Nucleic acids, such as DNA and RNA, are overall hydrophilic, meaning they dissolve readily in water. However, their structure is built from components that possess both water-loving (hydrophilic) and water-fearing (hydrophobic) properties. This amphiphilic nature is key to their unique double-helix structure and their stability within the cell’s watery environment.
Molecular Components of Nucleic Acids
The basic unit of a nucleic acid strand is the nucleotide, which links together to form long chains. Each nucleotide is composed of three distinct chemical parts: a five-carbon sugar (deoxyribose in DNA or ribose in RNA), a phosphate group, and a nitrogenous base. The sugars and phosphate groups alternate to form the long, continuous sugar-phosphate backbone. Attached to each sugar, projecting inward, is one of the four types of nitrogenous bases, which hold the genetic code.
The Hydrophilic Phosphate Backbone
The primary reason nucleic acids are highly soluble in water is the strong chemical nature of the sugar-phosphate backbone. The phosphate groups carry a negative electrical charge at physiological pH because they lose a proton in water, making the backbone highly anionic. Molecules with a full electrical charge are strongly attracted to the polar nature of water molecules. This attraction allows water to surround and dissolve the charged phosphate groups, making the entire backbone exceptionally hydrophilic and positioning it on the exterior of the molecule.
Characteristics of Nitrogenous Bases
In contrast to the charged backbone, the nitrogenous bases (A, G, C, and T or U) are largely hydrophobic. These flat, ring-shaped molecules are mostly non-polar, meaning they lack the charge separation needed to interact favorably with water. Because they repel water, they seek to minimize contact with the aqueous environment. This water-fearing nature drives the bases to aggregate together, a phenomenon known as base stacking, forcing a specific arrangement in solution.
Overall Solubility and the Double Helix
The opposing chemical properties of the hydrophilic backbone and the hydrophobic bases are directly responsible for the formation and stability of the DNA double helix. The molecule adopts a twisted, helical shape to satisfy the attraction of charged groups to water and the repulsion of non-polar groups from water. The negatively charged, hydrophilic sugar-phosphate backbones wind around the outside, maximizing exposure to the surrounding water. Simultaneously, the hydrophobic nitrogenous bases are tucked away in the center of the helix, minimizing unfavorable contact with the polar water and ensuring the overall molecule is highly soluble and hydrophilic.