DNA SIZE AND CHROMATIN STRUCTURAL ORGANIZATION IN THE MACRONUCLEUS OFA CILIATE BURSARIA TRUNCATELLA

As shown using pulse electrophoresis and electron microscopy, the size of DNA molecules in Bursaria truncatella macronucleus is 50 to 360 kb p. In mature differentiated cells, macronuclear DNA is packed in 60- t o 200-nm electron-dense clumps; consequently, the volume of one clump is sufficient for compaction of a DNA molecule. The interacting clumps may form higher-order structures. During encystment, large chromatin aggregates are formed: first, the size of chromatin clumps increases, then they fuse into long thick threads. The latter correspond to those described for Homalozoon macronucleus and for chromonemes in the chro mosomes of higher eukaryotes. The size of DNA molecules does not chang e in cysts compared with active cells. There is probably a mechanism c ontrolling the change in the clump size and formation of the chromonem e-like threads. This seems very interesting for further studies, becau se micronuclear DNA from ciliates compared with the nuclear DNA from h igher eukaryotes is of smaller size and contains no centromeric sequen ces.