What mechanism is utilized to add labeled nucleotides at the 3' end of DNA?

The addition of labeled nucleotides at the 3' end of DNA typically involves the use of T4 DNA polymerase. This enzyme is a DNA-dependent DNA polymerase that synthesizes new DNA strands by extending the 3' ends of a DNA template. T4 DNA polymerase can incorporate both natural and labeled nucleotides, enabling it to be used in various applications, such as creating radiolabeled or fluorescently labeled DNA for research or diagnostic purposes.

The other options are not directly responsible for the incorporation of labeled nucleotides at the 3' end in the same manner. T4 polynucleotide kinase is used primarily to add a phosphate group to the 5' end of nucleotides or to dephosphorylate them, while hexamer primers are short sequences designed to anneal to the template DNA, facilitating replication, but they do not themselves introduce labels. RNA polymerase, on the other hand, synthesizes RNA from a DNA template and is not involved in adding nucleotides to DNA directly in the context described. Therefore, the mechanism employed by T4 DNA polymerase aligns perfectly with the requirement for adding labeled nucleotides to the 3' end of DNA.