The Distribution of DNA Within the Nucleoli of the Amphibian Oocyte as Demonstrated by Tritiated Actinomycin D Radioautography

Abstract

The distribution of DNA within the nucleoli of the newt oocyte has been studied using tritiated actinomycin D (3H-AD) radioautography. In the course of the study further evidence was found for the specificity of this technique. In many of the cytological preparations the specificity of binding was enhanced by extraction with ethanol after exposure to 3H-AD. The actinomycin D-DNA complex is somewhat unstable in cytological preparations. Incubation in buffers causes some dissociation of the complex; divalent cations accentuate the effect. The technique of 3H-AD radioautography was again shown to be more sensitive than Feulgen staining by the detection of 3H-AD binding in the Feulgen-negative forms of the nucleoli of the amphibian oocyte. The binding of 3H-AD to all of the nucleolar forms confirms the presence of DNA in these organelles. Results from this study and from others using [3H]uridine in the same system provide evidence that the nucleolar DNA is a detached loop-chromomere complex.