Question

What is ovum activation?

Answer 1

Hint: Fertilization is the process of fusion of two types of gametes (spermatozoa and ovum) so as to form zygotes. Fertilisation occurs in human beings inside the ampulla-isthmus junction part of the oviduct. Ovum in a secondary oocyte state is released from the ovary during ovulation roughly in the middle of the menstrual cycle. Due to slow ciliary current the released ovum passes into a nearby oviduct through the ostium of its fimbriated funnel or infundibulum. Ovum reaches the ampulla -isthmus junction and rests there. It takes 12-24 hours for the ovum to reach the site of fertilisation.

Complete answer:
Insemination: It is the process of deposition of sperms in the genital tract of the female during coitus. The inseminating material is actually a viscid mixture called semen.
Capacitation of sperm: Sperm activation by which the sperm develops the ability to fertilise ova.
Acrosomal reaction: In contact with the surface of egg covering, the acrosome releases its contains hydrolytic enzymes.
Sperm entry: Head of sperm comes in contact with the fertilisation cone. It triggers a weak polarisation of the membrane and $Ca^{2+}$ wave inside the egg. Sperm and egg membranes dissolve in the region of contact. Components of the head, neck and middle piece of sperm enter the egg cytoplasm except the tail.
Cortical reaction: $Ca^{2+}$ wave causes extrusion of cortical granules out of the egg. They bring about modification of the plasma membrane and change the same into the fertilisation membrane which prevents the entry of second sperm.
Zona Reaction: Chemicals extruded from the egg act on zona pellucida and destroy the sperm receptors preventing attachment to any other sperm.
Activation of ovum: Egg is in the secondary oocyte stage in which meiosis II has started but arrested in the metaphase II stage due to development of a metaphase promoting factor. The factor is removed. Anaphase-promoting complex develops which results in completion of meiosis II and extrusion of a second polar body. This converts the oocyte into ovum.
Karyogamy: The nuclear envelope of male and female pronuclei breakdown. Mixing of paternal and maternal chromosomes form a diploid set. It converts the ovum into zygotes.

Note:
Significance of Fertilization: It provides the stimulus for completion of meiosis II. The diploid chromosome number of the species is restored. It is 46 in humans. It determines the sex of the baby, (22+ X) x (22+X) or (22+ Y), depending upon the chromosome complement of the fusing sperm. Karyogamy activates the fertilized egg to undergo cleavage. There is a mingling of characteristics between the two parents. A lot of variations appear due to chance separation of chromosomes, crossing over and mixing of chromosomes.