A molecular geneticist spools DNA out of a solution at the end of a DNA extraction, Great Ormond Street Hospital, London, May 2000. All genes are made of DNA. A copy of every gene is contained in most of the cells in the body, although only certain genes are used in each cell. Scientists can use cells from almost any part of the body (white blood cells for example) to obtain a sample of its entire DNA. Once it is out of the cells and into a test tube, DNA can be examined in detail.
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