Question

Define Fertilization.

Answer 1

Hint: Zygote is the product of this process. This process is responsible for variations in living organisms.
Syngamy is the final step of this process. In plants, pollination is the prerequisite of this process. This was explained by Aristotle.

Complete Answer:
Fertilization is defined as fusion of male and female gametes. This is the basis of sexual reproduction.

Sexual reproduction is the production of offsprings by fusion of male and female sex cells known as gametes. The gametes are haploid that fuse and form the diploid zygote. This then develops into a new organism.

The entire sexual reproduction takes place in three events:
1) Pre-fertilisation events
2) Fertilisation events
3) Post fertilisation events

1) Pre-fertilisation events :This includes
- Gametogenesis : This involves formation of gametes. In animals the male gametes are called sperms that are produced in testes and female gametes are called ovums that are formed in the ovaries. In flowers pollens are male gametes and egg is the female gamete.
- Gamete transfer : The male and female gametes should be brought together so that they can fuse. This is known as gametes transfer. In the majority of animals, this is known as copulation act. In some animals like frogs, water is a medium where the gametes contact each other. In flowers, pollination brings about this contact.

2) Fertilisation events :
As the male and female gametes come together, they fuse to form diploid zygote. This is known as fusion or syngamy. (syn means fusion and gamy means marriage). Fertilization includes all the events that ultimately leads to syngamy. Thus syngamy is the final step in the process of fertilization. The result of fertilization is diploid zygote which is a vital link that ensures continuity of species. The process of fertilization differs in different organisms.

In humans, the penis transfers the gametes to the female vagina. The sperms undergo functional maturation before it reaches the oviduct. It then pierces the corona layer, the zona and then the ovum membrane before it finally fuses with the nucleus.

In flowers, the pollens are deposited on the stigma of the female flower which then forms the pollen tube and then transfers the pollens to the ovary.

The fertilization can be external or internal. External fertilization is fusion of gametes outside the body as seen in frogs and internal fertilization is fusion inside the body as seen in humans.

3) Post-fertilization events :
After fertilization, the zygote formed undergoes many stages of development and thus forms new organisms. The developmental stages of zygote and included in post-fertilization events. These include embryogenesis and organogenesis in animals. In plants it includes fruit and flower formation.
Thus Sexual reproduction occurs in living organisms.

Note: There are some organisms in which fusion does not take place and the gametes directly form new individuals. This is known as parthenogenesis.
In some plants, the fruits are formed after fertilization and this is known as parthenocarpy.