How does fertilisation occur in sexual reproduction?

Fertilisation in sexual reproduction occurs when a sperm cell from the male unites with an egg cell from the female.

In more detail, sexual reproduction is a biological process that involves the combination of genetic material from two parent organisms to create offspring. This process begins with the production of sex cells, or gametes, through a process called meiosis. In males, this results in the production of sperm cells, while in females, it leads to the production of egg cells.

Fertilisation is the actual process where these two types of cells unite. This typically occurs in the female's reproductive system. The sperm cell, which is much smaller and more mobile than the egg cell, travels up the female's reproductive tract to reach the egg. This journey is not easy, and out of millions of sperm, only a few hundred may reach the egg.

Once a sperm cell reaches the egg, it must penetrate the outer layer of the egg, known as the zona pellucida. This is achieved through a process called the acrosome reaction, where enzymes in the head of the sperm break down this outer layer. Once a single sperm has penetrated the egg, changes in the egg's membrane prevent other sperm from entering. This ensures that only one sperm fertilises the egg, providing exactly half the genetic material for the new organism.

The sperm and egg then fuse, combining their genetic material. This results in a single cell called a zygote, which contains a complete set of chromosomes - half from the mother and half from the father. The zygote then begins to divide and grow into an embryo through a process called mitosis.

In summary, fertilisation in sexual reproduction is a complex process involving the production of sex cells, their journey towards each other, and their eventual fusion to create a new organism. This process ensures the genetic diversity that is a key feature of sexual reproduction.