Given below are two statements:
Statement I: The entry of the sperm into the secondary oocyte induces the completion of the meiotic division of the secondary oocyte.
Statement II: The second meiotic division of the secondary oocyte is unequal and results in the formation of a second polar body and a diploid ovum (ootid).
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Statement I is correct but Statement II is incorrect
Statement I: The entry of the sperm into the secondary oocyte induces the completion of the meiotic division of the secondary oocyte. — True. The ovum released at ovulation is a secondary oocyte that has not yet finished its meiotic division. The secretions of the acrosome help the sperm enter its cytoplasm, and this entry sets off the completion of the second meiotic division.
Statement II: The second meiotic division of the secondary oocyte is unequal and results in the formation of a second polar body and a diploid ovum (ootid). — False. The division is unequal and does give a second polar body, but the ovum it forms is haploid, not diploid. The secondary oocyte is already haploid after the first meiotic division, so the ovum formed from it is haploid too. The diploid stage comes only when the haploid nucleus of the sperm fuses with that of the ovum to form the zygote.
NCERT Extract:
Fertilisation and Implantation – “… 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.”