D3.1.6 Fertilization in humans Notes

Fertilization Restores Genetic Stability While Creating Diversity

1. Fertilization is the fusion of a sperm and an egg to form a zygote.
2. This process ensures the combination of genetic material from both parents, creating a new, genetically unique individual.

Fertilization Occurs in Four Key Steps

1. Sperm Navigation and Binding

1. Chemotaxis: Sperm are guided toward the egg by chemical signals.
2. Breaking Protective Layers:
   1. Cumulus Oophorus: A cloud of follicle cells surrounding the egg.
   2. Zona Pellucida: A glycoprotein layer that protects the egg.
   3. Acrosome Reaction: Sperm release enzymes to digest the zona pellucida and reach the egg.

2. Fusion of Sperm and Egg Membranes

1. Proteins on the sperm bind to receptors on the egg membrane.
2. The membranes fuse, allowing the sperm nucleus to enter the egg.

3. Prevention of Polyspermy

1. Once a sperm enters, mechanisms ensure no additional sperm can fuse with the egg:
2. Cortical Reaction:
   1. The egg releases enzymes that harden the zona pellucida, preventing further sperm entry.

4. Fusion of Genetic Material

1. After the sperm nucleus enters:
   1. Destruction of the Tail and Mitochondria: These are unnecessary for the zygote and are discarded.
   2. Dissolution of Nuclear Membranes: The membranes of both nuclei dissolve.
   3. Chromosome Alignment and Joint Mitosis: Chromosomes from both parents align and undergo mitosis, producing two diploid nuclei with 46 chromosomes.

Fertilization Balances Stability and Diversity

1. Restoring Genetic Stability:
   1. Fertilization combines haploid gametes to reestablish the diploid chromosome number, ensuring genetic consistency across generations.
2. Creating Genetic Diversity:
   1. Meiosis produces genetically varied gametes.
   2. Random fertilization multiplies the combinations of genetic material.

Why Mitochondria and the Tail Are Destroyed

1. The sperm's mitochondria and tail do not contribute to the zygote because:
   1. Mitochondria: Only maternal mitochondria are passed to offspring, ensuring mitochondrial DNA is inherited from the mother.
   2. Tail: The tail is unnecessary for the zygote and is broken down.

The Significance of Fertilization

1. Fertilization restores the diploid chromosome number and combines genetic material from both parents.
2. It triggers the egg to complete meiosis and begin embryonic development.
3. It ensures genetic variation, a key driver of evolution.