Question

Arrange the following events in the sequence as they would occur during fertilisation.

I. Metabolic activation
II. Formation of reception cone
III. Penetration of oocyte
IV. Amphimixis
V. Block to polyspermy

• A. I→ II→ III→ IV→ V
• B. V→ III→ I→ IV→ II
• C. III→ II→ I→ V→ IV
• D. IV→ III→ II→ I→ V

Answer 1

The correct option is C III→ II→ I→ V→ IV

The process of fertilisation refers to the fusion of male gamete with that of the female gamete. It is a multistep process.

First, the sperm penetrates various layers of the secondary oocyte such as corona radiata, zona pellucida. Then the plasma membrane of the oocyte (oolemma) rises slightly upwards and forms a reception cone to receive the incoming sperm.

The oocyte then undergoes metabolic activation where mRNA strands (previously inactivated by special proteins) are activated. This accelerates the protein synthesis.

After the sperm has entered into the plasma membrane, block to polyspermy (by fast block and slow block mechanisms) is achieved to prevent the entry of multiple sperms into the ovum.

After the sperm has successfully entered the oocyte, the oocyte completes its second meiosis. The remaining nuclear material of the oocyte reorganises to form the female pronucleus. The sperm nucleus swells and forms the male pronucleus.

Amphimixis is the final step, where there is fusion of male and female pronuclei to form a diploid zygote.