125 I-labeled DNA-RNA hybrids in cytological preparations.

RNA complementary to bulk humanplacental DNA was synthesized in vitro both in the presence and absence of (3)H-labeled ribonucleotides. The (3)H-labeled RNA was used directly for hybridization to the DNA of human metaphase chromosomes, whereas the unlabeled complementary RNA was labeled chemically with (125)I 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 (125)I-labeled nucleic acids give similar, if not identical, results as do (3)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.