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What changes turn a fertilized egg into a zygote?
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Fertilization
Fertilization occurs when a sperm cell penetrates and fuses with an egg (ovum).
A fertilized egg becomes a zygote through a series of complex biological events that occur after a single sperm penetrates the egg cell membrane.12
Key changes include:
- Surface Modification: Upon entry, the sperm triggers changes in the egg’s membrane, rendering it impermeable to other spermatozoa.1
- Pronuclei Formation: The sperm head loses its tail and swells to become the male pronucleus, while the egg completes its final maturation division to form the female pronucleus.12
- Genetic Reassociation: The two pronuclei meet in the center of the cytoplasm, where their nuclear membranes disappear. Their chromatin material organizes into a complete set of chromosomes, restoring the diploid number (46 in humans).123
- Activation: The union of these genetic materials creates the zygote, which contains the instructions for a new, genetically unique individual.14
This process concludes with the formation of the first mitotic spindle, which prepares the zygote for its initial cleavage into daughter cells.23