DNA chain:

C pairs with G and T pairs with A

There are only four molecules in the DNA chain: adenine (A), guanine (G), thymine (T) and cytosine (C). These As, Cs, Ts and Gs are also called "bases." 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.

http://www.affymetrix.com/corporate/media/genechip_essentials/dna_review/C_pairs_with_G_and_T_pairs_with_A.affx