The document discusses the process of fertilization in various organisms, emphasizing internal and external fertilization mechanisms, particularly in aquatic invertebrates, fish, amphibians, and plants. It details the steps involved in fertilization, including the penetration of the oocyte, fusion of gametes, and the formation of pronuclei, as well as the process of double fertilization in angiosperms. Key biochemical events such as enzyme activities, calcium efflux, and membrane fusion are highlighted as essential to the fertilization process.

1. FERTILIZATION
Internal External
Most aquatic invertebrates
 Most fish and amphibian
Most land animal Most plants
Allogamy Autogamy
Events
1. Chemotaxis
2. Sperm activation
3. Fusion of egg and sperm
2/17/2015 RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com

2. Sperm entry site The beginning of life
Fertilization site
2/17/2015 RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com

3. FERTILIZATION STEPS
 Hyaluronidase from acrosome
Mucosomal enzyme from uterine tube
Movements (mitochondria) of sperm tail
Step1: Penetration of corona radiata
The protective walls of oocyte
2/17/2015
RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com
The Mahabharata Chakravyuh

4. Step2: Zona lysis and polyspermy block
 Zona lysis; by esterases, neuraminidase and acrosine enzymes.
 Acrosomal GalT (receptor)- binds to Zona ZP3 protein (ligand)
 Sperm SED1 independently binds to Zona
 Mice CD9 is homologues to Human ZP3
 In sea urchin sperm bindin (ligand) binds to EBR1
 (receptor on egg vitelline membrane)
1. Ca+2 efflux
 MAP kinase activation- Restoration of mitotic cell cycle
 Cortical granule exocytosis-Fromation of hyaline layer
 NAD to NADP- formation of plasma membrane
2. PKC activation (IP3/DAG) :
 Na+ (influx)/H+ exchange
 Increase in cellular pH
 Cytoplasmic movement of morphogenetic content
 DNA replication and protein synthesis
Polyspermy block
Zona reaction
1. Ca+2 efflux
 MAP kinase activation- Restoration of mitotic cell cycle
 Cortical granule exocytosis-Fromation of hyaline layer
 NAD to NADP- formation of plasma membrane
2. PKC activation (IP3/DAG) :
 Na+ (influx)/H+ exchange
 Increase in cellular pH
 Cytoplasmic movement of morphogenetic content
 DNA replication and protein synthesis
2/17/2015 RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com
Sea urchin

5. Step3: Sperm and oocyte plasma membrane fusion
Gametes membrane fusion and break down at fusion point.
Entry of sperm head and tail to oocyte.
Cortical granules are important for polyspermy block
Sperm is not selfish like bacteriophage
2/17/2015 RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com
Fertilization in sea urchin T4 phage entry

6. Step5: Male pronuclei formation.
 Nucleus of sperm enlarges and tail
degenerates to form male pronuclei
Step4: Female pronuclei formation.
 Sperm entry activates second meiotic division of oocyte to
mature oocyte and polar body.
 Chromosomal decondensation leads to pronuclei formation
Step6: Ootid formation.
 Both pronuclei ( similar morphology) replicates its DNA
 Oocyte with two pronuclei is ootid
Step6:Zygote formation
 Fusion of pronuclei
 Arrangement of chromosome on cleavage spindle2/17/2015 RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com

7. Double fertilization
Occurs only in angiosperm (flowering plants)
Two sperm fuses with egg cells (n) and central cell (2n) resulting to
diploid embryo (2n) and triploid (3n) endosperm.
Nuphar (cow lily) endosperm is diploid (only one fertilization)
2/17/2015 RANJEET SINGH MAHLA
ranjeet@iiserb.ac.in/mahlaranjeet@gmail.com
Nuphar (cow lily) Fertilization in angiosperm