The Fine Structure of Mesokaryote Genome: Multiple Nucleolar-Organizing Chromosomes inProrocentrum Micans

Abstract

Using TEM technique for studying the fine structure of dinoflagellate chromosomes in Prorocentrum micans, we have obtained data supporting the model of stacked series of nested-transversal DNA arcs, held together by a presumable ribonucleoprotein matrix and representing a higher order of organization of a single, or few, very long DNA fibrils folded in a spiral manner around a virtual axis. Both interchromosomal connections and the attachment of chromosomes to the nuclear envelope suggest a superchromosomal organization of the mesokaryote genome, well evidenced when the chromosomes are in a despiralized, relaxed state. We have also obtained proof for the existence of several nucleolar-organizing chromosomes in the same mesokaryote nucleus, all being involved in the organization of the same nucleolus. At the level of nucleolus-organizer region of such NO-chromosomes, DNA fibrils are extensively unfolded, a situation commonly encountered in typical eukaryotes.