A base pair is the combination of two nucleotides whose bases are bonded with each other and lie on opposing strands of a DNA molecule. Adenine bonds with thymine, whereas cytosine bonds with guanine, as shown in the figure below (illustrating Watson-Crick base pairing).
During transcription and translation, RNA is involved in base pairing interactions, in which case uracil replaces thymine, and so the base pair AU is formed instead of AT. This base pair can also arise during RNA folding.
The practical purpose of DNA base pairing is to protect the vital DNA molecule from foreign bonding, which could disrupt transcription. As a result, many researchers believe that DNA actually evolved from RNA.