Question

Starting from the earliest, sequentially arrange the following events occurring during fertilization:
(A). Penetration of corona radiata and zona pellucida by spermatozoon
(B). Contact of spermatozoon with vitelline membrane leading zona reaction
(C). Formation of male and female pronuclei
(D). Fusion of pronuclei
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (B), (A), (C), (D).
• C. (B), (A), (D), (C).
• D. (C), (B), (D), (A).

Hint:

To remember the sequence of fertilization:
First penetrate the outer barriers, then make membrane contact to block other sperm, form the pronuclei, and finally fuse them together to create the zygote.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Fertilization is the biological process where male and female gametes fuse to initiate the development of a new organism.
This process involves a highly coordinated sequence of cellular, biochemical, and genetic events.

Step 2: Detailed Explanation:
Let us arrange the events of fertilization in chronological order:
1. Penetration of outer layers (A): The released spermatozoon must first navigate the protective outer cellular layer (corona radiata) and penetrate the glycoprotein matrix (zona pellucida) using enzymes released during the acrosome reaction.
2. Membrane fusion and block to polyspermy (B): After crossing the zona pellucida, the spermatozoon binds to the oocyte's plasma membrane (vitelline membrane).
This contact triggers the cortical reaction, releasing granules that alter the zona pellucida (zona reaction) to prevent other sperm from entering (polyspermy).
3. Pronuclei formation (C): Once inside the oocyte, the sperm head decondenses to form the male pronucleus, while the oocyte completes its second meiotic division to form the female pronucleus.
4. Karyogamy (D): The male and female pronuclei migrate toward the center of the cell and fuse, restoring the diploid chromosome number and forming the zygote.
This chronological sequence corresponds to the order: (A) \(\rightarrow\) (B) \(\rightarrow\) (C) \(\rightarrow\) (D).

Step 3: Final Answer:
The correct sequential order of events is (A), (B), (C), (D). This corresponds to option (A).