PATRICK LANDMANN / SCIENCE PHOTO LIBRARY PATRICK LANDMANN / SCIENCE PHOTO LIBRARY
Genetic material. Labelled diagram showing the structure of human genetic material. Nearly every cell (upper centre) has a nucleus (orange) with 23 pairs of chromosomes (blue, right). Chromosomes are made of condensed chromatin (lower right). Chromatin is composed of nucleosomes, where DNA (deoxyribonucleic acid) is wrapped around histone proteins (blue spheres). DNA is a double helix molecule (lower centre) that comprises two backbone strands (brown) joined by nucleotide bases (multicoloured). There are four bases: guanine, cytosine, adenine and thymine. DNA contains sections, called genes, which encode each cell's structure, function and behaviour.
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