Mechanism of fertilization, Biology

Assignment Help:

MECHANISM OF FERTILIZATION -

The process of fertilization complete into 5 steps -

1.      APPROACH OF SPERM TO OVUM

  1. For fertilization sperm & ova interaction is essential.
  2. Sperm are motile, sperm reach to the ovum, with the help of tail movement but it is a chance.
  3. In animal which exhibit internal fertilization, the sperms are deposited close to the fertilization site with the help of copulatory organ. In many animals the ovulation take place after copulation.

2450_mechanism of fertilization.png

2.       CAPACITATION AND CONTACT -

  1. Capacitation is a type of chemotaxis process.
  2. Fertilization is a specific process & it takes place in the gametes of same species.
  3. Lillie (1919) proposed the fertilization theory.

LILLIE THEORY -

  1. According to it, there is a substance called fertilizin on the ovum surface which is a glyco-protein. It is different in different species.
  2. Anti-fertilizin present on the surface of sperms. It is a simple acidic protein.
  3. It is attached to the proteins of sperm plamalemma. Fertilizin & anti-fertilizin react like antigen antibody.
  4. As a result the sperms get attached to ovum surface. It is called as sperm agglutination.
  5. The fertilizin & anti-fertilizin of the same species can react. Hence, sperms identify the ovum of same species.
  6. The vitelline membrane of the ovum has many binding receptor sites which react with the binding protein of the sperms.

340_capacitation and contact.png

3.      ACROSOME REACTION & PENETRATION -

  1. This reaction take place after the attachment of the sperm to the egg surface.
  2. There are specific changes in the acrosome region which are collectively called as acrosomal reaction.
  3. According to colwin & colwin, the acrosome first elongates to form an acrosomal tubule.
  4. The length of the acrosomal tubule varies from species to species.
  5. Normally its length is double than the length of the nucleus.
  6. The sperm with acrosomal tubule is called as active sperm.
  7. The acrosomal tubule release sperm lysins eg. hyaluronidase enzyme in mammals.
  8. The sperm lysins digest egg membranes to create a passage for sperm entry.
  9. Finally, there is a contact between the plasmalemma of both sperm & ovum.

4.      ACTIVATION OF EGG -

  1. When acrosomal membrane contract to plasmalemma of ova, these membranes are fused.
  2. During this period many changes take place in the ooplasm, these changes are collectively called activation of egg.
  3. It involves -

(i) Colour change -

  • Surface of egg white (previously yellow) due to distribution of fat, equally after the contact of sperm.
  • Colour changes initiate to contact site of sperm.

(ii) Formtion of fertilization cone -

  • The ooplasm at the point of contact of sperm bulges out to form a reception cone or fertilization cone.
  • The fertilization cone may be provided with pseudopodia -like or cylendrical processes.
  • The fertilization cone gradually engulfs the sperm & get withdrawn.
  • The fertilization cone is also called as receptor cone.
  • In ova, nucleus of sperm, periacrosomal material, proximal centriole and mitochondria are enter.

(iii) Cortical reaction and formation of fertilization membrane -

  • After the entry of the sperm into the ova so many changes in the ooplasm. These changes are physio-chemical type, which are collectively called as cortical reaction.
  • There are cortical granules just below the plasmalemma which are made up mucopolysaccharides.
  • These cortical granules release mucopolysaccharides into the perivitelline space.
  • The perivitelline space is found between the plasmalemma of ovum & vitelline membrane.
  • These mucopolysaccharides fuse with the vitelline membrane & form a fertilization membrane.
  • The fertilization membrane prevents the further entry of the sperm.

(iv) Metabolic activation -

Entry of sperm into the ovum stimulates many metabolic changes which are as follows -

(a) The permeability of plasmalemms for water, ethylene glycol, phosphate, potassium is increased.

(b) Beginning of synthesis of cAMP which inturn stimulates other metabolic activities.

(c) Activation of ETS to form ATP.

(d) Beginning of protein synthesis with the help of mRNA & other RNA.

(e) The rate of Oconsumption is changed. For example, it increase in the eggs of sea urchin, decreases in molluscs & chaetopterus & remain unchanged in Bufo.

5.       FUSSION OF GAMETIC PRONUCLEI -

  1. The nuclei of both ovum & sperm absorb water and they swell to become vesicular or jelly-like. Such nuclei are termed as pronuclei.
  2. Simultaneously, the sperm rotates to 180° so that the mid-piece assumes the leading position.
  3. The male pronuclei moves towards the female pronuclei.
  4. The path of the movement of male pronuclei is called as fertilization path or copulation path.
  5. Both the pronuclei fuse to form zygote which is called as amphimixis.
  6. Amphimixis take place in the centre of the yolk free ooplasm.
  7. Entry way of sperm in the ova is called as penetration path.
  8. Zygote move toward the one specific site, this way is called cleavage path.

1999_fusion of gametic pronuclie.png


Related Discussions:- Mechanism of fertilization

Phospholipids - conjugated lipids, PHOSPHOLIPIDS Most abundant lipid...

PHOSPHOLIPIDS Most abundant lipid present in cell membrane, also called membrane lipid. It is made up of lipid & phosphoric acid. The basic phospholipid is phosphat

Explain the factors on which spoilage of fish depends, Q. Explain the facto...

Q. Explain the factors on which Spoilage of fish depends? i) The type offish. Different types of fish are susceptible to spoilage due to shape and size. A fat fish is suscept

Which is the first heart chamber into which blood enters, Q. Which is the f...

Q. Which is the first (human) heart chamber into which blood enters? Where does the blood go after passing that chamber? What is the name of the valve that separates the compartmen

Adaptations, Adaptations Every organism can live and reproduce within a...

Adaptations Every organism can live and reproduce within a certain range of climatic conditions. Organisms that live in hot or cold environments have behavioural and physiologi

What is embryo culture, What is the basis of classifying 'cancer? Name and ...

What is the basis of classifying 'cancer? Name and describe the different Categories of cancer. Mention any two approaches for cancer treatment. What is embryo culture? What is

Explain the amphibian heart and the reptile heart, What is the difference b...

What is the difference between the amphibian heart and the reptile heart? The reptiles have double and incomplete circulation too, three heart chambers (two atria and one ventr

Genetic resources of biodiversity, Q. Genetic resources of biodiversity? ...

Q. Genetic resources of biodiversity? Genetic diversity or the gene pools are important for promoting and ensuring high agricultural yield. Genetic diversity provides crops wit

Why is complementary base pairing important in dna structure, Why is comple...

Why is complementary base pairing important in DNA structure? Complementary base pairing is important because the hydrogen bonds between the bases hold the two strands of DNA

What is the form that produces gametes, In the metagenesis of Aurelia and O...

In the metagenesis of Aurelia and Obelia what is the form that produces gametes? What is the form that reproduces asexually? In the metagenesis of some coelenterates, like Obel

Eexplain importance of mitosis for the embryonic development, What is the i...

What is the importance of mitosis for the embryonic development? Every embryo grows from a single cell that suffers mitosis and operates other cells that also divide themselves

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd