[The localization of DNA in the nucleoli of mammalian cells]

The localization of DNA within nucleoli of mammalian cells was studied in human diploid fibroblasts (strain MRC-5), stained with chromomycin A3--the fluorochrome specific for G-C DNA base pairs. To change the rate of rDNA transcription, actinomycin D (0.08 mkg/ml) was added to log phase cells for 30-90 min. The stained cells were analysed with a highly sensitive video camera and deblurring algorithm (Jordan, Rawlins, 1990). The pattern of staining was compared with the ultrastructural peculiarities of the cells fixed in the same conditions. As the result, it is inferred that the intranucleolar DNA (rDNA) is consistent with the fibrillar centres as well as with the dense fibrillar component. The localization of rDNA may alter due to the transcriptional activity of ribosomal genes. The data obtained support the idea on the transcription of ribosomal genes inside the dense fibrillar component of the nucleolus.