The base pairs in DNA are adenine to thymine and guanine to cytosine. In RNA, they are adenine to uracil and guanine to cytosine.
A base pair is made of two nucleotides. The nucleotides, located on opposite strands of DNA or RNA, are drawn to each other in a hydrogen bond. These bonds are what hold the strand together in a double helix formation. The double structure is a redundancy that acts as a backup system to store genetic information.
Base pairs facilitate transcription, which is the process whereby genetic information encoded in DNA is transferred to RNA. The information is carried in only one of the two strands of DNA, which is called the coding strand. Every nucleotide in the coding strand has a complementary nucleotide in the other strand, called the template strand.
The Watson-Crick pairs are the standard DNA and RNA base pairs. In DNA, adenine bonds to thymine while guanine bonds with cytosine. The same pairs apply to RNA, except that uracil replaces thymine. Uracil and thymine molecules are very similar in shape, allowing them to form the same kinds of hydrogen bonds with adenine.
There are, however, some alternate bond pairs which result from other hydrogen bonds. In a wobble base pair, guanine bonds to uracil, hypoxanthine bonds to uracil, hypoxanthine bonds to adenine, and hypoxanthine bonds to cytosine. A Hoogsteen base pair is another alternate formation. Hoogsteens and wobbles occur most often in RNA and are highly complex.