Vocabulary
DNA
The carrier of genetic information for all complex organisms.
DNA is shaped like a twisted step-ladder -- the famous "double helix." The genetic information is carried on the rungs of the ladder.
In the figure above, you will see that each rung of the ladder is labelled with two letters. There are only four possible letters: A, C, G, T. These four letters are the "alphabet" of DNA. Technically, they are called "nucleotide bases," but we will continue to call them letters.
On any given rung, if you know one letter, you also know the other. This is possible because A always goes with T, and C always goes with G.