The submicroscopic basophile structure of chromosomes in Ehrlich ascites tumor cells.

The cytochemistry and morphology of interphase and con densed mitotic chromosomes are described in ultrathin-seetioned specimens prepared by freezing and drying, postfixation with protein-sensitive bifunctional reagents of low density, basophile staining with an electron-dense dye lake, and enzy matic digestions. In specimens not subjected to enzymes, interphase chromatinic threads were observed as paired bodies with a seemingly granular structure which may represent cross-sec tioned fibrils. At the nucleolus-organizer region, the chromatin is extremely dense and consists of occasional, large, true gran ules. Metaphase chromosomes consist of tightly crowded, strongly basophilic fibers about 160 A wide, with no evidence of fibrillar subunits; the chromosomal mass also comprises an extremely fine, longitudinally oriented substructure of obscure morphologic features. Sister chromatids have been observed in cross-section at one of the earliest stages of a split. The chroma tinic nature of stainable basophile structure was in each instance established by its resistance to RNase and digestibility with DNase. The residuum after DXase action on metaphase chromo somes consists mainly of 60-80 A particles. These occur among tubular elements measuring about 250 A in width. Trypsin was capable of digesting neither the chromatin nor the 60-80 A particles but the tubules were not visualized after enzymatic extraction. The trypsin-resistant particles correspond to those observed in DNase-digested chromosomes and probably consist, for the most part, of non-histone protein together with smaller amounts of basophile substance than are present in the chromatin and in ribosomes.