Select the statements which are true for oogenesis?
(A). Ovulation occurs at the secondary oocyte stage.
(B). Polar body is larger than the secondary oocyte.
(C). No more oogonia is formed after birth in the ovary.
(D). The sperm and the ovum nucleus fuse before the formation of the second polar body.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (C) and (D) only
Note: None of the given options appears to be correct, as explained below. As per NCERT, only (A) and (C) are true, and no option gives “(A) and (C) only”.
Oogenesis is the formation of the mature female gamete (ovum). It begins in the ovary before birth and is completed only at fertilisation.
(A) Ovulation occurs at the secondary oocyte stage. — True. The primary oocyte finishes its first meiotic division inside the follicle and becomes the secondary oocyte. The Graafian follicle then bursts and releases the secondary oocyte. So the cell released at ovulation is not yet a finished ovum.
(B) Polar body is larger than the secondary oocyte. — False. The first meiotic division is unequal. The secondary oocyte keeps almost all the nutrient-rich cytoplasm, and the first polar body is a tiny cell. The statement has the two sizes reversed.
(C) No more oogonia is formed after birth in the ovary. — True. Oogonia are formed during foetal life, a couple of million in each ovary. No new oogonia are formed and added after birth.
(D) The sperm and the ovum nucleus fuse before the formation of the second polar body. — False. The order is the other way round. The second polar body forms first, and the two haploid nuclei fuse after it.
NCERT Extract:
Fertilisation and Implantation – “… During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum (Figure 2.10) and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote.”