Practice question

Diaminobenzidine, abbreviated DAB, is classical chromogenic substrate for horseradish peroxidase used in immunoblotting and immunohistochemistry. In presence of hydrogen peroxide, HRP catalyzes oxidative polymerization of DAB into insoluble phenazine polymer that deposits as reddish-brown precipitate precisely at site of antibody-antigen complex on membrane or tissue section. Precipitate is stable, insoluble in organic solvents, and archival, facilitating permanent record. Tetramethylbenzidine yields transient blue product turning yellow after acid stop, while BCIP/NBT with alkaline phosphatase yields purple-black deposit, distinguishing substrate-color relationships fundamental to assay interpretation.