What two alternating molecules make up the backbone of DNA?

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DNA, or Deoxyribonucleic Acid, is a double helix, with a backbone that is made up of alternating molecules of deoxyribose, a five-carbon sugar with the chemical formula C5H10O4 and molecules of phosphate, an inorganic salt with the formula PO4. The DNA molecule is made up of two such strands, which formed the backbone, rather like the side rails of a ladder. The rungs of the ladder are formed by bonded pairs of nitrogenous bases known as adenine, thymine, cytosine, and guanine.

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