Fertilisation

Fertilisation is one of the fundamental processes in biology that allows reproduction, maintaining the living characteristics of an organism. When it occurs by the fusion of male and female gamete, it is said to be a part of sexual reproduction. There are variants to the process, such as the complete development of individuals from a single cell. It is a specific, intricate and orchestrated process involving a series of events occurring at the cellular and molecular levels.

Table of Contents

Characteristics of Fertilisation
Types of Fertilisation
Steps of Plant Fertilisation
Steps of Human Fertilisation

Characteristics of Fertilisation

Fertilisation is a carefully curated series of events marked by key features enlisted below:

It begins with gamete fusion in sexual reproduction. The process of fertilisation results in the formation of a zygote with a complete set (2n) of chromosomes obtained from both parents (n).
It is the specific process that allows the fusion of gametes of particular species. The interaction between different species is often sterile. Specific recognition and gametic interaction allow the occurrence of fertilisation.
The fertilisation is accompanied by the activation of an egg that disables polyspermy or fusion with multiple sperms, which may lead to multiple sets of chromosomes, further leading to disability.
Membrane fusion allows the fusion of genetic material of two gametes. The subsequently formed zygote carries all the information for it to grow and develop.
Genetic variation, which is crucial for long-term survival, is introduced through fertilisation, often absent in other modes of reproduction.
Fertilisation occurs at a specific timeframe and at a specific location.

Types of Fertilisation

Fertilisation occurs through the fusion of male and female gametes. However, nature has multiple other modes of reproduction, such as:

Parthenogenesis: It prefers to develop an individual without male involvement or fertilisation of gametes. The ovum develops into an embryo by egg activation, which further might occur due to spontaneous chromosomal duplication or external stimuli. The process is seen in plants like bananas, reptiles, fish, invertebrates and amphibians.
Self-fertilisation: The hermaphrodite plants exhibit such a reproduction method where ovum and pollen belong to the same plant. It is also seen in invertebrates. Such organs have both male and female reproductive organs. It results in the accumulation of harmful mutations.
External Fertilisation: Generally, fertilisation occurs in the female reproductive system. The exceptions are seen in fertilisation occurring in the environment outside the parents’ body. Generally, it is seen in moist environments and water. The participating organisms are fishes, amphibians and invertebrates.
Hybridogenesis: It is the rare fertilisation technique where hybrid offspring are produced. Here the genetic material of one offspring is passed to the next generation, while the other parent’s genetic material is retained in subsequent generations. It is seen in a few fishes and water frogs.

Steps of Plant Fertilisation

Strasburger discovered the process of fertilisation in plants. It involves the following steps:

Germination of male and female reproductive organs occurs. The pollen grain develops in male plants, while pollen tube growth occurs in female plants. Pollen grains reach the stigma and germinate to form a pollen tube that grows into the stigma to reach the ovarian cavity.
The pollen tube enters into the ovule, which can occur either through the micropyle, chalazal region or the integument.
Regardless of the mode of entry, the pollen tube enters the embryo sac only from the micropylar end. The pollen tube further penetrates, degenerating the synergid and releasing two male gametes and vegetative nuclei.
Out of the two male gametes, one fuses with an egg and forms a diploid zygote that further develops into an embryo and hence a plant. This is called generative fertilisation or syngamy.
The remaining male nucleus fuses with the secondary nucleus or polar nuclei to form the triploid endosperm. The process is called triple fusion or vegetative fertilisation. It is a significant step that serves to form the nutritional source for developing embryos.

Steps of Human Fertilisation

Human fertilisation is a complex process following the stated steps:

The sperm travels to the ovum. On meeting, the acrosomal reaction releases chemicals to lyse the cell membrane of the secondary oocyte enabling the fusion of gametes.
Then begins the cortical reactions by the cortical granules of the oocyte. The release of cortical enzymes between zona pellucida and the plasma membrane hardens the former to further prevent polyspermy.
The cone of reception received the sperm. The sperm entry follows the completion of the secondary meiotic division leading to the haploid ovum and second polar body formation.
Karyogamy occurs through the fusion of male and female pronuclei. The fertilised ovum is then called a zygote.
The egg activation and protein synthesis occurs, and cellular respiration increases.
The meiosis begins within 24 hours to increase the cell number. The developing zygote travels from three to four days and travels to the uterus and is now referred to as an embryo.
It further develops for implantation, followed by preparation for pregnancy.

Practice Problems

Q1. How is polyspermy prevented?

a. Depolarisation of both gametes
b. Depolarisation of plasma membrane of oocyte
c. Depolarisation of plasma membrane of sperm
d. Both b and c are correct

Ans. b. Depolarisation of the plasma membrane of the oocyte

Depolarisation occurs in the oocyte after the binding of sperm with it.

Q2. At cellular, which parent is responsible for the sex of the child?

a. Male
b. Female
c. Both of them
d. None of them

Ans. a. Male

The female has XX chromosomes, while the male has XY chromosomes. The gametic fusion of the X or Y chromosomes from the male determines the sex of the embryo.

Q3. What is the lifespan of sperm?

a. 24 days
b. 24 hours
c. 72 hours
d. 72 days

Ans. c. 72 hours

The sperm is capable of surviving 72 hours to reach the ovum.

Q3. What happens in the absence of fertilisation in humans?

a. No zygote formation
b. Menstruation
c. Parthenogenesis
d. Self-fertilisation

Ans: Both a and b are correct

The phenomenon of parthenogenesis and self-fertilisation is not seen in humans.

Frequently Asked Questions

Q1. What is the reason for the limited days of the possibility of fertilisation among women?
Ans. The short span of the ovum, about 12 to 24 hours, requires quick fertilisation. Else the ovum begins to degenerate, leading to no chance of fertilisation.

Q2. What is the gestation period?
Ans. The gestation period is defined as the time between the foetus' development to childbirth.

Q3. Who discovered double fertilisation or triple fusion?
Ans. Nawaschin and Guignard discovered the process of double fertilisation, which occurs only in angiosperms.