The backbone of the double helix consists of alternating phosphate and deoxyribose sugar molecules. If you picture DNA like a twisted ladder, the sides of the ladder would be the backbone and the rungs would be complementary base pairs.
A single unit of DNA is called a nucleotide. A nucleotide contains a phosphate group, a sugar called deoxyribose and one of four nitrogenous bases--adenine, guanine, cytosine or thymine. If one nucleotide contained the base adenine, then the complementary base in the adjacent nucleotide would contain thymine. That is because adenine pairs to thymine. Similarly, the base cytosine pairs to guanine. A chemical bond called a hydrogen bond holds the base pairs together in the double helix.
I have attached two links with nice diagrams of the DNA molecule. Both are color coded to illustrate the location of the base pairs and the backbone of the molecule.
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