DNA is a macromolecule with 4 nucleotides types as monomers. These pair always in the same way: Thymine with Adenine; Cytosine with Guanine. The pairing system is not based on a covalent bond, but on a hydrogen bridge between the bases. Both thymine and adenine offer two possible sites for hydrogen bridge interactions, while cytosine and Guanine offer three. The interactions between another base than the ones described before are more unstable and because of it, they do not last.