PATRICK LANDMANN / SCIENCE PHOTO LIBRARY PATRICK LANDMANN / SCIENCE PHOTO LIBRARY
Chromosome structure. Artwork of a chromosome (left), showing how it is formed from chromatids (centre) and deoxyribonucleic acid (DNA, right). Humans have 23 pairs of chromosomes, which are made of condensed chromatin. Chromatin is composed of nucleosomes, where DNA is wrapped around histone proteins (blue spheres). DNA is a double helix molecule that comprises two backbone strands (brown) joined by nucleotide bases (multicoloured). There are four bases: guanine, cytosine, thymine and adenine. DNA contains sections, called genes, which encode each cell's structure, function and behaviour.
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