The chromosomes are functionally indissociable from the entire cell. Their activities in metabolism and in cell morphogenesis take place in the working nucleus of interphase. In the transition to interphase the nuclear sap and the nuclear membrane are formed. The latter contains protein and lipids; electron microscopy is beginning to tell us about its structure. The membranes of large nuclei from amphibian oocytes and from Amoeba proteus have two layers: a porous layer which presumably serves for mechanical support and a continuous layer which may regulate permeability8. Now the nucleoli form. Surely these events involve the intense exchange of substance between nucleus and cytoplasm. Differences in the appearance of the nuclei of various tissues and from various functional states of the same cell indicate nucleocytoplasmic interactions. The changing behavior of the heterochromatin has already been mentioned (p. 17). The ratio of protein to DNA in “resting nuclei” in various tissues varies in the few determinations that have been made between 8:1 and 20: 1 according to the cell type, while the ratio is about 1.5:1 in isolated chromosomes. The ratio of RNA to DNA also varies in various types of interphase nuclei.
KeywordsMetaphase Chromosome Heterochromatic Region Special Segment Euchromatic Region Meiotic Metaphase
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