DNA Molecules: The Building Blocks of the Genome
A genome is made up of DNA molecules that contain only four chemical bases: Adenine, Thymine, Cytosine, and Guanine--called A, T, G, and C, for short. Each DNA molecule, also known as a chromosome, is made up of two related strands. Since A only likes to match up with T, and C only with G, the order on one strand dictates the order on the other. This is called base pairing and enables the genome to make copies of itself.
The order of the bases spells out a code that will be used to make the organism.