RNA complementary to bulk humanplacental DNA was synthesized in vitro both in the presence and absence of 3H-labeled ribonucleotides. The 'Hlabeled RNA was used directly for hybridization to the DNA of human metaphase chromosomes, whereas the unlabeled complementary RNA was labeled chemically with l26l before hybridization. A comparison of autoradiographs produced by either isotope revealed no qualitative differences in the chromosomal annealing sites of the same population of RNA molecules. Since "15I-labeled nucleic acids give similar, if not identical, results as do 'H-labeled nucleic acids in in situ hybridization experiments, their use should make possible the localization of genetic elements for which tritium labeling methods are either inadequate or not possible.
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