DNA is a chain of chemical "building blocks", called "bases", of which there are four types: these can be abbreviated A, T, C, and G. Each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand;
