Fertilization

Fertilization is when a sperm from a male fuses with an egg (ovum) from a female to form a zygote, marking the beginning of a new organism.

Calcium Wave: Sperm entry triggers a Ca²⁺ surge in the egg, initiating metabolic activation and preventing polyspermy.
Cortical Reaction: Release of enzymes modifies the zona pellucida, blocking other sperm.
Zygotic Genome Activation (ZGA): Initially, the zygote uses maternal mRNA; the embryonic genome becomes active around the 8-cell stage (in humans).]