CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY
DNA within sperm. Computer artwork of DNA within the head of a sperm cell (spermatozoon) on a background of spermatozoa. Sperm cells carry one half of the gentic material (DNA) for a new human. The other half is in the female sex cell (ovum, not seen), with which one sperm merges during fertilisation. A molecule of DNA (deoxyribonucleic acid) consists of two helical strands coiled around the same axis. There are four nucleotide bases that hold the two helical strands together in a double helix; adenine (A), cytosine (C), guanine (G) and thymine (T). The sequence of the bases in the molecule forms the genetic code.
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