The four building blocks of DNA are nucleotides. Each nucleotide consists of a nitrogenous base, a deoxyribose sugar, and a phosphate group. The four nitrogenous bases are adenine (A), cytosine (C), guanine (G), and thymine (T). These bases pair with each other in a specific way: A with T, and C with G. This pairing is what gives DNA its double helix structure.