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  • Title: [DNA-containing polygonal structures detected in somatic nuclei of Bursaria ovata during preparation to cryptobiosis].
    Author: Sergeeva GI, Samoshkin AA.
    Journal: Tsitologiia; 2002; 44(10):1015-28. PubMed ID: 12498081.
    Abstract:
    Supramolecular chromatin organization of the somatic nucleus (macronucleus--Ma) was studied in a free-living unicellular eukaryotic organism, the ciliate Bursaria ovata Beers 1952, at two late successive stages of its encystment (in the state of preparation to cryptobiosis). A modified Miller's method (Sergejeva et al., 1987) and the same technique in combination with high resolution DNA autoradiography were used. In chromatin spread preparations of Ma, not labeled by 3H-thymidine, numerous electron dense structures (rounded, stick-like and polygonal) were revealed, along with rarely occurring typical supramolecular chromatin structures, such as nucleosomic and not nucleosomic threads, nucleomeres, chromomeres, rosette-like looping chromatin, and electron dense chromonemes (Fig. 1). For DNA visualizing in the revealed polygonal structures, the vegetative cells (trophonts) of B. ovata were inoculated into the culture medium, containing 3H-thymidine and food (ciliates Paramecium caudatum). Here, the ciliates passed through 3-4 successive cell division cycles, thus progressively accumulating the radioactive DNA precursor in Ma. After washing the ciliates in 3H-thymidine-free culture medium, the process of their encystment was induced, and Ma were isolated by hand from the ciliates being at two late successive stages of encystment. Isolated Ma were dispersed in the low ionic solutions, as described elsewhere (Sergejeva et al., 1987). The carbon shadowed electron grids, that contained spread Ma preparations, were individually coated with photographic emulsion, according to the loop interference method (Angelier et al., 1976a; Bouteille, 1976). After a 6 month exposure at 4 degrees C, thymidine incorporation was revealed in fibril crowds, rosette-like structures (Fig. 2), and crystal-like plates of different size and morphology (Fig. 3). In all our experiments, non-specific localization of radioactive DNA precursor was not observed. The above data confirm undoubtedly our earlier assumption (Sergejeva et al., 1987; Sergejeva, Bobyleva, 1988) that the Ma chromatin of Bursaria may undergo crystallization during encystment, i.e. in the state of preparation to cryptobiosis. The present data enable us first to suggest that the looping rosette-like chromatin may be transformed into crystal-like structures ("exotic liquid crystal structures") by means of a peculiar loop packing within the limits of an individual resette (Fig. 2-4), these structural transformations taking place without any visible loop destruction. In this paper, we first describe new morphological types of polygonal plates, differing from those earlier reported elsewhere for the Ma of Bursaria (Sergejeva, Bobylova, 1988), and also from the plates earlier described in studies on liquid crystals both in vivo and in vitro (see: Gianonni et al., 1969; Lerman, 1974; Livolant, 1991: Leforestier et al., 1993, 1997, 1999). The technical approaches used in the present work enabled us to obtain, for the first time, a direct evidence of the presence of DNA in the crystallized structures of somatic nuclei of ciliates during their preparation to cryptobiosis, the DNA-containing polygonal structures being highly morphologically diverse. Further studies into the reasons of this diversity are needed.
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