What is DNA made of?

DNA is made of small subunits called bases that join together to form a long chain. The bases in each chain join to the bases in another chain to form a structure that looks a bit like a ladder, with each chain running parallel to the other. There are 4 different types of bases called Adenine, Thymine, Cytosine and Guanine; or A, T, C and G for short. When a base on one strand bonds to a base on a second strand, it is called a base pair. Base pairing is not random; each base can only bond to one other type of base. For example A can only bind to T and C can only bind to G. This is called complementarity: A is complementary to T and C is complementary to G.