Natural human fertilisation requires the presence of an oocyte, obtained from the ovary by ovulation, and a spermatozoon in the Fallopian tube. The two gametes can only meet and fuse after recognising specific factors on the two cells, which then prevents other spermatozoa from entering the ovum.
The oocyte can then begin the final stages of meiosis. The two haploid nuclei (or rather the two pronuclei) can finally fuse to form a single diploid nucleus (with two sets of chromosomes). The egg cell can then begin to divide to form a new person, genetically different from its parents.
Fertilisation can also be forced by inserting a spermatozoon directly into the oocyte (this is intra-cytoplasmic injection of spermatozoa).
Fertilisation is the fusion of a male gamete with a female gamete. © DR