THE PARTICULATE ORGANIZATION OF THE CHROMOSOME

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Abstract
Salivary glands of Drosophila melanogaster and testes of the grasshopper (Melanoplus femur rubrum femur rubrum) were frozen by solid CO2, then extracted in the cold (1) in distilled water, (2) in .05 [image] sodium citrate, (3) in .05 [image] sodium citrate for 3 hours, followed by distilled water for 3-7 hrs. The tissues were fixed in Carnoy''s soln. and stained by the Feulgen method. The grasshopper tissues were sectioned, and the salivary glands were also squashed in 50% acetic acid and observed under phase-contrast. The chromosomes were not soluble in distilled water alone or in .05 [image] citrate alone, but were completely dissolved by the combination of citrate and distilled water. The deoxyribonucleoprotein from the dispersed chromosomes was deposited in the cytoplasm and on the nuclear membrane. In another test Drosophila salivary glands were frozen with solid CO2 and extracted in the cold for 6 hrs. at pH 7 in (1) distilled water, (2) .001 [image] Versene, (3) .01 [image] Versene and (4) .1 [image] Versene. The chromsomes were not dissolved by distilled water or .1 [image] Versene, but were completely dispersed in .001 [image] Versene and partly dispersed by .01 [image] Versene. The conditions needed for dispersal of chromosomes are twofold, an agent which can bind Ca, Mg, or both, and remove them from the system, and a medium of low ionic strength. Based on this evidence the chromosome is assumed to be composed of complex macromolecules with a DNA and protein composition comparable to that of the whole chromosome. These particles are held together by bridges of divalent cations (Ca, Mg, or both), and by interactions producing "insolubility" at moderate ionic strengths such as exist in a cell. This hypothesis predicts that intergenic phenomena (chromosome breakage and rearrangement, crossing-over, etc.) will be sensitive to the ionic environment of the chromosome, whereas intragenic phenomena ("point" effects) will not, and that the effects of ionizing radiations in producing chromosome breakage will be sensitive to the ionic environment of the chromosomes.

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