DNA will be fragmented during apoptosis and this method detects the hydroxylated 3' terminus of damaged DNA. It used deoxynucleotidyl transferase dUTP to add labelled dUTP to the end of the DNA fragments which can be visualized by fluorescence or colormetric methods.

More info available at: https://www.thermofisher.com/au/en/home/life-science/cell-analysis/cell-viability-and-regulation/apoptosis/tunel-assays.html