You're missing the list of reactions! Please provide the list of reactions you're referring to so I can help you identify the one used for radioactively labeling DNA.

For example, some common methods for radioactively labeling DNA include:

* Nick translation: Uses DNA polymerase to incorporate labeled nucleotides into a DNA strand.

* Random priming: Uses random oligonucleotide primers to initiate DNA synthesis with labeled nucleotides.

* End-labeling: Uses enzymes like T4 polynucleotide kinase to add labeled nucleotides to the 5' ends of DNA.

Once you provide the list, I can tell you which one is used for radioactive DNA labeling!