The backbone of DNA is made up of a repeating sugar-phosphate framework. Each strand consists of deoxyribose sugar molecules alternating with phosphate groups, forming the sides of the double helix. Attached to each sugar is one of the four nitrogenous bases—adenine (A), thymine (T), cytosine (C), or guanine (G). The two DNA strands are held together by hydrogen bonds between complementary bases (A with T, C with G), while the sugar-phosphate backbones create the stable outer ladder that supports the molecule.