Daniel Mazia

Nucleus-cytoplasm relationships in the action of ultraviolet radiation on amoeba proteus☆

Abstract 1. 1. The effects of ultraviolet irradiation on the division rate, capacity to divide, and survival of whole amoebae and nucleate and enucleate half cells have been observed. The overall effects are summarized in Table I. 2. 2. The first effect of irradiation of whole amoebae and nucleated half amoebae is a delay in the divisions immediately following. This is completely reversible; later progeny of the irradiated amoebae have a normal division rate. 3. 3. Amputation of half of the cytoplasm greatly increases the radiation sensitivity as measured by delayed division or by the dose required for permanent inhibition of division (sterilization dose). 4. 4. Amoebae that have received the sterilization dose may survive for 20–30 days but not longer. 5. 5. The survival time of enucleate fragments is very much reduced by small (200–500 ergs/mm2) doses of ultraviolet radiation. 6. 6. It is concluded that the overall radiation effect may have both nuclear and cytoplasmic components.

FIBER PROTEIN STRUCTURE IN CHROMOSOMES AND RELATED INVESTIGATIONS ON PROTEIN FIBERS

The numerous problems of chromosome function fall into two major groups: (a) those concerning the chemical composition of the chromosome and directed toward an ultimate comprehension of the nature of genic material and (b) those concerning the molecular architecture of the chromosomes as expressed in their visible and mechanical properties, their movements and reproduction in mitosis and meiosis, and the phenomena of breakage and recombination. This paper is concerned with the latter question at its most elementary level, the question of the intermolecular forces responsible for the organization of proteins and nucleoproteins into the tangible, elastic, “insoluble” body observed under the microscope, and subject to manipulation.’ Direct means are not available for a study of this question, nor, because the classes of compounds found in chromosomes are generally studied in solutions, do we have even a clear conception of the forces involved in the realization of structure at this level. The action of proteolytic enzymes on dipteran salivary gland chromosomes has been studied from this point of view? Once it was shown that the nucleic acids, as such, did not seem to be essential to the maintenance of the form and material continuity of salivary gland chromosomes, i t was possible, applying knowledge of the peptide specificity of certain proteases; to consider the protein constitution of the chromosomes in relation to their structure. The basic observation is that of Caspersson4 that trypsin effects complete dissolution of the chromosomes, implying a continuous structure of proteins containing basic amino acids. The action of crystalline pepsin, shown in FIGURES 1 and 2, presents a different picture. Represented in FIGURE 1 is the action of crystalline pepsin (5 mg. per ml. at pH 1.5 for 1.5 minutes) on chromosomes in the whole freshly dissected gland of Drosophila melanogaster. The control, FIGURE 2, was exposed to HC1 of pH 1.5 alone for the same time. The effect of enzyme is to cause a drastic shrinkage of the chromosomes to less than half of their original volume. The loss of material is largely in the interband regions, these virtually disappearing as the bands pull together. Some of the digested material is also contained in the bands, which become narrower. The general impression is that the enzyme has not affected the staining desoxyribonucleoprotein of the chromosomes, but has removed another component which is normally evident in the interband regions, a protein which contains IittIe or no nucleic acid, but which occupies the larger part of the chromosome volume. It is to be noted that the removal of this large volume of material does not cause any breaks

Radioactive Copper and the Mechanism of Oligodynamic Action

INASMUCH as the stimulation of organisms by extremely low concentrations of heavy metal ions (oligodynamic action) is a well-known phenomenon, it seemed desirable to use the artificially radioactive isotope 29Cu64 (Cu*) to enable one readily to determine the fate of the radioactive copper added to cells in concentrations of the order of 10−10 moles per litre.