Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) method was developed to detect DNA fragmentation in situ.
In apoptotic cells, the chromosomal DNA is cleaved by endonucleases at linker DNA sites between nucleosomes. Thereafter, many DNA fragments appear as oligomers of approximately 180 bp in the nuclei.
In the TUNEL method, the 3'-OH ends of DNA fragments are nick-end labeled with FITC-dUTP (or dUTP-biotin and avidin-FITC); this process is mediated by terminal deoxynucleotidyl transferase (TdT).
TdT catalyzes the addition of deoxynucleotides at the 3'-OH terminus of oligodeoxyribonucleotides, single-stranded, and double-stranded DNA.