Question

Describe the events of fertilization.

Short answer

Fertilization is a process that begins when million of sperms reach the egg, and the one that penetrates the egg's outer layer is the one that fertilizes the egg. The nuclei of the sperm and the egg then merge to create a complete set of chromosomes, called a zygote. This zygote divides and grows to form a multi-cell organism, marking the start of pregnancy when it implants into the wall of the uterus.

Step-by-step solution

The Arrival of Sperm

The process of fertilization begins when sperm cells reach the egg. During intercourse, millions of sperm are deposited into the female reproductive tract, but only a few will reach the egg. The sperm swim up the reproductive tract and towards the egg using their tails, guided by chemical signals.

Penetration of Sperm into the Egg

Once a sperm reaches the egg, it needs to penetrate the egg's outer layers to fertilize it. The sperm uses enzymes located in its head to penetrate the egg's outer layers, which is known as the zona pellucida. The first sperm to penetrate these layers will be the one to fertilize the egg.

Merging of Sperm and Egg Nuclei

After the sperm has penetrated the egg, the nuclei of the sperm and egg merge, a process known as syngamy. This is the moment of fertilization, when the genetic material of the sperm and egg combine to create a single cell with a complete set of chromosomes. At this point, the fertilized egg is called a zygote.

Formation of a Zygote

The zygote now begins to divide and grow. The zygote splits into two cells, then four, then eight, and so on, becoming a multi-cell organism. This early stage of embryonic development is known as cleavage. The ball of cells continues to grow and eventually implants into the wall of the uterus, marking the start of pregnancy.