Cell Generation Cycle Research Articles

RNA synthesis in synchronously growing populations of HeLa S3 cells. I. Rate of total RNA synthesis and its relationship to DNA synthesis

AbstractThe rate of RNA synthesis in synchronously growing HeLa S3 cells was determined as a function of position in the cell generation cycle. Measurements throughout the cycle of both the rate of incorporation of radioactively‐labeled uridine and of the total amount of RNA indicate that (1) the rate of RNA synthesis is constant (or increases only slightly) during G1, approximately doubles during the first half of S, and then remains constant during the remainder of S and G2, and (2) cells attain the average G1 rate of RNA synthesis very early in G1, and maintain the average G2 rate until mitosis.If the initiation of DNA synthesis is blocked, the acceleration of RNA synthesis is markedly reduced or eliminated. Further experiments in which DNA synthesis was inhibited at different times in S, or to varying degrees from the beginning of S, suggest that the extent to which RNA synthesis is accelerated depends on the amount of DNA duplicated. These data also indicate that duplication of the first half, and in particular the first few per cent, of the DNA complement results in a disproportionate acceleration of RNA synthesis.The possibility that fluctuations in the sizes of precursor pools may lead to misinterpretation of labeled‐uridine incorporation data was examined. Experiments indicate that in this system pool fluctuations do not cause invalid measures of RNA synthesis.It is concluded that RNA synthesis occurs throughout interphase, but undergoes a two‐fold increase in rate which is dependent on the duplication of DNA.

Effects of metabolic inhibitors on Paramecium aurelia during the cell generation cycle

The ability of a number of metabolic inhibitors (namely, p- dl-fluorophenylalanine, puromycin, chloramphenicol, 5-fluoro-2-deoxyuridine, actinomycin D, triparanol citrate, and 2,4-dinitrophenol) to interfere with cell division has been studied in the species Paramecium aurelia. First, the lowest concentrations sufficient to prevent further division in newly-divided paramecia continuously exposed to the compound were found. Then the effects of exposure to these concentrations for 30 and/or 60 min were tested throughout the interval between two successive divisions, and assessed in terms of prolongation of the cell generation cycle. In all cases the most characteristic feature was an abrupt change from long to short prolongations, which occurred with small increases in the age of the cells, about an hour prior to completion of division. In a discussion on the possible mechanism underlying this response, comparison has been drawn with the parallel behaviour of Tetrahymena pyriformis.

The proliferation of pulp cells in rat incisors

Twenty immature August rats were injected with tritiated thymidine and killed at intervals between 1 and 48 hr. Autoradiographs of saggital sections of the maxillary and mandibular incisors were prepared. The distribution of mitoses and labelled nuclei at 1 hr enabled the extent of the proliferative area of the pulp to be defined. This was strictly basal in position and within it gradients of proliferative activity were observed. Labelling indices of the labial and lingual aspects of the basal pulp were not found to be significantly different; there were, however, significant differences between values determined for the corresponding areas in maxillary and mandibular incisors. The components of the cell generation cycle were established for the mandibular pulp ( T G 2 minimum, 1.3 hr; T G 2 maximum, 1.7 hr; T M , 1.5 hr; T S , 8.2 hr; T G 1 , 10.9 hr; T C , 22.1 hr) and for the maxillary pulp ( T G 2 minimum, 1.3 hr; T G 2 maximum, 1.9 hr; T M , 1.7 hr; T S , 8.0 hr; T G 1 , 12.2 hr; T C , 23.5 hr). Small regional differences between the labial aspect ( T S , 7.8 hr) and lingual aspect ( T S , 8.5 hr) of the maxillary pulp were noted; these were not found in the mandibular pulp. A correlation between high cell population density and high cell proliferative activity was found to exist in the basal pulp. The presence of a cell-dense band of fibroblasts of low proliferative activity between the extremities of the odontogenic epithelia was noted. Migration of pulp cells from the proliferative area in a coronal direction was observed. Brief observations were made on the proliferative activity, migration and maturation of the ameloblasts and odontoblasts.

Effects of dl- p-fluorophenylalanine on Paramecium aurelia during the cell generation cycle

Effects of dl- p-fluorophenylalanine on Paramecium aurelia during the cell generation cycle

DNA synthesis and the mitotic cycle in frog kidney cells cultivated in vitro

Frog kidney cells in tissue culture exhibit a lag period of about 48 hr before the onset of DNA synthesis. During this time the number of cells incorporating nucleoside into rapidly labeled RNA decreases for 10–12 hr, then increases until 48 hr. Coincident with the start of DNA synthesis there is a sharp drop in the synthesis of rapidly labeled RNA. The generation cycle of frog kidney cells was determined: G1 + telophase = 16 hr, S = 22.3 hr, G2 + prophase = 7.7 hr, T = 46 hr.

Quantitative analysis of cell proliferation and differentiation in the cortex of the postnatal mouse cerebellum.

The generation cycle of germinative cells (external matrix cells) in the external granular layer of the cerebellar cortex of the 10-to 11-day-old mouse was studied by radioautography following repeated injections of H(3)-thymidine. The generation time is 19 hr, presynthetic time 8.5 hr, DNA-synthetic time 8 hr, postsynthetic time 2 hr, and mitotic time 0.5 hr. These proliferating cells occupy the outer half of the external granular layer and make up the external matrix layer. Neuroblasts are differentiated from the external matrix cell, migrate out from the layer and accumulate in the inner half of the external granular layer to form the external mantle layer. The transit time of the neuroblasts in the external mantle layer is 28 hr. Thereafter, they migrate farther into the molecular layer and the internal granular layer. By means of long-term cumulative labeling, the rate of daily production of neuroblasts from the external matrix cell is studied in quantitative terms. It becomes clear that the entire population of the inner granule neurons arises postnatally in the external granular layer between 1 and 18 days of age and that 95% of them is produced between postnatal days 4 and 15. Finally, the fate of the cells in the external granular layer at its terminal stage was studied by marking the cells with H(3)-thymidine during 15-16 days of life and following their subsequent migration and developmental changes up to 21 days of life. Comparison of radioautographs taken before and after the migration disclosed that the external matrix cells give rise to a small number of neuroglia cells. This finding revealed their multipotential nature.

X-ray Sensitivity of HeLa S3 Cells in the G2 Phase: Comparison of Two Methods of Synchronization

The sensitivity of HeLa S3 cells to 220 kv X-rays was measured in terms of cell survival (colony development) during the G2 phase of the cell generation cycle, employing two procedures designed to free G2 cultures from contaminating cells from other phases of the cycle. Treatment of synchronous cultures (obtained initially by mitotic selection) with high specific activity tritiated thymidine (HSA-(3)HTdR) selectively eliminated S phase cells, while addition of vinblastine permitted removal of cells as they entered mitosis. It was found that HeLa S3 cells become increasingly sensitive as they progress through G2. The pattern of sensitivity fluctuations observed in synchronous HeLa S3 populations selected by the foregoing method was compared with that found in synchronous cultures prepared by the HSA-(3)HTdR method of Whitmore. The latter method had been used previously with mouse L cells, which were found to undergo a different pattern of sensitivity fluctuations. The two methods yield similar results for HeLa cells in the S and G2 phases of the cycle. It may be concluded, therefore, that the discrepancies between HeLa and mouse L cells do not arise from methodological factors, but represent fundamental differences between the cell types.

X-Ray Sensitivity during the Cell Generation Cycle of Cultured Chinese Hamster Cells

X-Ray Sensitivity during the Cell Generation Cycle of Cultured Chinese Hamster Cells

Effect of Continuous Gamma Irradiation of the Generation Cycle of the Duodenal Crypt Cells of the Mouse and Rat

In a previous experiment on the effect of daily protracted Co60 y-irradiation on the generation cycle of the duodenal crypt cells of the mouse (1), the disturbance of the cycle after 1 day of exposure was found to be similar to that produced by a single brief exposure (2). There is a prolongation of G2 + M, and the shape of the curve for percentage of labeled mitotic figures versus time suggests that movement of cells through other phases of the cycle is retarded. On subsequent days, however, the rapidly dividing crypt cells appear to compensate for the radiation damage. Mitotic arrest is no longer observed, and the crypt cells undergo repeated divisions at virtually the normal rate, maintaining an intact epithelium for 3 weeks in spite of continued irradiation to accumulated doses in excess of 3000 R within 15 days (1). Similar results were found at 12 R/day (3). In the following experiments, the generation cycle of the duodenal crypt cells was studied in both rats and mice exposed to 50 rads/day at various intervals after the start of irradiation. At this exposure level, mice can accumulate mean lethal doses of 6000 to 7000 rads, and rats can accumulate doses exceeding 10,000 rads (4, 5). Thus, it is possible with this exposure condition to study the effects of large accumulated doses on the generation cycle and to compare the cycles of two species that differ markedly in their resistance to chronic exposure.

The cell generation cycle of the eleven-day mouse embryo.

The incorporation of tritiated thymidine into the DNA of erythroblasts, primitive ependymal cells, and mesenchymal cells of 11-day mouse embryos was studied by radioautography at different times between 25 minutes and 18 hours after injection intraperitoneally. There was no labeling of mitotic figures until 1 hour after injection. Following this, mitotic figures were labeled for about 5.5 hours in primitive ependymal cells and mesenchymal cells, and for a longer period in erythroblasts. The percentage of the labeled primitive ependymal cells at various times after injection indicate a periodic migration into and out of the mitotic zone. The cell generation cycle of primitive ependymal cells and mesenchymal cells is similar to some kinds of adult cells. The cycle of the erythroblasts is more like that of the cells of aging mice.

Changes induced in the first post-irradiation generation cycle of human cells studied by double labeling

Changes induced in the first post-irradiation generation cycle of human cells studied by double labeling

INHIBITION BY A CARCINOGENIC HYDROCARBON OF INCORPORATION OF TRITIATED CYTIDINE INTO MOUSE EPIDERMAL CELLS.

EARLY effects of carcinogenic hydrocarbons on the cell cycle and on deoxyribonucleic acid (DNA) synthesis have been noted by several authors. McCarter and Quastler1,2 found that 7,12-dimethylbenz[α]anthracene (DMBA) decreases the rate of incorporation of 3H-thymidine into DNA and prolongs the DNA synthetic phase of the generation cycle of mouse epidermal cells. Evensen3 found that 3-methylcholanthrene alters the proportion of epidermal cells labelled with 3H-thymidine in the hairless mouse. Jensen et al.4 reported a reduction in uptake of 3H-thymidine in various tissues of the mouse following the administration of DMBA, including some tissues in which tumours do not usually arise after treatment with this agent. Elgjo5 reported that 3-methylcholanthrene changes the mitotic phase of the cell cycle over an eleven-week period. We now report an early effect of DMBA on the synthesis of ribonucleic acid (RNA) in mouse skin.

Aging and the generation cycle of intestinal epithelial cells in the mouse.

Aging and the generation cycle of intestinal epithelial cells in the mouse.