Structure of the organellar nuclei in Euglena revealed by immunoelectron microscopy

The localization of DNA in the condensed interphase chromosomes and of DNA in the nucleoids of chloroplasts and mitochondria in cells of Euglena was studied by immunoelectron microscopy. Small colloidal gold(5-6nm in diameter)was conjugated with anti-mouse IgM antibodies. Deposits of gold particles on the interphase chromosomes that coincided with the localization of DNA followed threads that corresponded to the chromatin fibers. The threads were50-80nm in diameter. The localization of gold deposits on chromosomes that had been sectioned in various directions suggested that the chromatin fibers coiled around the surface of chromosomes, with a wide central axial region of the chromosomes remaining free of DNA. The structure of the nucleoids in chloroplasts and in mitochondria was very different from those previously observed by conventional electron microscopy. Nucleoids in chloroplasts appeared to be composed of twisted threads50-70nm in diameter. Mostnucleoids in mitochondria were spherical or ovoid, 70-130nm in diameter. Nucleoids both in chloroplasts and in mitochondria contained cores with which DNA threads were in tight contact.