TIM VERNON / SCIENCE PHOTO LIBRARY TIM VERNON / SCIENCE PHOTO LIBRARY
Fertilisation. Computer artwork of sperm cells (purple) attempting to penetrate a human egg. Each sperm (spermatozoan) has a rounded head and a long tail which it uses to both swim and burrow into the egg. Only one of these sperm cells fertilises the egg. Fertilisation occurs when the sperm's genetic material (deoxyribonucleic acid, DNA) fuses with the egg's DNA forming a zygote.
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