Question 31 (Choice 1) - Biology - CBSE Class 12 Sample Paper for 2023 Boards - Solutions - Solutions to CBSE Sample Paper - Biology Class 12

Trace the events from copulation to zygote formation in a human female.

Answer:

1. During copulation (coitus) semen is released by the penis into the vagina (insemination).
2. The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube.
3. The ovum released by the ovary is also transported to the ampullary region where fertilization takes place.
4. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary region. This is the reason why not all copulations lead to fertilisation and pregnancy.
5. The process of fusion of a sperm with an ovum is called fertilisation.
6. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum.
7. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
8. This induces the completion of the meiotic division of the secondary oocyte.
9. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid).
10. Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote.