Fertilization / Syngamy
Reproduction in Animals | Biology Class 12th Biology | Fertilization | Syngamy Mechanism of fertilization | Significance of fertilization
- Fertilization is the process in which fusion of the haploid male and female gametes takes place and form diploid zygote
- In sexual reproducing organisms this process is common.
- In mammal the fertilization is internal, and it usually takes place in the ampulla of the fallopian tube .
- The zygote develops into embryo in uterus.
Mechanism of fertilization
- Coagulated Semen (Sperm and some secretion) are ejaculate during ejaculation
- undergo liquefaction and sperms become active.
- The mechanism of fertilization is as follows
A. Movement of sperm towards egg
B. Entry of sperm into the egg
C. Activation of ovum
1. Movement of sperm towards egg
Involves
Capacitation of sperms Making it possible for penetrate the
ovum
Enter millions of
sperm 50% are destroyed.
Few are undergo
capacitation. This process requires 5-6 hours
Tail show rapid
whiplash movements acrosome membrane becomes thin become extra active start moving to the oviducts
The prostaglandins
activate the sperms and vestibular secretions enhance sperms motility
The movement of sperm to word egg
is 1.5 to 3.0 mm/min.
Sperm
reach to amulla by self movement and contraction
of uterus and fallopian tubes this is
stimulated by oxytocin of female.
After capacitation the sperms may reach ampulla within 5 minutes.
Sperms can remain viable for 24-48
hours (ovum for about 24 hours).
B. Entry of sperm into the egg
Out of millions of sperm few hundred reach to ampulla and only one of
them fertilize the ovum.
After reaching to egg acrosome relases the lysins:
hyaluronidase and corona penetrating enzymes.
Dissolve all
membranes and sperm head passes through the zona pellucida of egg.
Zona pellucida has
fertilizin receptor proteins (zp3, zp2).
The fertilizin
binds to specific acid protein- antifertilizin of sperm.
It brings about
attraction of sperms to the egg to enhance fertilization.
Fertilizin-antifertilizin
interaction is species specific.
Thus, the fertilizin-antifertilizin reaction is also called compatibility reaction.
Acrosome Reaction
Acrosome covering rupture and lytic enzyme
Acrosine or zona lysine is release.
They
act on the zona pellucida at the point of contact.
Fertilization
cone is formed on egg membrane.
Sperm
head connect to egg and weak wave of depolarization is produce.
Membrane
of both cell are dissolve and sperm nucleus and the centrioles enter the egg
After
sucssefule entry of sperm head the vitelline membrane change into a fertilization membrane by deactivating
the sperm receptors of zona pellucid.
A
distinct perivitelline space is created around the fertilization membrane
This prevent entry of other sperm and avid polyspermy.
C. Activation of ovum
When sperm head contact to vitelline membrane of egg
the ovum complete its meiosis II
Before
the fertilization it was at metaphase II
stage.
It
gives out the second polar body.
The
germinal vesicle organises into female pronucleus.
Now it is the true ovum or egg.
Fusion of egg and sperm
The
coverings of male and female pronuclei degenerate allowing the chromosomal
pairing.
This
results in the formation of a synkaryon by the process called syngamy
or karyogamy.
The
zygote is thus formed.
The proximal centriole received from the sperm
helps in formation of the synkaryon spindle and cleavage of cell into two
blastomeres.
Fertilization / Syngamy Mechanism of fertilization
Significance of fertilization
Secondary
oocyte completes the process of oogenesis and is transformed into a mature ovum
(n).
The
diploid chromosome number is restored in the zygote by the process of syngamy.
The
ovum lacks the centrioles necessary for further divisions, are received from
the sperm during fertilization.
Fertilization involves fusion of male and female gametes from the two parents.
It results in
variations which are significant to evolution.
Reproduction in Animals | Biology Class 12th Biology | Fertilization | Syngamy Mechanism of fertilization | Significance of fertilization
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