WIDELY DISPERSED DNA WITHIN PLANT AND ANIMAL NUCLEOLI VISUALIZED BY 3-D FLUORESCENCE MICROSCOPY

Ribosomal RNA (rRNA) genes in eucaryotes hold out the possibility of a temporal and spatial understanding of structural events in genetic regulation, since they are present in very large copy numbers and are extensively transcribed within a highly organised and specialised organelle - the nucleolus. However, although the existence of the genes within active nucleoli has been beyond dispute for many years, their position and arrangement has been the subject of considerable recent controversy. We have made use of the very sensitive and photometrically accurate imaging given by a cooled solid state CCD camera, coupled with computer deconvolution, to image DNA directly within nucleoli by three dimensional (3-D) optical microscopy. Our results show that DNA is more widely distributed than immunogold techniques have indicated and that in addition to brightly staining structures, corresponding to fibrillar centres, there is also a large amount of diffuse DNA, consistent with DNA location in other nucleolar components; this dispersed DNA is likely to correspond to the sites of gene transcription.