Fertilization is the process of fusion of male and female germ cells, resulting in the formation of a zygote.
An embryo develops from the zygote, which gives rise to a new organism.
Fertilization in animals
In animals, the fertilization process begins with the penetration of the sperm into the egg.
When the sperm head comes into contact with the egg membrane, the enzymes contained in the acrosome are released onto the surface of the membrane. Under their action, the egg shell dissolves at the point of contact. The contents of the sperm cell penetrate into the egg cell.
The oocyte membrane becomes impenetrable for the rest of the spermatozoa, and two nuclei merge in it. As a result, the diploid nucleus of the zygote is formed.
In a fertilized egg, DNA duplication takes place, and it prepares for division.
In animals, there are two ways of fertilization: external and internal.
With external fertilization, the female lays eggs (eggs), and the male – sperm – into the external environment. Fertilization takes place there. This method is typical for aquatic inhabitants (fish, amphibians).
With internal fertilization, the fusion of gametes occurs in the genital tract of the female. This method is typical for terrestrial and some aquatic inhabitants (worms, insects, reptiles, birds, mammals).
The biological significance of fertilization lies in the fact that when gametes merge, the diploid set of chromosomes is restored, and the new organism acquires genetic information and features of both parents.
Parthenogenesis is a type of sexual reproduction in which an adult develops from an unfertilized egg.
Parthenogenesis occurs in lower crustaceans (daphnia), insects (bees, aphids), in some birds (turkeys) and alternates with sexual reproduction.
A new organism develops from unfertilized eggs with a haploid set of chromosomes. During the first division of mitosis after DNA duplication, the chromosomes do not diverge, and the diploid set is restored.
Parthenogenesis can take place under both favorable and unfavorable conditions.
Example: in aphids, daphnia, females develop in summer, and males develop from unfertilized eggs in autumn. In bees, from unfertilized eggs, males always develop – drones, and from fertilized eggs – females (queens) and worker bees.