The internal order of the interphase nucleus

Summary This paper has two parts. The first one is theoretical, whereas in the second, some experimenteal results are reported. Part 1: Theoretical Considerations. Comings' considerations on an ordered arrangement of chromatin in the interphase nucleus are used as a basis for further investigations and calculations in order to establish a preliminary model of the interphase nucleus. Information on the amount of DNA of a diploid human nucleus, on the degree of spiralization of chromatin threads found in electron microscopy, and measurements of salivary gland chromosomes was used to estimate the lengths of the entire interphase chromosomes. The number of fixing points-pores—was indirectly calculated proposing a model of an internal order of the chromatin threads. This number was found in concord with a direct calculation of the number of pores in the nuclear membrane based on results from electron microscopy. Part 2: Experimental Results and Discussion. In the second part of this study, an approach was made as to how to arrange chromosomes and chromosome segments in their proximity to each other. Results of cytogenetic studies of newborn babies and abortions, of cells from patients with Bloom's syndrome and Fanconi's anemia and normal cells treated with Mitomycin C and Trenimon, are thought to be informative under certain suppositions for the problem, which chromosome or chromosome parts are situated in proximity to each other. The symmetrical and equal interchanges seen, for example, in Bloom's syndrome are an indication of somatic pairing during the time of reunion. Therefore, the unequal interchanges in the same syndrome in which different chromosomes are involved should give evidence for proximity of nonhomologous chromosomes. Arguments for and against a temporal and spacial hypothesis for somatic pairing are discussed. The differing frequencies of chromosomes involved in Robertsonian translocations in man are informative for proximities of satellite regions at the nucleolus. Nucleolus and sex chromatin could be used as fixed points in a model of the interphase nucleus in which finally the absolute localization of the chromosomes will be discovered. The discussion points out promising methods for further investigations on the subject and mentions problems which could be attacked if the approach described here leads to a model of internal order in the interphase nucleus.This work was supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 35, Klinische Genetik.