There are only four molecules, or "bases", in every DNA chain: adenine (A), guanine (G), thymine (T) and cytosine (C). These four molecules partner: C partners with G and T partners with A. Pairing is a natural state for DNA and if you pulled the double helix apart, it would inevitably move back together, like two long chains of magnets that are attracted to each other.

Like DNA, RNA is composed of 4 bases: A, G, C and uracil (U) instead of thymine. Uracil is related to its DNA equivalent, thymine, such that in RNA, C partners with G and U partners with A. However, unlike DNA, RNA is usually single stranded, meaning that it can easily bind to any other single stranded matching sequence, whether it's DNA or RNA.