MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Computer illustration of a sperm (centre right) fertilising a human egg (far right). 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 egg's thick outer layer (zona pellucida) using enzymes, and fertilises the egg by fusing with its DNA (deoxyribonucleic acid) genetic material. When this occurs, the egg membrane forms a barrier to other sperm.
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