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The Submicroscopic Basophile Structure of Chromosomes in Ehrlich Ascites Tumor Cells¹

Section of Genetics and Cell Biology, Biological Sciences Group, University of Connecticut, Storrs, Connecticut 06268

The cytochemistry and morphology of interphase and condensed mitotic chromosomes are described in ultrathin-sectioned specimens prepared by freezing and drying, postfixation with protein-sensitive bifunctional reagents of low density, basophile staining with an electron-dense dye lake, and enzymatic digestions. In specimens not subjected to enzymes, interphase chromatinic threads were observed as paired bodies with a seemingly granular structure which may represent cross-sectioned fibrils. At the nucleolus-organizer region, the chromatin is extremely dense and consists of occasional, large, true granules. Metaphase chromosomes consist of tightly crowded, strongly basophilic fibers about 160 Å 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 chromatinic nature of stainable basophile structure was in each instance established by its resistance to RNase and digestibility with DNase. The residuum after DNase action on metaphase chromosomes consists mainly of 60–80 Å particles. These occur among tubular elements measuring about 250 Å in width. Trypsin was capable of digesting neither the chromatin nor the 60–80 Å 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.

¹ Work supported by Research Grants GM-08326 and CA-08732 from the NIH, USPHS. Contribution No. 136 from the Institute of Cellular Biology, University of Connecticut.

² Recipient of a Research Career Development Award (5-K3-GM-8492) from the NIH.

Received 6/ 6/66. Accepted 11/28/66.