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1.
The response of polycythaemic mice to a standard dose of erythropoietin has been measured at various, time intervals after single or repeated injections of hydroxyurea. The results exclude S phase of the cell cycle as the period responsive to erythropoietin. They suggest the existence of feedback mechanisms within the cell cycle, operating at the G1-S boundary and within the G1 phase. Hydroxyurea given to polycythaemic mice at various time intervals after erythropoietin induced characteristic changes in the response. These changes can be explained if both gradual transit of differentiated cells into the DNA synthesis (S phase) and changes in amount of the erythropoietin sensitive cells caused by the feedback mechanisms operating in the cell cycle are considered. 相似文献
2.
HeLa cells were synchronized by a double thymidine block and pulse labeled at different stages of the cell cycle with 3H-choline. The specific activity of phospholipids extracted from the cell, the nucleus and the nuclear membrane showed a progressive increase from S to G1; the incorporation of choline into phospholipids of asynchronous cells showed a specific activity intermediate between the values of S and G1 cells. Similar results were obtained when 32phosphorus was used as a precursor instead of choline. Thin layer chromatographic analysis of phospholipids extracted from cells in S and from cells in G1 failed to show any difference in the distribution of radioactivity among the various phospholipid classes. Choline uptake by HeLa cells in different phases of the cell cycle did not show significant variations. However, during the synchronization process, shortly after the addition of excess thymidine, an increased uptake of choline by cells and an increased incorporation of choline into phospholipids were found. The results indicate that some of the changes occurring in phospholipids synthesis may not be cell cycle dependent, but may be the effect of the synchronizing process. 相似文献
3.
THE SYNTHESIS OF ACIDIC CHROMOSOMAL PROTEINS DURING THE CELL CYCLE OF HELA S-3 CELLS : I. The Accelerated Accumulation of Acidic Residual Nuclear Protein before the Initiation of DNA Replication 总被引:40,自引:4,他引:36
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The synthesis and accumulation of acidic proteins in the tightly bound residual nuclear fraction goes on throughout the cell cycle of continuously dividing populations of HeLa S-3 cells; however, during late G1 there is an increased rate of synthesis and accumulation of these proteins which precedes the onset of DNA synthesis. Unlike that of the histones, whose synthesis is tightly coupled to DNA replication, the synthesis of acidic residual nuclear proteins is insensitive to inhibitors of DNA synthesis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of acidic residual nuclear proteins shows different profiles during the G1, S, and G2 phases of the cell cycle. These results suggest that, in contrast to histones whose synthesis appears to be highly regulated, the acidic residual proteins may have a regulatory function in the control of cell proliferation in continuously dividing mammalian cells. 相似文献
4.
Growth deceleration of an Ehrlich ascites tumor with increasing mass is associated with a prolongation of the cell cycle and a decline in the growth fraction. These effects are reversed upon transfer of cells from an older tumor into a new host. Studies were made to locate the stages at which a cell cycle could be suspended or resumed. Transplantation caused a prompt rise in both mitotic and flash H3TdR labeling indices. When all the cells in cycle including mitoses were prelabeled with H3TdR in older tumors, the fraction of labeled mitoses did not decline for a considerable period after transplantation into new hosts. This suggests that the early rise in mitoses is not due to a flow of resting (Go) cells from a G2 store (G2-Go transition). It appears rather to be a reflection of a lag of the mitotic process relative to other stages during the initial readjustment of the cycle. A prompt rise in flash H3TdR indices in the transplants suggested cell entry into S from either a suspended GI (G1-Go transition) or a suspended S (S-Go transition). These possibilities were examined by relating micro-spectrophotometric estimates of DNA to the cell cycle stage as revealed by H3TdR autoradiography. Since Go cells had DNA values corresponding to GI, it was concluded that decycling or recycling could occur only after mitosis and before DNA synthesis. 相似文献
5.
The effects of irradiation (300, 500 and 1500 rads) on mitosis and DNA synthesis in the estrogen primed vaginal epithelium have been studied. Dose-effect relations and the time sequence of effects on the two processes were investigated. The technique of tritiated thymidine labeling of DNA with autoradiography was used, in conjunction with the mitotic count, to study alterations in the generative cycle. Prior to irradiation, ovariectomized female rats were treated daily with diethylstilbestrol for a period of 2 weeks to create a steady state in the vaginal cell population. It was observed that:
- 1 Within 1 hr following ionizing radiation, mitotic figures disappear from the population and reappear at a time that is dose dependent. Those cells that have begun mitosis at the time of irradiation were able to complete that phase but no cells which were in G2 were able to begin mitosis. Therefore, a G2 block occurs within 1 hr post-irradiation.
- 2 Radiation reduces the rate of DNA synthesis thus prolonging the S phase. There is no evidence of a radiation-induced G1 to S block in this system.
- 1 Cells in G1 at the time of irradiation are relatively insensitive and continue to progress through the generative cycle at a rate primarily determined by the level of estrogen stimulation.
- 2 Radiation may interfere with the estrogen priming mechanism in this hormonedependent system thereby reducing the effective level of estrogen stimulation. This is seen in the behavior of cells which were in G1 at the time of irradiation. The extent of the blockage of estrogen increases with radiation dose and after 1500 rads, estrogen stimulation is essentially at castrate level.
6.
EFFECT OF METHOTREXATE ON THE CELL CYCLE OF L1210 LEUKEMIA 总被引:1,自引:0,他引:1
The influence of methotrexate (MTX) on the proliferative activity of cells in different phases of cell cycle has been studied. MTX (5 mg/kg) was injected i.p. 3 days after the inoculation of 5 × 106 leukemia cells into F1 (DBA × C57 BL) mice. It was shown that MTX causes degeneration of cells, being in G1- as well as in S-phase at the time of drug injection. Incorporation of 3H-TdR was suppressed for a period ranging from 2 to 12 hr after MTX administration, which is demonstrated by the decrease in the number of grains per cell. The number of cells labeled after 3H-TdR injection was also sharply decreased during this period. For a period of 3 until 15 hr after MTX administration the mitotic index decreased significantly as a result of inhibition of DNA synthesis. The blocking of the G1-S transition was evident during 4 hr after MTX. Thereafter the G1-S transition proceeds at a rate which is practically equal to that for nontreated controls. MTX did not inhibit transition to mitosis of cells being in G2-phase and in a very late S-phase at the time of drug injection. The sensitivity of G1-cells to the cytocidal effect of MTX shows that for L1210 leukemia cells MTX can be classified as a cycle-specific drug killing both G1 and S-cells rather than S-phase specific agent with self-limitation. 相似文献
7.
A COMPARISON OF THE HETEROGENEOUS NUCLEAR RNA OF HELA CELLS IN DIFFERENT PERIODS OF THE CELL GROWTH CYCLE 总被引:3,自引:2,他引:1
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HeLa cells synthesize heterogeneous nuclear RNA (HnRNA) in the G1, S, and G2 portions of the cell cycle. HnRNA prepared from these various periods was compared by RNA-DNA hybridization experiments. The results indicated that some of the HnRNA molecules were equivalent at all times in the cell cycle, but limitations in the sensitivity of the hydridization reactions, as well as in the spectrum of hybridizing molecules, restrict the conclusions that can be drawn from these comparisons. 相似文献
8.
The toxic and inhibitory properties of hydroxyurea (HU) have been studied in asynchronous and synchronized populations of mouse L-cells. Hydroxyurea is a potent growth inhibitor and appears to be specifically lethal for cells which are in the early part of S phase at the time the compound is introduced. Cells in late S phase, G2, mitosis and G1 appear to progress normally around the cycle in the presence of the compound until they reach the G1/S boundary. There are indications that at least some G1 cells are able to enter the S phase even in the presence of the drug; however their flow into S is much slower than that of control cells and therefore they are killed at a slow rate. Upon prolonged exposure to the drug a second phase of more rapid killing is observed, beginning at about the time division would occur in uninhibited cells. Hydroxyurea exhibits a rapid and marked inhibition on DNA synthesis but its effect on RNA synthesis is much less pronounced and may be a consequence of the inhibition of DNA synthesis. The effects of hydroxyurea on cell viability and DNA synthesis can be partially prevented by the addition of deoxyribonucleosides which in sufficient concentration appear to compete temporarily with the drug. The fact that the protection is only temporary would appear to rule out the hypothesis that the primary mode of action of the drug is the inhibition of the reduction which converts ribonucleotides to deoxyribonucleotides. The data presented in this communication taken together with observations of other workers would appear to suggest that the effect of the drug may be directly on the DNA molecule. 相似文献
9.
The action of tubulosine on the mitotic cycle was studied using continuous labelling with tritiated thymidine. This alkaloid provokes a lengthening of the G1 and S phases and a blocking of G2 is totally reversible when the treatment is followed by recovery in normal medium. At a dose of tubulosine which induces a reversible mitostasis in the shortest possible time the lengthening of the phases of the cell cycle was estimated by three different techniques: labelled mitoses for the determination of G2; labelling intensity for the determination of S; binucleate cells for the determination of T, and an original technique using labelling index of binucleate cells for the determination of G1. The limits of the technique of labelled mitosis together with the interest of the technique aiming at the direct determination of G1 in the case of a perturbed cycle are then discussed. 相似文献
10.
SEPARATION OF NUCLEI REPRESENTING DIFFERENT PHASES OF THE GROWTH CYCLE FROM UNSYNCHRONIZED MAMMALIAN CELL CULTURES 总被引:5,自引:3,他引:2
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Nuclei have been isolated from unsynchronized cultures of Chinese hamster fibroblasts after varying intervals of growth following the incorporation of thymidine -3H for 20 min. These nuclei were fractionated by unit gravity sedimentation in a stabilizing density gradient of sucrose, and fractions were analyzed for the concentration of nuclei, DNA, and radioactivity. A more rapidly sedimenting population of nuclei in the G2 phase of the cell cycle was separated from a group of nuclei in the G1 phase, and nuclei in progressive stages of DNA synthesis (S phase) were distributed between these two regions. The fractionation of intact cells by sedimentation according to their position in the cell cycle was found to be less satisfactory than the corresponding separation of nuclei. This probably results from the continuous accumulation of mass within individual cells throughout the entire cell cycle, whereas most of the mass of a nucleus is replicated during a relatively narrow interval of the total cell cycle. 相似文献
11.
Near-ultraviolet and visible radiations increased the duration of the mitotic cycle in excised pea root meristems primarily by lengthening the duration of the pre-DNA synthetic period (G1). All radiations tested shortened the duration of the post-DNA synthetic period (G2). The most pronounced effects were exhibited by green radiation, which lengthened the duration of the cell cycle, G1, DNA synthesis (S), and mitosis (M), and shortened the duration of G2. Progression of cells arrested by starvation in G1 and G2 into DNA synthesis and mitosis was also affected by light treatments. Green radiation appeared to arrest a group of cells in DNA synthesis as well as in G1 and G2. Meristems receiving green and near-ultraviolet radiations exhibited the most rapid progression of G1 cells through S and G2. 相似文献
12.
CELL MITOTIC CYCLE SYNTHESIS OF NIL HAMSTER GLYCOLIPIDS INCLUDING THE FORSSMAN ANTIGEN 总被引:2,自引:1,他引:1
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The synthesis of phospholipids and glycolipids during the cell mitotic cycle of an established hamster line, NIL, has been studied. Cells were synchronized with excess thymidine and mitotically harvested by shaking. Cells were radioactively labeled for 4 h with palmitate, glucosamine, or galactose. Lipids were analyzed by thin-layer chromatography. As cells progressed through the mitotic cycle, incorporation into phospholipids increased but the fraction represented by each remained constant. Similarly, ceramide monohexoside, dihexoside, and hematoside were labeled equally in all phases. Ceramide trihexoside and tetrahexoside were labeled only during G1 and S. Ceramide pentahexoside (the Forssman antigen) shows density-dependent synthesis, accumulation, and reactivity. Ceramide pentahexoside was labeled during all phases of the mitotic cycle but the rate of incorporation decreased in S and G2. The total amount of lipid assayed immunologically in cell extracts gradually increased. Exposure of the Forssman antigen in untreated or trypsin-treated cells was studied using binding of chemically labeled antiForssman antiserum. The amount of antigen detected in trypsinized cells increased during G1 and early S but then remained constant. Mitotic cells exposed all detectable antigen. As cells progressed through the mitotic cycle, a large fraction of the Forssman antigen became cryptic. 相似文献
13.
CHANGES IN THE DNA SYNTHESIS PATTERN OF PARAMECIUM WITH INCREASED CLONAL AGE AND INTERFISSION TIME 总被引:1,自引:1,他引:0
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The clonal age in paramecia refers to the total number of vegetative divisions a clone has undergone since its origin at autogamy (self-fertilization). As clonal age increases, the interfission time usually increases. The DNA synthesis pattern of cells of different ages was compared by autoradiographic analysis of the DNA synthesis of synchronized cells at various time intervals during the cell cycle (from one division to the next). The study showed that the G1 period (the lag in DNA synthesis post division) was constant, irrespective of interfission time or clonal age; but the duration of the DNA synthesis period increased with increased interfission time or clonal age. Therefore, we have shown for the first time that the G1 period is fixed, and the S period is increased in a eukaryotic unicellular organism as a function of interfission time and clonal age. 相似文献
14.
15.
Colin K. Franker 《Journal of phycology》1971,7(2):165-169
Division cysts of zooxanthellae harbored by Anthopleura elegantissima were isolated from random host-associated populations by their preferential attachment to plastic surfaces. By selecting cysts at similar stages in the cell cycle, synchronous division, excystation, and daughter cell reencystment were obtained in a medium enriched with amino acids. The interval of DNA synthesis in a single growth cycle was determined by pulse-labeling with radioactive thymidine. Analysis of the sequence of morphologic changes within such cycles suggested that the initial selection procedure isolates G2 cells. The transit time from first generation G2 division cysts to subsequent reentry into G2 was about 70 hr. 相似文献
16.
W. Nagl 《American journal of botany》1972,59(4):346-351
The treatment of root tips of Allium carinatum, Allium cepa, and Allium flavum with colchicine, abscisic acid, kinetin, and indole-3-acetic acid, applied in appropriate concentrations, combinations, and durations, makes possible the selective blockade of the cell cycle in G1, G2, any mitotic stage, and between karyokinesis and cytokinesis. Moreover, treatment with abscisic acid followed by a recovery period stimulates polyploid nuclei in mature tissues to divide. Colchicine, kinetin, and indole-3-acetic acid applied together cause end-to-end association of metaphase chromosomes. These results together with earlier findings suggest that any step of the cell cycle is independently controlled both by specific balance of the growth regulators and by specific synthesis of the nucleic acids. 相似文献
17.
The influence of cisplatin, an anticancer agent, on DNA synthesis and cell cycle progression of a cisplatin-resistant cell line was investigated. Cell cycle analysis using flow cytometry showed that cytotoxic concentrations of cisplatin caused a transient inhibition of parental HeLa cells at S phase, followed by accumulation at G2 phase. In contrast, the resistant cells progressed through the cell cycle without being affected by the same treatment. However, cell cycle distributions were the same in the resistant and the parental cells at IC50, the drug concentration inhibiting cell growth by 50%. Studies using a [3H]thymidine incorporation technique also demonstrated a transient inhibition of DNA synthesis in HeLa cells by cisplatin; such inhibition was greatly reduced in the resistant cells. These data argue for the hypothesis that the inhibition of DNA synthesis is important in determining cisplatin-induced cytotoxicity. In addition, the accumulation of cells at G0/G1 by serum starvation was not effective in the resistant cells compared to the parental cells, suggesting that the control of cell cycle exiting is also altered in the resistant cells. Taken together, these results support the notion that alterations in cell cycle control, in particular G2 arrest, are important in determining the sensitivity or resistance of mammalian cells to cisplatin and may have a role in clinical protocols. 相似文献
18.
Archibald W. Hopkins 《Journal of phycology》1977,13(4):321-323
A Feulgen microspectrophotometric analysis of successive generations of Fritschiella tuberosa Iyengar revealed an asexual repeating of one generation rather than an obligate alternation of isomorphic generations as previously described. F. tuberosa regularly reproduces asexually by the release of one quadriflagellate zoospore produced/cell in all parts of the upright and prostrate portions of the thallus. In the vegetative thalli, nuclei in the G1 (pre-synthesis), synthesis, and G2 (post-synthesis) phases of the mitotic cycle are all well-represented. 相似文献
19.
THE MECHANISM OF 5-AMINOURACIL-INDUCED SYNCHRONY OF CELL DIVISION IN VI CIA FABA ROOT MERISTEMS
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Cessation of mitosis was brought about in Vicia faba roots incubated for 24 hours in the thymine analogue, 5-aminouracil. Recovery of mitotic activity began 8 hours after removal from 5-aminouracil and reached a peak at 15 hours. If colchicine was added 4 hours before the peak of mitoses, up to 80 per cent of all cells accumulated in mitotic division stages. By use of single and double labeling techniques, it was shown that synchrony of cell divisions resulted from depression in the rate of DNA synthesis by 5-aminouracil, which brought about an accumulation of cells in the S phase of the cell cycle. Treatment with 5-aminouracil may have also caused a delay in the rate of exit of cells from the G2 period. It appeared to have no effect on the duration of the G1 period. When roots were removed from 5-aminouracil, DNA synthesis resumed in all cells in the S phase. Although thymidine antagonized the effects of 5-aminouracil, an exogenous supply of it was not necessary for the resumption of DNA synthesis, as shown by incorporation studies with tritiated deoxycytidine. 相似文献
20.
Millie Hughes-Fulford 《Journal of cellular biochemistry》1994,54(3):265-272
Our previous studies have implied that prostaglandins inhibit cell growth independent of cAMP. Recent reports, however, have suggested that prostaglandin arrest of the cell cycle may be mediated through protein kinase A. In this report, in order to eliminate the role of c-AMP in prostaglandin mediated cell cycle arrest, we use the-49 lymphoma variant (cyc?) cells that lack adenylate cyclase activity. We demonstrate that dimethyl prostaglandin A1 (dmPGA1) inhibits DNA synthesis and cell growth in cyc? cells. DNA synthesis is inhibited 42% by dmPGA1 (50 μM) despite the fact that this cell line lacks cellular components needed for cAMP generation. The ability to decrease DNA synthesis depends upon the specific prostaglandin structure with the most effective form possessing the α,β unsaturated ketone ring. Dimethyl PGA1 is most effective in inhibiting DNA synthesis in cyc? cells, with prostaglandins PGE1 and PGB1 being less potent inhibitors of DNA synthesis. DmPGE2 caused a significant stimulation of DNA synthesis. S-49 cyc- variant cells exposed to (30–50 μm) dmPGA1, arrested in the G1 phase of the cell cycle within 24 h. This growth arrest was reversed when the prostaglandin was removed from the cultured cells; growth resumed within hours showing that this treatment is not toxic. The S-49 cyc? cells were chosen not only for their lack of adenylate cyclase activity, but also because their cell cycle has been extensively studied and time requirements for G1, S, G2, and M phases are known. Within hours after prostaglandin removal the cells resume active DNA synthesis, and cell number doubles within 15 h suggesting rapid entry into S-phase DNA synthesis from the G1 cell cycle block. The S-49 cyc? cells are known to have a G1/S boundary through M phase transition time of 14.8 h, making the location of the prostaglandin cell cycle arrest at or very near the G1/S interface. The oncogenes, c-fos and c-myc which are normally expressed during G1 in proliferating cells have a 2–3 fold enhanced expression in prostaglandin G1 arrested cells. These data using the S-49 variants demonstrate that dmPGA1 inhibits DNA synthesis and arrests the cell cycle independent of cAMP-mediated effects. The prostaglandin arrested cells maintain the gene expression of a G1 synchronous cell which suggests a unique molecular mechanism for prostaglandin action in arresting cell growth. These properties indicate that this compound may be an effective tool to study molecular mechanisms of regulation of the cell cycle. 相似文献