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Fertilization involves several key processes in sexual reproduction:
1. Sperm Activation and Attraction: Sperm cells undergo activation to become capable of fertilizing an egg. Chemical signals from the egg attract the sperm.
2. Sperm-Egg Binding: The sperm binds to the outer layer of the egg, called the zona pellucida in mammals.
3. Acrosome Reaction: Enzymes from the sperm's acrosome (a cap-like structure) are released to digest a pathway through the zona pellucida, allowing the sperm to reach the egg's plasma membrane.
4. Fusion of Membranes: The plasma membranes of the sperm and egg fuse, allowing the sperm to enter the egg.
5. Sperm Entry: The sperm's nucleus enters the egg, while the rest of the sperm is typically left outside.
6. Cortical Reaction: The egg releases enzymes that modify the zona pellucida to prevent additional sperm from entering, ensuring only one sperm fertilizes the egg.
7. Nuclear Fusion: The sperm and egg nuclei (each containing half the number of chromosomes) merge to form a zygote with a full set of chromosomes.
8. Zygote Formation: The fertilized egg, now called a zygote, begins cell division and development into an embryo.
These steps collectively ensure the successful combination of genetic material from both parents, initiating the development of a new organism.