Complementary base pairing refers to the structural pairing of nucleotide bases in deoxyribonucleic acid, which is commonly known as DNA. DNA is made up of four nucleotide bases, each of which pairs with only one of the other bases.
The four nucleotide bases in DNA are guanine, cytosine, adenine and thymine. The guanine base is always paired with the complementary cytosine base, and the adenine base is always paired with the complementary thymine base.
A DNA molecule is composed of two connected strands of nucleotide bases, which form a spiraling double helix structure. The two strands of nucleotide bases are arranged such that every base in the first strand is paired to its complementary base in the second strand. Since every nucleotide base is always paired with its complement, you can always deduce the sequence of the second strand if you can identify the sequence of bases in the first strand.
The two sets of complementary base pairs are commonly represented in an abbreviated form that takes the first letter of each base. The guanine-cytosine base pair is represented as G–C, and the adenine-thymine base pair is represented as A–T. In a DNA molecule, the G–C base pair is linked by two hydrogen bonds, and the A–T base pair is linked by three hydrogen bonds.