Fertilization Process

During fertilization, the fusion of a sperm cell with an egg cell occurs, resulting in the formation of a zygote. This process involves various steps that ultimately lead to the successful fertilization of the egg.

1. Capacitation and Acrosome Reaction
- Before fertilization can occur, the sperm cells undergo a process called capacitation. This involves changes in the sperm membrane that enable them to penetrate the egg.
- As the sperm approaches the egg, the acrosome, a specialized structure at the tip of the sperm, undergoes an acrosome reaction. This reaction releases enzymes that help the sperm penetrate the protective layers surrounding the egg.

2. Penetration of the Zona Pellucida
- The zona pellucida is a thick layer surrounding the egg. Only a few sperm are able to reach this layer, and they compete to penetrate it.
- The enzymes released during the acrosome reaction help the sperm to break through the zona pellucida. However, only one sperm is able to successfully penetrate and reach the egg's cytoplasm.

3. Fusion of Sperm and Egg
- Once a sperm has successfully penetrated the zona pellucida, it binds to receptors on the egg's plasma membrane.
- This binding triggers changes in the egg's membrane that prevent other sperm from binding and fertilizing the egg.
- The sperm then fuses with the egg, and their genetic material combines to form a zygote.

4. Formation of the Zygote
- After fusion, the genetic material from the sperm and egg combines, resulting in the formation of a zygote.
- The zygote contains the complete set of chromosomes necessary for the development of a new individual.
- The zygote then undergoes a series of cell divisions, forming an embryo that implants into the lining of the uterus.

Answer Explanation
In the given options, only option D accurately describes what happens during fertilization. The secretion of acrosome helps one sperm enter the cytoplasm of the egg. This is a crucial step that allows the sperm to release enzymes necessary for penetrating the egg's protective layers, including the zona pellucida. Once a sperm successfully penetrates the zona pellucida, it can fuse with the egg's cytoplasm and contribute its genetic material to form a zygote.

Options A, B, and C are not accurate descriptions of what happens during fertilization. While it is true that the sperm lose their tails after reaching close to the ovum, this occurs before they reach the zona pellucida and is not a determining factor in fertilization. Similarly, option C suggests that the cells of the corona radiata trap all the sperm except one, but in reality, only a few sperm reach the zona pellucida. Option B states that only two sperm nearest to the ovum penetrate the zona pellucida, which is incorrect as only one sperm is able to successfully penetrate and fuse with the egg.