The structure of a mesokaryote chromosome

Abstract

Condensed and dispersed forms of the chromosomes of the dinoflagellate, Prorocentrum micans, deposited on grids by the microcentrifugation technique were studied by electron microscopy. In the normally condensed form, the chromosomes appear as banded rods surrounded by a peripheral cloud of partially dispersed fibers. Single fibers in these and in extensively dispersed preparations appear as smooth threads of uniform diameter (55–65 Å). The chromosome fibers are contrasted by positive-group-specific stains, indicating the presence of cationic moieties associated with the DNA. Occasionally Y-shaped chromosomes are seen; these may be replicating structures. These observations are in general agreement with studies of dinoflagellate chromosomes by other techniques, and provide support for the suggestion that these organisms possess a genome organization whose structure is typical of neither prokaryotes nor eukaryotes, and hence may be intermediate forms.