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# Long description

Three diagrams of DNA, labelled (a), (b) and (c). Diagram (a) shows a small section of a DNA strand as though it were untwisted, which looks like a ladder with two boxes on each rung. Each box represents a base (A, C, G or T). Each pair of bases forms one nucleotide. Several nucleotides make up a gene. Diagram (b) shows how the strand of DNA in (a) is twisted into the famous double helix. Diagram (c) shows a chromosome formed from one DNA strand.