Fertilisation. Computer artwork of one of a group of human sperm (spermatozoa) successfully penetrating an egg (ovum). Only one sperm can penetrate the egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when DNA (deoxyribonucleic acid) released by the sperm fuses with DNA contained in the egg. Women produce a single egg at a time, whilst men release around 500 million sperm. The sperm use their flexible tails to travel through the female reproductive tract. Only a few thousand sperm reach the egg, located in one of the Fallopian tubes.