In the Fuelgon reaction, which reagents are used to demonstrate DNA?

• A. 1N HCl, Schiff's Reagent (basic Fuchsin, sodium metabisulfite, 1N HCl), and sulfurous acid
• B. Methyl orange and sulfuric acid
• C. Hematoxylin and eosin
• D. Periodic acid and Schiff's Reagent

Answer: (A)

DNA is demonstrated in the Feulgen reaction by first converting the deoxyribose in DNA into aldehyde groups through acid hydrolysis, and then staining those aldehydes with Schiff's reagent to produce a magenta color. The hydrolysis step uses 1N hydrochloric acid to generate the aldehyde groups. Schiff's reagent, which is prepared with basic fuchsin and a sulfite source, is what binds to those aldehydes; sulfurous acid (or sodium metabisulfite) in the reagent bleaches the dye until it meets an aldehyde, enabling a specific pink/magenta stain of DNA. So the essential combination is acid hydrolysis with 1N HCl and Schiff's reagent containing basic fuchsin and sulfite (sulfurous acid or equivalent). The other options do not provide this DNA-specific reaction: they either involve general tissue stains, pH indicators, or reactions that target carbohydrates rather than DNA.