The presence of extrachromosomal nucleoli in amphibian oocytes has permitted isolation and electron microscopic observation of the genes coding for ribosomal RNA precursor molecules. Visualization of these genes is possible because many precursor molecules are simultaneously synthesized on each gene. Individual genes are separated by stretches of DNA that apparently are not transcribed at the time of synthesis of precursor rRNA in the extrachromosomal nucleoli.

The morphology of active structural and putative ribosomal RNA genes was observed by electron microscopy after lysis of fragile Escherichia coli cells. Conclusions drawn are: most of the chromosome is not genetically active at any one instant; translation is completely coupled with transcription; the 16S and 23S ribosomal RNA cistrons occur in tandem, in regions which are widely spaced on the chromosome.

Preparative techniques are detailed for isolating nuclear contents of amphibian oocytes and HeLa cells for electron microscopy, and the ultrastructure of presumptive ribosomal RNA (rRNA) genes and other active chromosomal loci is described. The rRNA genes from the two cell types present strikingly similar structural configurations, both being fully loaded with RNA polymerases and exhibiting gradients of short to long nascent ribonucleoprotein (RNP) fibrils. Other chromosomal loci in the two cell types are strikingly different; those on lampbrush chromosomes of amphibian oocytes show closely-spaced polymerases and extremely long RNP fibrils, whereas those on HeLa genomes have widely-spaced polymerases and relatively short attached RNP fibrils.Numerous studies using thin-section autoradiography of nuclei fixed in situ and of isolated nuclear fractions have demonstrated RNA synthesis on chroma¬ tin within both nucleolar and nonnucleolar compartments of nuclei (e. g. Faken

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