Ultrastructural studies on nucleic acids of nucleolar granular components in Novikoff hepatoma cells

The nucleolar ribonucleoprotein particles of Novikoff hepatoma cells were studied by means of electron microscopy at high magnifications after enzymatic digestion carried out in fixed cell suspensions. The ribonucleoprotein particles in nucleoli and in the cytoplasm contained thin filaments about 20 Å in width. These filaments were sensitive to ribonuclease and they were also similar to the filaments (molecules) of ribonucleic acid in ultrathin sections. Therefore, these dense filaments in ribonucleoprotein particles appear to be molecules of ribonucleic acid in ribonucleoprotein particles. The possibility exists that the nucleolus contains some polysomes since a few ribonucleoprotein particles in the nucleolus were connected by a filament 20 Å wide, as were also some of the ribosomes in the cytoplasm. Perinucleolar and intranucleolar chromatin fibers about 70–100 Å in width were composed of coiled filaments about 20–25 Å in width. Uncoiled chromatin filaments 20–25 Å in width were observed within the nucleolus. Along with the biochemical evidence for rapid synthetic reactions in the nucleolus, the evidence that the nucleolus contains ribonucleoprotein particles virtually indistinguishable from the cytoplasmic ribosomal ribonucleoprotein particles provides support for the concept that the latter are assembled in the nucleolus [6, 7].