How is the absorption of ultraviolet light by DNA and RNA important in the analysis of nucleic acids?

When DNA and RNA molecules are exposed to ultraviolet (UV) light, they absorb the light energy at a specific wavelength of around 260 nm. This process is primarily caused by the presence in their structure of aromatic rings, specifically the nitrogenous bases (adenine, guanine, thymine, cytosine, and uracil). The absorption of UV light can be measured using spectrophotometry, which provides valuable information about the nucleic acids.

The amount of UV light absorbed by a sample at 260 nm can be used to determine the concentration of nucleic acids present. Since the absorbance is directly proportional to the concentration, this provides a simple and quick method to measure the concentration of DNA or RNA in a given sample. For example, a spectrophotometer reading (absorbance) of 1 corresponds to a concentration of 50 µg/mL of double-stranded DNA or 40 µg/mL of RNA.

Apart from determining the concentration of nucleic acids, the absorption of UV light can also be used to assess their purity. In biological samples, contaminants such as proteins, phenols, or solvents might also absorb UV light at different wavelengths. By measuring the absorbance at both 260 nm and 280 nm (the wavelength where proteins absorb) and calculating the ratio A260/A280, the purity of the sample can be estimated. A ratio of ~1.8 indicates pure DNA, while a ratio of ~2.0 indicates pure RNA.

In addition to concentration and purity assessment, the absorption of UV light by nucleic acids allows for the study of their structure and function. Changes in the UV absorption can be observed in different states of the nucleic acids, such as single-stranded, double-stranded or triple-stranded conformations, or when they are involved in interactions with other molecules (e.g., proteins or ligands). These changes can be used to analyze folding, conformational changes, and binding events of nucleic acids. In conclusion, the absorption of ultraviolet light by DNA and RNA is important in the analysis of nucleic acids as it allows for determining the concentration, purity assessment, and studying the structure and function of these biological molecules.