A base pair is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA. In DNA, adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C) /05%3A_DNA/5.04%3A_Base_Pairing_in_DNA_and_RNA). RNA is the same, except that adenine always pairs with uracil (A-U) . The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same, and similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same/05%3A_DNA/5.04%3A_Base_Pairing_in_DNA_and_RNA). The total number of base pairs is equal to the number of nucleotides in one of the strands, and the human genome, for example, is made up of an estimated three billion base pairs.