Answer: Hydrogen bonds
Explanation: A DNA is a molecule with double-stranded helical structure. It is made up of three components: A deoxyribose, a phosphate group and a nitrogenous base. The deoxyribose and phosphate group makes up the phosphodeoxyribose backbone that forms the rail of the helix while the bases form the rungs or steps of the helix. In base pairing, adenine pairs with thymine while cytosine pairs with guanine. The pairing between the bases involves the formation of hydrogen bonds between the bases. Adenine forms two hydrogen bonds with thymine while cytosine forms two hydrogen bonds with guanine. The hydrogen bonds between the bases hold the two strands of the DNA together, when these bond are broken, the two strands separate.