[Study of DNA-containing structures in the nucleus and cytoplasm of the Crithidia flagellates using the Miller...

Weakly condensed interphase chromosomes made of chromatin, and the tightly packed kinetoplast DNA (kpDNA) of a single mitochondrion of Crithidia (Kinetoplastidea, Trypanosomatida) flagellates were studied on the Miller-type spread preparations by electron microscopy. Chromatin of organisms lysed at low ionic strength conditions for 5-10 min unfolds up to 10-nm nucleosomal filaments and 20-nm chromatin fibers. The initial indications of kpDNA decompaction become visible after a 10-13 min dispersion of lysed flagellates. However, at least a 15 min long procedure is required for well-defined identification of intrakinetoplast structures. In this case, the kinetoplast looks like a heterogeneous network disposed close to the kinetosome of a single flagellum. Cells with diameters of 162.5 nm and contour wall length of 510 nm dominated within the network. With the prolongation of the dispersion time up to 20 min both these parameters increased up to 218 and 686 nm, respectively. Further prolongation of the treatment up to 60 min results in wall disruption in many cells. Within these cells, some isolated circular kpDNA molecules appear with the contour length of 588-792 nm. The circles of this size correspond to individual minicircles of the Crithidia kpDNA. Partly unfolded maxicircles of the kpDNA can be found only at early stages of dispersion (10 min). Special features of compaction of DNA-containing structures in both the nucleus and cytoplasm of Crithidia are discussed.