THE NATIVE STRUCTURE OF DINOFLAGELLATE CHROMOSOMES AND THEIR STABILIZATION BY CA2+ AND MG2+ CATIONS

Abstract

Native chromosomes from the primitive dinoflagellate Prorocentrum micans were isolated without recourse to chemicals from purified nuclei frozen in liquid nitrogen. Thawing of the nuclear suspension resulted in the disruption of nuclear membrane, and the released chromosomes were incubated in various conditions or directly observed by EM after spreading. Observations of whole mounted chromosomes confirmed that they were organized in a double-helical bundle of chromosomal fibers. Investigations of these high-ordered chromosomal fibers at high magnification revealed helical compaction of nucleofilaments in a hierarchy of 6 organization levels (from the DNA to the double helical bundle forming the chromosome). This peculiar architecture was shown to be stabilized mainly by divalent cations; the behavior of native chromosomes incubated in a low concentration of either EDTA or EGTA [ethyleneglycol-bis(.beta.-aminoethylether)-N,N-tetraacetic acid] strongly suggested that the chromosome has 2 different cation sites, 1 for Ca2+ and 1 for Mg2+.