Question

Extrusion of second polar body from egg nucleus occurs:

• A. after entry of sperm but before fertilization
• B. after fertilization
• C. before entry of sperm into ovum
• D. simultaneously with first cleavage

Answer 1

The Correct Option is A

To solve the question about the timing of the extrusion of the second polar body from the egg nucleus, we need to understand the process of oocyte maturation and fertilization in human biology.

The sequence of events is as follows:

1. During oogenesis, the primary oocyte undergoes the first meiotic division to become a secondary oocyte and a first polar body. The secondary oocyte is arrested at metaphase II.
2. Ovulation occurs, and the secondary oocyte is released into the fallopian tube.
3. When a sperm enters the secondary oocyte, the second meiotic division resumes. However, fertilization (fusion of the sperm and egg nuclei) has not yet occurred.
4. The second polar body is extruded following the entry of the sperm into the oocyte. This indicates the completion of the second meiotic division but occurs before the actual fusion of the sperm and egg nuclei, which constitutes fertilization.

Hence, the correct answer is: after entry of sperm but before fertilization.

Let's consider why the other options are incorrect:

• **After fertilization:** The second polar body is extruded before fertilization, specifically after sperm entry but before the nuclei fuse.
• **Before entry of sperm into ovum:** The second meiotic division, leading to the extrusion of the second polar body, is triggered after the sperm enters the oocyte.
• **Simultaneously with first cleavage:** The first cleavage occurs after fertilization has been completed, which is after the second polar body is extruded.