MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Fertilisation. Computer artwork of one of a group of human sperm (spermatozoa) penetrating an egg (ovum). Only one sperm can penetrate the egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when DNA (deoxyribo- nucleic 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.
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