C Moretti’s scientific contributions

The localization, structure and activity of nuclear organizers were studied in quail and mouse oocytes during prophase I of meiosis using electron microscopy and in situ hybridization with tritium labelled 18S and 28S RNA. The newly synthesized nucleolus consists of a fibrillar centre surrounded by a layer of electron-opaque fibrils. DNP fibres with a diameter of about 50 Å extend from the nucleolar organizer to the fibrillar centre and penetrate into the latter. At the onset of nucleolar synthesis, the position of the fibrillar centre coincides with that of the ribosomal cistrons revealed by in situ hydridization. Following brief incorporation of tritiated uridine, labelling grains are localized over the electron-opaque fibrillar layer adjacent to the fibrillar centre. These observations indicate that rDNA is present in both the fibrillar centre and the dense fibrillar layer which are always in association, and that transcription of the ribosomal cistrons essentially occurs in the dense fibrillar layer.