Deoxyribonucleic acid, or DNA, carries the genetic instructions for all known living organisms. In a laboratory, DNA is commonly dissolved in watery solutions. However, when alcohol is introduced, DNA often separates and becomes visible as a white, stringy precipitate. This observation prompts the question of why DNA behaves so differently in these two common liquids.
The Basics of DNA Structure
DNA is a complex molecule made of repeating units called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. These nucleotides link to form long strands, with the sugar and phosphate groups forming the DNA backbone.
The sugar-phosphate backbone is important for DNA’s interaction with solvents. The phosphate groups along this backbone carry a negative electrical charge. These negative charges are important for its interactions with other molecules. The nitrogenous bases, while central to DNA’s genetic function, have a lesser direct impact on its overall solubility compared to the charged backbone.
Understanding Solubility: The “Like Dissolves Like” Rule
Solubility, the ability of a substance to dissolve in another, is governed by fundamental chemical principles. One of the most important is the “like dissolves like” rule. This principle states that substances with similar chemical properties tend to dissolve in each other. Polar substances dissolve other polar substances, and nonpolar substances dissolve other nonpolar substances.
Polar molecules have an uneven distribution of electrical charge, meaning one part has a slight positive charge and another a slight negative charge. Water, for example, is a polar solvent that dissolves other polar compounds like table salt. Nonpolar molecules have an even distribution of charge and do not mix well with polar substances; oil and water do not dissolve in each other because oil is nonpolar and water is polar.
Water’s Polarity and Its Interaction with DNA
Water is a highly polar molecule, due to its bent molecular shape and the strong electronegativity of its oxygen atom. This arrangement results in the oxygen atom pulling electrons closer, creating a partial negative charge, while hydrogen atoms develop partial positive charges. These positive and negative regions allow water molecules to form attractions, known as hydrogen bonds, with other charged or polar molecules.
The negatively charged phosphate groups on the DNA backbone are highly attracted to the partial positive charges on water’s hydrogen atoms. Water molecules surround each DNA molecule, forming a “hydration shell” or “solvation shell” around the DNA, with their positive ends oriented towards the negatively charged phosphates. This network of water molecules shields the DNA’s charges, preventing the DNA strands from clumping and allowing them to remain dispersed and dissolved.
Alcohol’s Role in DNA Precipitation
Alcohols are polar molecules, but they are significantly less polar than water. While alcohols can form some hydrogen bonds, their ability to interact with and surround highly charged molecules like DNA is weaker compared to water. Larger nonpolar hydrocarbon chains in alcohol molecules reduce their overall polarity and their capacity to form electrostatic interactions with charged structures.
When alcohol is added to a DNA solution, it competes with water molecules for interactions with the DNA. Alcohol is less effective at forming a hydration shell around the negatively charged phosphate groups, disrupting existing water-DNA interactions. As alcohol concentration increases, water molecules are displaced from the DNA’s surface, and the exposed negative charges are no longer shielded. This causes DNA molecules to aggregate and clump together, especially when salts are present to neutralize the DNA’s negative charges. The aggregated DNA then becomes visible as a precipitate.