Abstract
The nuclei of dinokaryotes contain numerous, quasi-permanently condensed, cylindrical chromosomes which, by electron microscopy of thin sections, present characteristic banded and archshaped patterns. In this work, the ultrastructure of the chromosomes of the primitive dinoflagellate Prorocentrum micans is studied in situ , in specimens prepared by various conventional fixation and embedding methods and by more recently developed procedures, such as cryosubstitution or cryosectioning of vitrified specimens. These new procedures give a better ultrastructural preservation than conventional ones; the texture of the chromosome sections appears thinner and more regularly distributed. Comparison of these results with those previously reported in the literature provides a characterization of the different levels of possible artifacts resulting from preparation and/or observation and allows a geometric analysis of chromosomal ultrastructure. It confirms the plywood representation of Y. Bouligand, M.-O. Soyer, and S. Puiseux-Dao (1968, Chromosoma 24 , 251–287) and supports the hypothesis that the linear double-stranded DNA is packed into a twisted nematic (cholesteric) liquid crystal.
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