Question

The figure given below shows 3 sperms A, B and C.

1. Which one of the three sperms will gain entry into the ovum?
2. Describe the associated changes induced by it on P and Q.

Ovum surrounded by few sperms

Answer 1

1. Sperm A
2. In the figure given, Sperm ‘A' has come in contact with the zona pellucida layer (P) of the ovum (Q), which will induce changes in the membrane that will block the entry of additional sperms (B and C). Thus, it ensures that only one sperm can fertilise the ovum.
   The secretions of the acrosome of sperm A will help it to enter into the cytoplasm of the ovum (Q) through the zona pellucid (P) and the plasma membrane, this will induce the completion of the meiotic division of the secondary oocyte (Q).
   The second meiotic division in Q being unequal will result in the formation of a second polar body and a haploid ovum. Then, the haploid nucleus of sperm ‘A’ and that of the ovum (Q) will fuse together to form a diploid zygote.