensuringtheformationofanormaldiploid
embryo.
•ThesesurfacealterationsresultfromtheCa
2+-
inducedexocytosisofsecretoryvesiclesthatare
presentinlargenumbersbeneaththeeggplasmamembrane.Releaseofthecontentsofthese
vesiclesalterstheextracellularcoatoftheeggsoastoblocktheentryofadditionalsperm.
TheincreaseincytosolicCa2+followingfertilizationalsosignalsthecompletionofmeiosis.
•IneggsarrestedatmetaphaseII,themetaphasetoanaphasetransitionistriggeredby
activationoftheanaphase-promotingcomplex,resultingfromCa2+-dependentphosphorylation
anddegradationofaninhibitoryprotein.

5. •TheresultantdegradationofcyclinBandcondensinleadstocompletionofthesecondmeiotic
division,withasymmetriccytokinesis(asinmeiosisI)givingrisetoasecondsmallpolarbody.
Followingcompletionofoocytemeiosis,thefertilizedegg(nowcalledazygote)containstwo
haploidnuclei(calledpronuclei),onederivedfromeachparent.
•Inmammals,thetwopronucleithenenterSphaseandreplicatetheirDNAastheymigrate
towardeachother.Astheymeet,thezygoteentersMphaseofitsfirstmitoticdivision.Thetwo
nuclearenvelopesbreakdown,andthecondensedchromosomesofbothpaternalandmaternal
originalignonacommonspindle.Completionofmitosisthengivesrisetotwoembryoniccells,
eachcontaininganewdiploidgenome.Thesecellsthencommencetheseriesofembryoniccell
divisionsthateventuallyleadtothedevelopmentofaneworganism

6. Image source: Molecular Biology of
cell by Albert

7. •Anembryoexclusivelyreliesonmaternalgeneproducts,RNAs,andproteinsforitsearlydevelopment
untilactivationofitsowngenome.Theprecisedevelopmentaltimeperiodanddevelopmentalprocesses
undermaternalcontrolwhentheembryoislargelytranscriptionallysilentvaryamongorganisms.Insome
animals,suchasmice,humans,andnematodes(Caenorhabditiselegans),onlythefirstorfirstcoupleof
cleavagecyclesareaccomplishedbeforetranscriptionoftheembryonicgenomeisactivated.
•Maternalgenesarethoseexpressedbycellsofeggchamberandinvolvedinearlydevelopmentofthe
oocyteandsyncytialblastoderm.Theproductsofthesegenessuchas,RNAorprotein,areproducedor
depositedintheoocyteorarepresentinthefertilizedeggorembryobeforeexpressionofzygoticgenesis
initiated.Theproteinsofthesegenesthenstimulateothergenes,whichinturnstimulateyetothergenes
inacascadeofcontrol.Asmightbeexpected,mostofthegeneproductsinthecascadeareregulatory
proteins,whichbindtoDNAandactivateothergenesThesematernalgeneproductsregulatemeiosis,
oocytedevelopment,andearlydevelopmentoftheembryoincludingfertilization,transitionsbetween
meioticandmitoticcellcycles,andtheswitchfromutilizationofmRNAsandproteinsprovidedbythe
mothertotheembryo’sowngeneproductsduringzygoticgenomeactivation.
Role of Maternal gene in early embryonic development

8. Practice questions*
1. Fertilization may also be defined as
A. A sequence of coordinated molecular events involving the merging of the sperm with the egg.
B. The fusion of the pronucleiand the intermingling of the maternal and paternal chromosomes.
C. Both (a) and (b)
D. None of these
2. One effect of elevation in intracellular Ca
2+
in egg cytoplasm during fertilization is
A. The induction of surface alterations that prevent additional sperm from entering the egg.
B. Migration of zygote from falopiantube to uterus
C. Fertilization of egg by multiple sperm simultaneously
D. Recycling of sperm receptor
3. Which of the following is the function of maternal gene?
A. Genes expressed by cells of sperm and involved in early development of the oocyteand syncytial
blastoderm.
B. Genes expressed by cells of egg chamber and involved in early development of the oocyteand syncytial
blastoderm.
C. Production of sperm receptor for attachment of sperm on to egg.
D. None of these
4.The term ‘zonapellucida’ is associated with
A. Sperm
B. Ovum or egg
C. Zygote
D. None of the above
5. The sperm binds to zonapellucidaof egg through…………..
A. N-linked oligosaccharide on ZP3
B. O-linked oligosaccharide on ZP3
C. N-oligosaccharide on ZP1
D. O-oligosaccharide on ZP1

9. 1.http://www.vivo.colostate.edu/hbooks/pathphys/reprod/fert/fert.html
2.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5029953/
3.https://www.slideshare.net/kaushal2683/molecular-event-during-fertilization?qid=36f6c9e5-
6cc8-4569-9eed-b683ab7dff74&v=&b=&from_search=1
4.Molecular Biology of cells. Albert
MCQ Practice questions
1. Practice and Learn Animal cell Science and Technology: Multiple choice question for learning.
Author: ShailendraSingh Shera. Publisher: Amazon Kindle.
Available on: amazon.com
References & Further reading