The type of bond that keeps the bases paired together in a DNA double helix is called a hydrogen bond. Hydrogen bonds are weak chemical bonds that form between an electronegative atom and a hydrogen atom that is bonded to an electronegative atom. In DNA, the hydrogen bonds form between the nitrogen atoms in the purines (adenine and guanine) and the oxygen atoms in the pyrimidines (thymine and cytosine). The hydrogen bonds between the base pairs are what hold the two strands of DNA together in the double helix.