MIKE AGLIOLO / SCIENCE PHOTO LIBRARY MIKE AGLIOLO / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Artwork of a sperm (brown, at centre) fertilising a human egg (red). The sperm's DNA (deoxyribonucleic acid) genetic material is seen as a double helix. Women usually produce one large egg (ovum), whereas men release some 300 million much smaller sperm (spermatozoa) which travel with whiplash movements of their tail through the uterus to reach the egg. A few hundred sperm survive this journey and encounter the egg in the fallopian tube. However, only one sperm manages to penetrate the wall of the egg and fertilise it by fusing with the egg's DNA in the nucleus. When this occurs, the egg membrane forms a barrier to other sperm.
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