Fertilization

Fertilization is the union of haploid gametes to form a diploid zygote, beginning the development of a new organism. It restores the species’ chromosome number and combines genetic material from two parents, contributing to genetic variation. Fertilization can occur internally or externally, depending on the species and environment.

In animals, fertilization typically involves a sperm penetrating the egg's protective layers and fusing its genome with the oocyte's. After recognition and binding, the sperm undergoes an acrosome reaction to reach the egg membrane. The two haploid pronuclei then fuse to form a diploid zygote. Preventive mechanisms, such as cortical and zona pellucida changes, reduce polyspermy. The zygote then enters embryonic development through cleavage.

In flowering plants, fertilization takes place after a pollen grain germinates on a stigma and a pollen tube delivers sperm to the ovule. One sperm fertilizes the egg, forming the zygote that develops into the embryo. A second sperm fuses with the central cell to form the endosperm, a triploid tissue that nourishes the embryo—a process known as double fertilization.

In fungi and some algae, fertilization involves distinct steps such as plasmogamy, the union of cytoplasm, followed by karyogamy, the fusion of nuclei. Across groups, fertilization integrates genetic material and initiates the life-cycle’s growth phase.