The two polynucleotide chains that comprise each molecule of deoxyribonucleic acid, or DNA, are held together by two forces: hydrogen bonding and hydrostatic forces. Hydrogen bonds form between complementary base pairs, while hydrostatic forces determine the orientation of the hydrophilic phosphate backbone relative to the more hydrophobic base pairs.
Each polynucleotide chain is comprised of a negatively charged phosphate backbone that connects a sequence of base pairs. The base pairs are nonpolar, planar, organic rings containing various ammonia and oxygen groups. Polar water molecules naturally form hydrogen bonds with other charged molecules. When dissolved in water, DNA chains naturally orient themselves such that the more hydrophilic phosphate backbones face outward, forming a helical structure, with the more hydrophobic base pairs stacked inside the helix. When complementary sequences of base pairs on opposing DNA strands are aligned, hydrogen bonds form between positively charged hydrogen atoms and negatively charged oxygen atoms within the structure of the base pairs. These hydrogen bonds add additional stability to the DNA double helix.