In DNA, the complementary bases are adenine and guanine ; thymine and cytosine. Each nucleotide in DNA is composed of a nitrogen containing base, either guanine (G), adenine (A), thymine (T), or cytosine (C) as well as a monosaccharide sugar called deoxyribose and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone.
According to base pairing rules (A with T and C with G), hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA.