Arrange the following events of fertilisation in humans in the correct sequence:
A. The secondary oocyte completes its meiotic division, forming a second polar body and a haploid ovum (ootid).
B. A sperm comes in contact with the zona pellucida layer of the ovum.
C. The haploid nuclei of the sperm and the ovum fuse together to form a diploid zygote.
D. The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B, D, A, C
B. A sperm comes in contact with the zona pellucida layer of the ovum — this is first. The contact changes the membrane so that no other sperm can enter, which makes sure only one sperm fertilises the ovum.
D. The secretions of the acrosome help the sperm enter the cytoplasm of the ovum — this is second. The acrosome at the tip of the sperm releases secretions that carry it through the zona pellucida and the plasma membrane.
A. The secondary oocyte completes its meiotic division — this is third. The ovum released at ovulation is a secondary oocyte. The entry of the sperm induces its second meiotic division, which is unequal and gives a second polar body and a haploid ovum (ootid).
C. The haploid nuclei of the sperm and the ovum fuse to form a diploid zygote — this is last. Fusion can happen only after the ootid’s haploid nucleus has formed, and it completes fertilisation.
NCERT Extract:
Fertilisation and Implantation – “… The process of fusion of a sperm with an ovum is called fertilisation. 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.”