Although DNA is held together by several different kinds of chemical interactions, it is still a rather fragile molecule. The nitrogen bases that constitute the “rungs” of the ladder are held together by hydrogen bonds. The “sides” of the ladder (the phosphate and deoxyribose molecules) are held together by a type of covalent bond called a phosphodiester bond. Because part of the DNA molecule is polar (the outside of the ladder) and the rungs (nitrogen bases) are nonpolar, other interactions—called hydrostatic interactions—occur between the hydrogen and oxygen atoms of DNA and water. The internal part of the DNA tends to repel water, while the external sugar-phosphate molecules tend to attract water. This creates a kind of molecular pressure that glues the helix together.