Fertilization is the union of haploid sperm with haploid ovum to form a diploid zygote.

Fertilization is a crucial process in sexual reproduction that leads to the formation of a new individual. It involves the fusion of male and female gametes, resulting in the formation of a zygote. Let's understand the process in detail:

1. Gametogenesis:
Before fertilization can occur, both males and females undergo gametogenesis, a process that produces haploid gametes. In males, this process is known as spermatogenesis, which takes place in the testes and produces spermatozoa. In females, the process is called oogenesis, which occurs in the ovaries and produces ova or eggs.

2. Release of Gametes:
During sexual intercourse, millions of sperm cells are released into the female reproductive tract through ejaculation. On the other hand, a single mature ovum is released from the ovary during ovulation and enters the fallopian tube.

3. Capacitation:
After ejaculation, the sperm cells undergo a process called capacitation. It involves changes in the sperm's membrane and its ability to penetrate the egg. Capacitation occurs in the female reproductive tract and prepares the sperm for fertilization.

4. Penetration of the Egg:
As the ovum travels through the fallopian tube, it is enveloped by a protective layer known as the zona pellucida. When sperm reach the egg, they undergo the acrosome reaction, where the acrosome (a structure on the sperm's head) releases enzymes that help penetrate the zona pellucida. The first sperm to successfully penetrate the egg's protective layers will fertilize it.

5. Fusion of Genetic Material:
Once a sperm penetrates the egg, fusion of genetic material occurs. The nuclei of the sperm and egg fuse, resulting in the formation of a diploid zygote. The zygote contains a complete set of chromosomes, half from the sperm and half from the egg, and has the potential to develop into a new individual.

6. Implantation:
After fertilization, the zygote undergoes a series of cell divisions called cleavage. It travels down the fallopian tube and eventually reaches the uterus. Here, it implants itself into the uterine lining and continues to develop into an embryo.

In conclusion, fertilization is the union of a haploid sperm with a haploid ovum to form a diploid zygote. This process marks the beginning of a new life and is essential for sexual reproduction.