Question

Extrusion of second polar body from egg nucleus occurs
(a) After entry sperm before fertilization.
(b) After fertilization
(c) Before entry of sperm into ovum
(d) Simultaneously with first cleavage

Answer 1

Hint:After the first meiotic division the secondary oocyte is formed with the expulsion of the first polar body. The second polar body is not formed until the arrested secondary oocyte gets a stimulus by the sperm for continuing its meiosis-II.

Complete answer:
The primary oocyte within the tertiary follicle grows and completes its first meiotic division at puberty. It is an unequal division resulting in the formation of a large secondary oocyte and a tiny first polar body. The secondary oocyte retains the bulk of the nutrient- rich cytoplasm of the primary oocyte. The entry of sperm into the ovum induces the completion of the meiotic division of secondary oocytes. The entry of sperm causes a breakdown of the Metaphase promoting factor (MPF) which turns into the anaphase- promoting complex (APC). This results in the completion of meiosis- II. The second meiotic division is also unequal and it results in the formation of a second polar body along with a haploid ovum. Soon the haploid nucleus of sperm and that of the ovum fuse to form a diploid zygote.

So, the answer is, ‘After entry sperm before fertilization.’.

Note: - The secondary oocyte is released during ovulation and not the ovum. Ovum is formed after the completion of the second meiotic division. - The secondary oocyte is arrested in the metaphase II stage. - The first and second polar body both do not undergo meiosis- II and usually disintegrate. - There is an unequal division of cytoplasm seen in the meiotic division as the mature egg retains most of the cytoplasm, which provides nourishment for the ovum during development. - In humans from one primary oocyte, a single ovum and two polar bodies are formed.