32P radioactive isotope would be the most useful to mark a nucleic acid.
What is 32P radioactive isotope?
Since DNA has a phosphate backbone, the radioactive isotope 32P is perfect for tracking DNA because it is not present in significant quantities in other macromolecules.
By integrating 32P into phage DNA and seeing that the radioactive phosphorous (as part of DNA) was transported within infected bacterial cells during the Hershey-Chase experiment, it was demonstrated that DNA is the genetic material of bacteriophages (while 35S, incorporated in the protein coat of the bacteriophage, remained in the liquid surrounding cells).
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