Nirmal K. Das

Bio: Nirmal K. Das is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: RNA & Mitosis. The author has an hindex of 12, co-authored 17 publications receiving 564 citations. Previous affiliations of Nirmal K. Das include University of Wisconsin-Madison & University of Kentucky.

Synthetic activities during spermatogenesis in the locust.

Abstract: Isolated testes of the locust Schistocerca gregaria were immersed in solutions of tritiated thymidine, cytidine, uridine, or arginine for short periods to study nucleic acid and protein synthesis during spermatogenesis. DNA synthesis in this tissue is completed prior to initiation of meiosis. Protein synthesis continues throughout the whole meiotic cycle as well as during spermatid development. Meiotic cells, except those in metaphase through early telophase, and early spermatids are also actively synthesizing RNA. The heteropycnotic X-chromosome does not produce RNA at any stage of spermatogenesis. The rates of protein and particularly RNA synthesis decrease as chromosome condensation progresses. Depression of RNA synthesis, however, is not always accompanied by cytologically detectable condensation of chromatin, since very little or no RNA is synthesized in spermatids in which chromatin condensation has barely begun.

The relation of DNA synthesis and mitosis in tobacco pith tissue cultured in vitro.

Abstract: DNA synthesis and mitosis were initiated in cultured tobacco pith tissue by means of IAA and kinetin. DNA classes were determined by microspectrophotometric measurements (Feulgen); autoradiographs (tritiated thymidine) served to ascertain whether or not nuclei had undergone DNA synthesis during culture. All mitoses in “new” cells (resulting from divisions in culture) were diploid and had been preceded by DNA synthesis in culture. Whereas many of the “old” cells (which had not previously divided in culture) found in diploid or polyploid mitosis had undergone DNA synthesis during culture, others had not. Such non-radioactive mitoses still occurred after 16 days. In view of this, a 4 C nucleus in differentiated tissue should be considered as potentially both diploid and tetraploid, for it appears impossible to predict whether it would, upon restoration of conditions conducive to DNA synthesis and mitosis, enter a diploid mitosis or, after undergoing DNA synthesis, a tetraploid one. A high nuclear DNA content seems to have a much more inhibiting effect on the onset of DNA doubling than on that of mitosis. Somatic polyploidization is understood as the result of two DNA doublings between which mitosis was omitted, or aborted, or in effect undone by a failure of cytokinesis leading to fusion during a later mitosis.

Chromosomal and nucleolar RNA synthesis in root tips during mitosis.

Abstract: Comparative rates of RNA synthesis in chromatin and nucleolar fractions during mitosis in root-tip cells of Allium and Nigella were studied by pulse-labeling of cells with tritiated cytidine. Although the rate of RNA synthesis decreases in the condensing chromosomes during prophase, it remains normal in the nucleolar fraction as long as nucleoli are maintained. RNA synthesis stops in mitotic cells lacking distinct nucleoli. In the late telophase or very early interphase cells, RNA synthesis resumes at a faster rate in the pronucleolar bodies than in the chromatin.

Synthetic capacities of chromosome fragments correlated with their ability to maintain nucleolar material

Abstract: Onion (Allium cepa) and bean (Vicia faba) root tip cells containing many micronuclei, derived from x-ray-induced chromosome fragments, were exposed to H3-thymidine and H3-cytidine to determine the ability of such fragments to undergo DNA and RNA synthesis. Only a few micronuclei in onion and many in bean roots synthesize nucleic acid simultaneously with their main nuclei. A few micronuclei labeled with H3-thymidine undergo mitotic chromosome condensation along with the main nuclei, while the unlabeled ones never do so. The onset of nucleic acid synthesis as well as mitosis in micronuclei appears to be under generalized cellular control. Although all chromosomes and chromosome fragments at telophase give a positive reaction for a silver stainable nucleolar fraction, in the subsequent interphase only some micronuclei, derived from such chromosome fragments, are found to maintain nucleoli; others lose them with time. Those micronuclei which maintain nucleoli, perhaps due to the presence of specific chromosomal regions, are also active in DNA and RNA synthesis. These results are compatible with the concept that nucleoli and associated chromosome regions play an important role in the primary biosynthetic processes of the cell.

Differentiation of the mammary gland and susceptibility to carcinogenesis

Abstract: It has been demonstrated that in humans certain factors such as early menarche, late pregnancy, and nulliparity are associated with a higher risk of developing breast cancer, while early pregnancy acts as a protective factor. Induction of mammary cancer in rats by administration of the chemical carcinogen 7, 12-dimethylbenz(a)anthracene reveals that the same factors influencing human breast cancer risk also affect the susceptibility of the rat mammary gland to the chemical carcinogen. Nulliparous rats and rats undergoing pregnancy interruption are more susceptible to developing carcinomas. This fact has been attributed to the incomplete differentiation of the gland at the time of carcinogen administration. Parous rats are resistant to the carcinogenic effect of DMBA, which is explained by the complete development of the gland attained during pregnancy and lactation. This development is manifested by the differentiation of terminal end buds into secretory units, which have a smaller proliferative compartment; the epithelial cells of these secretory units have a longer cell cycle, less avidity for binding DMBA, and possess a more efficient DNA excision repair capacity.

Maintenance and induction of morphological differentiation in dissociated mammary epithelium on floating collagen membranes

Abstract: Dissociated normal mammary epithelial cells from prelactating mice were plated on different substrates in various medium-serum-hormone combinations to find conditions that would permit maintenance of morphological differentiation. Cells cultured on floating collagen membranes in medium containing insulin, hydrocortisone and prolactin maintain differentiation through 1 month in culture. The surface cells form a continuous epithelial pavement. Some epithelial cells below the surface layer rearrange themselves to form alveolus-like structures. Cells at both sites display surface polarization; microvilli and tight junctions are present at their medium-facing or luminal surface and a basal lamina separates the epithelial components from the gel and stromal cells. Occasinal myoepithelial cells, characterized by myofilaments and plasmalemmal vesicles, are identified at the basal surface of the secretory epithelium. In contrast, cells cultured on plastic, glass or collagen gels attached to Petri dishes form a confluent epithelial sheet showing surface polarization, but lose secretory and myoepithelial specializations. If these dedifferentiated cells are subsequently maintained on floating collagen membranes, they redifferentiate. There is little DNA synthesis in cells on collagen gels, in contrast to Petri-dish controls. Protein synthesis in cells on floating collagen membranes increases over T0 values and remains constant through 7 days in culture whereas it decreases on attached gels; however, if the gels are freed to float, protein synthesis increases sharply and parallels that seen on floating membranes.

Cell polyploidy: its relation to tissue growth and function.

Abstract: Publisher Summary This chapter discusses the cell polyploidy and its relation to tissue growth and function. Cytophotometry established a fundamental similarity between polyploidy and polyteny, their common feature being multiple doubling of DNA in the cell. The occurrence and properties of polyploid cell populations is discussed. Polyploid nuclei have been found in protozoa and in the cells of many metazoa, both animals and plants. The common property of polyploid cell populations is retention of ability to reproduce DNA by differentiating and often fully differentiated cells. The stem cell phenomenon, which is clearly defined in many diploid cell populations, is in polyploid ones deprived of its main function being the only source of proliferation. The mode of polyploidy is briefly explained. Different types of mitotic polyploidization proper, complete abolition of the mitotic mechanism, and other cases of increases in the DNA content of cells are considered. The mechanism of supplanting mitotic syntheses by tissue-specific ones as the cause of reduction in the stages of mitosis may be common for different cell populations. But polyploidy in different tissues may have a different meaning associated with the specific function of the tissue. Polyploidization, as a rule, accompanies cell differentiation.