Which labeling involves a hapten and requires secondary detection?
Labeling methodologies are classified as direct or indirect based on visualization requirements. Direct methods link a reporter that is immediately detectable, such as radioisotope or fluorophore covalently attached to the probe. Indirect methods incorporate a hapten, a small immunogenic molecule like biotin or digoxigenin, which itself produces no signal. After hybridization, a secondary detection reagent such as enzyme-conjugated streptavidin or anti-digoxigenin antibody binds the hapten and generates colorimetric, chemiluminescent, or fluorescent output. This two-step approach allows signal amplification and flexibility in reporter choice, enhancing sensitivity for low-copy targets.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.