DNA and RNA may be easily distinguishable by:

Question

Accepted Answer

The correct option is 2: Measurement of their ultra violet spectra

Nucleic acids, which include DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid), are fundamental biological macromolecules composed of nucleotide units. These units contain nitrogenous bases—purines (Adenine and Guanine) and pyrimidines (Cytosine and Thymine/Uracil). Crucially, these bases possess aromatic rings and conjugated double-bond systems, which are chromophores that absorb ultraviolet (UV) light strongly, with a maximum absorption peak consistently found near 260 nanometers (nm). This inherent physical property allows for easy distinction and routine quantification through spectrophotometry. Although both absorb at 260 nm, the subtle but reliable differences in their molecular architecture—such as the presence of the 2'-hydroxyl group on the ribose sugar in RNA, the different base composition (Uracil in RNA vs. Thymine in DNA), and the different secondary and tertiary structures—cause slight variations in their specific absorption maxima and the characteristic absorption ratios (like A260/A280 and A260/A230), making their spectra easily distinguishable in a laboratory. This technique is non-destructive, rapid, and highly sensitive.

The incorrect option is 1: Extraction of DNA by hot trichloro-acetic acid

The use of hot trichloro-acetic acid (TCA) is a component of classical cell fractionation and extraction protocols, often employed to achieve the selective or differential hydrolysis of RNA while precipitating the more stable DNA. Therefore, this method is primarily a harsh chemical procedure used for quantitative *extraction* or *differential hydrolysis* of nucleic acids from a cell homogenate, not a straightforward method for simple, rapid *distinction* between a purified DNA and RNA sample. The process is lengthy, destructive, and does not offer the instantaneous spectral signature of the molecules that spectrophotometry does.

The incorrect option is 3: Estimation of rate of trypsin digestion

Trypsin is classified as a serine protease, and its highly specific function is to catalyze the hydrolysis of peptide bonds in proteins, breaking them down into smaller peptides. DNA and RNA are nucleic acids, which are polymers of nucleotides linked by phosphodiester bonds, not peptide bonds. Consequently, trypsin has no enzymatic activity toward either DNA or RNA molecules. Estimating the rate of trypsin digestion would therefore be completely irrelevant for distinguishing between the two nucleic acids, as any observed digestion rate would only be attributable to contaminating protein, not the nucleic acids themselves.

The incorrect option is 4: Colour reaction for the specific sugar

While DNA and RNA are structurally differentiated by their specific pentose sugars—DNA having 2'-deoxyribose and RNA having ribose—and specific colorimetric chemical tests exist for each sugar (e.g., Dische reaction for deoxyribose), these are chemical assays. They are typically destructive, require heating, and are more cumbersome and time-consuming than spectrophotometry. The question asks for the method by which they may be *easily* distinguishable, and in a modern context, the measurement of the UV spectrum offers a faster, simpler, and more efficient analytical method than performing a chemical colour reaction.