One method is using a purified DNA polymerase enzyme that can label all the nucleotides in a DNA molecule, resulting in highly radioactive DNA probes. Another method is using nonradioactive DNA molecules that are produced by incorporating specific chemical markers into DNA using DNA polymerase. Other chemical labels, like biotin, can also be attached to nucleotides and detected in a similar manner. A third method is using polynucleotide kinase, which selectively labels only the 5′ ends of DNA strands. This method allows for the production of DNA molecules with a single 5′-end-labeled strand, which can be obtained by subsequent restriction nuclease cleavage.