endlabeling

Endlabeling is a molecular biology technique that attaches a detectable label to the terminus of a nucleic acid or protein, most commonly DNA or RNA. The label is bound to either the 5' or 3' end and can be a radioactive isotope, a fluorescent dye, biotin, or a hapten. End-labeled molecules are used for detection, quantification, and isolation in a range of assays.

In nucleic acids, two principal approaches are used. 5' end labeling typically employs T4 polynucleotide kinase to transfer a radioactive or fluorescent phosphate from ATP to the 5' hydroxyl, producing a labeled 5' end. Non-radioactive variants often use kinase with labeled ATP analogs or substitute labels on the oligonucleotide during synthesis. 3' end labeling is commonly achieved with terminal deoxynucleotidyl transferase, which can add a short tail of labeled nucleotides to the 3' terminus. Chemical methods, such as periodate oxidation of the terminal ribose followed by conjugation to a label, provide alternative end-labeling strategies.

In proteins, end-labeling can refer to attaching a label to a protein terminus, such as N-terminal or C-terminal labeling, typically achieved through specific reactive groups, enzymatic tags, or genetic fusion to affinity or fluorescent tags. This is distinct from internal labeling, which labels amino acids within the protein sequence.

Applications include detection on blots and in situ assays, generation of labeled primers or probes for PCR and sequencing library preparation, autoradiography, and immunoassays. Considerations include labeling efficiency, potential disruption of nucleic acid or protein function, stability of the label, and safety concerns with radioactive materials.