Ultrastructure analysis of germinal nucleus and its differentiation to the vegetative nucleus in Paramecium bursaria.

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In Paramecium bursaria, micronuclei are composed of dense chromosomal chromatin, microlamella and cytoskeletal components, such as microtubules and intermediate filaments. Chromosome consists of 50 nm chromatin filaments, which are intertwisted to make 200 nm chromosomes even at interphase. At least a part of the chromosomes are covered by the microlamella. During cell division, 200 nm chromosomes are moved to the pole together with each other. On the other hand, in the conjugating cells, typical telocentric chromosomes of 300 nm in diameter and up to 5 mum in length are observed. These 300 nm chromosomes are less dense than the 200 nm one observed in dividing cells. Micronuclear chromatin are loosened at the log-phase of growth, but the 50 nm fiber are still electron dense, suggesting the micronuclear chromatin are basically condensed as the 50 nm heterochromatic filaments. In contrast to the micronucleus, the macronuclear chromatin are not organized into chromosomes even at cell division, but construct chromatin bodies of 150 nm in diameter. Length of the chromatin body changes during growth stage. In the log-phase, they are short, but in the stationary phase, they long and branched. Macro-and micronuclei are differentiated from the synkaryon of the conjugating cells. Recognizable macronuclear anlage shows degenerated microlamellae and decondensed chromosomal chromatin. During development, anlage is filled with 20 nm filaments, then they aggregate to construct primary chromatin bodies. The manner of packing the macronuclear DNA into chromatin bodies are analyzed by the chromatin spreading method using isolated macronuclei. Spread chromatin showed that the 11 nm nucleosomes are coiled to form 20-30 nm filaments. These filaments further coiled to form 50-60 nm filaments, which are folded to build up the 150 nm chromatin bodies. Because of difficulty of mass-isolation, micronuclear chromatin can not yet be analyzed.