THE ANNUAL INVITATION LECTURE ULTRASTRUCTURE AND MOLECULAR ORGANIZATION OF GENETIC SYSTEMS

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Abstract
A comparative ultrastructural analysis of genetic systems in cellular organisms has led to the recognition of two major types: In the complex cells of animals and plants the primary genetic system takes the form of chromosomes, which are present in specific types and number (Karyo-type). In the non-dividing cell they form a nucleus contained in a nuclear envelope with characteristic ultrastructure (eukaryon of Dougherty, 1959). In the light microscope the chromosomes appear subdivided into two and possibly more strands (chromonemata) which undergo cyclic changes in the degree of helical coiling. The electron microscope reveals still further subdivision into elementary chromosome fibrils about 100A thick. This fibril consists of two subunits about 40A thick, corresponding to nucleohistone macromolecules linked end to end through a non-nistone protein, into long fibrils. In monera (bacteria and blue-green algae) we find instead of chromosomes or nuclei, chromatin bodies which are variable in shape, lack a membrane, and are divided by simple constriction (prokaryon of Dougherty, 1959). In the electron microscope these appear as areas of low density containing fibrils approximately 25A thick (nucleoplasm). These fibrils are essentially DNA macromolecules and are not combined with histones. The term genophore is suggested for the physical equivalent of a linkage group, restricting "chromosome" to the complex multi-stranded structure found in higher cells. Two kinds of "plasmagenes" or "plasmids" are discussed, namely Kappa particles of Paramecium and the chloroplast of Chlamydomonas. Both contain DNA and electron microscopy reveals the presence of atypical nucleoplasm with DNA fibrils 25A thick. The genetic system of these "plasmids" thus resembles that found in monera. The ultrastructure of these plasmids supports the idea that they originated as endosymbiotes from monera.
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