The "rungs" of the DNA ladder are made up of nitrogenous bases. These bases are of four types: adenine (A), guanine (G), cytosine (C), and thymine (T) . Each nucleotide, which is the basic building block of DNA, consists of a phosphate group, a sugar molecule, and a nitrogenous base. The nitrogenous bases pair up to form the rungs of the DNA ladder, with adenine always pairing with thymine and guanine always pairing with cytosine. The two strands of DNA run in opposite directions so that they are always complementary to each other, and they are held together by hydrogen bonds between the complementary nitrogenous bases.