Postsynthetic Phase Research Articles

Cell cycle arrest induced by inhibitors of epigenetic modifications in maize (Zea mays) seedling leaves: characterization of the process and possible mechanisms involved.

Epigenetic modifications play crucial roles in the regulation of chromatin architecture and are involved in cell cycle progression, including mitosis and meiosis. To explore the relationship between epigenetic modifications and the cell cycle, we treated maize (Zea mays) seedlings with six different epigenetic modification-related inhibitors and identified the postsynthetic phase (G2 ) arrest via flow cytometry analysis. Total H4K5ac levels were significantly increased and the distribution of H3S10ph signalling was obviously changed in mitosis under various treatments. Further statistics of the cells in different periods of mitosis confirmed that the cell cycle was arrested at preprophase. Concentrations of hydrogen peroxide were relatively higher in the treated plants and the antioxidant thiourea could negate the influence of the inhibitors. Moreover, all of the treated plants displayed negative results in the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labelling (TUNEL) and γ-H2AX immunostaining assays after exposure for 3d. Additionally, the expression level of topoisomerase genes in the treated plants was relatively lower than that in the untreated plants. These results suggest that these inhibitors of epigenetic modifications could cause preprophase arrest via reactive oxygen species formation inhibiting the expression of DNA topoisomerase genes, accompanied by changes in the H4K5ac and H3S10ph histone modifications.

Chromosome Sensitivity to Bleomycin in Peripheral Blood Lymphocytes from Unexposed and Irradiated Persons

With the help of modifying G2-bleomycin sensitivity assay (treatment of human peripheral blood lymphocytes culture in late post-synthetic phase of mitotic cycle by bleomycin in concentrations 0,05 and 5,00 mcg/ml) the investigation of hidden chromosome instability in 9 unexposed donors as well as in 32 persons with different intensity of radiation exposure (patients recovered from acute radiation sickness - ARS and Chernobyl liquidators) had been fulfilled. The basic criterion of chromosomes’ sensitivity to bleomycin exposure the total frequency of chromosome aberrations, mainly acentric fragments had been considered. In all examined groups the individual levels of chromosome aberrations under identical mutagenic exposure varied in wide range and didn’t depend on their initial values in intact cultures. In control donors the mean-group frequencies of chromosome aberrations were 10.37+0.66 (3.0 - 32.0) and 15.54+0.78 (5,0 - 35,0) per 100 cells, additions to background rate of aberrations were 9,14 and 14,31 per 100 cells under bleomycin exposure in concentrations 0.05 and 5.00 mcg/ml, accordingly. Among control donors three hypersensitive persons had been identified with aberrations rate 35.0, 34.0 and 16.0 per 100 cells that can be considered as genetically caused phenomenon. Similar situation had been determined in 10 liquidators with low radiation exposure - the mean-group frequencies of chromosome aberrations were 11.26+0.63 (2.3 - 34.0) and 15.10+0.79 (3.0 - 34.5) per 100 cells, above-background level of aberrations were 9.14 and 14.31 per 100 cells under bleomycin exposure in concentrations 0.05 and 5.00 mcg/ml, correspondingly. In liquidators group three hypersensitive persons had been identified with aberrations rate 34.5, 20.0 and 19.3 per 100 cells. In 19 ARS patients increased mean-group frequency of aberrations induced by bleomycin in both concentrations had been revealed - 16.80±0.50 (8.7 - 38.2) and 28,04±0,63 (6.0 - 60.0) per 100 cells with addition to background aberrations level 13.47 and 24.71 per 100 cells, accordingly. 13 persons with increased chromosome instability (from 22.2 till 60.0 aberrations per 100 cells) had been found. The data received permit to assume that high radiation doses can modify the inherited human susceptibility to mutagen exposure.

Extra DNA synthesis in the dedifferentiating cells ofVicia faba roots

Cell dedifferentiation was induced inVicia faba root tissues by removing the whole root meristem (decapitation) and the behaviour of the nuclear DNA in the dedifferentiating cells was studied by means of cytophotometric and autoradiographic analyses. Cytophotometric determination after Feulgen-staining showed that: 1. the vast majority of nuclei in differentiated cells were in the DNA postsynthetic phase, but their Feulgen absorption was lower than that of DNA postsynthetic nuclei (G2, 4 C) in the meristem; 2. such a Feulgen absorption was detected in certain nuclei after root decapitation; 3. all the mitoses in the dedifferentiating tissues were diploid, fully matching the Feulgen absorption of mitoses in the meristem.

Proliferationskinetik regenerierender Urothelzellen in der Rattenharnblase nach Applikation von Cyclophosphamid

Regenerative cell proliferation of the urinary bladder urothelium has been investigated in 303 female Wistar rats after damage by single or multiple applications of cyclophosphamide. Between 2 h and 14 h after administering 100 mg/kg, the3H-TdR-index of the entire urothelium was clearly lesser than the control value. After 22 h the proliferative activity increased rapidly and after 30 h a peak value with a mean3H-TdR-index of 17.8% was obtained. Thereafter the labeling index fell to an average value of 4.1% at 2 days. After 3 days there was a renewed rise in the3H-TdRindex, which reached a second peak of 12.2% on the 7th day. After application of 200 mg/kg the3H-TdR-index increased in a similar manner at 22 h, following an initial depression of DNA synthesis. After 32 h the highest labeling index of 12.2% was obtained. At about 4 weeks both doses of cyclophosphamide yielded values corresponding with those in the control animals. The3H-TdR-index rose initially in the trigone region and also reached its highest value there. After about 1 week the maximum proliferative activity had shifted upwards into the dome of the bladder. The basal cells showed a considerably higher absolute labeling index than the suprabasal cells. After multiple intermittent applications a slightly increased3H-TdR-index was found with the increase in the total dose administered. Using the percentlabeled-mitoses method, average values were obtained of 24 h for the generation time (TC), 15 h for the DNA-synthesis phase (TS), 7 h for the postsynthetic phase together with the mitotic phase (TG2 + M) and 2 h for the presynthetic phase (TG1). Stimulation of the proliferative activity of rat bladder urothelium by cyclophosphamide provides itself as an experimental animal model for testing weak carcinogenic agents.

Etude de la teneur en DNA nucléaire dans le méristème apical des tiges du Celosia cristata (Amarantacées) en fonction de la photopériode

Two samples of plants of Celosia cristata, a quantitative short-day plant, were grown in an 8-h or a 16-h day. The shoot apices were collected at 11, 15, 18, and 28 days in order to carry out a cytophotometric study of the nuclear DNA in the corpus. This distribution of the DNA values was analyzed statistically using the χ2 test. At the beginning of development, for both 8-h day and 16-h day plants, the DNA content of the majority of the nuclei is at the 2C level. Then the percentage 2C–4C or 4C nuclei increases, slowly for the plants grown under a 16-h photoperiod and quickly for the others. At the 28th day, the Gaussian distribution of DNA values indicates that the state reached by the meristems is quite different. Between the 18th day and the 28th day of culture, ontogenesis of the shoot apex goes through a critical period. At the end of this period, shoot apices of the plants grown under a 16-h photoperiod are in the intermediate phase with a majority of nuclei in the synthetic phase (S) or in the postsynthetic phase (G2). Simultaneously, structural modifications of the axial zone give rise to a central zone characteristic of the fasciation of the shoot apex. In plants grown with an 8-h photoperiod, the shoot apex goes through the prefloral phase with more numerous nuclei in the presynthetic phase (G1) or in the synthetic one (S). For each sample the nuclei clustered in a particular phase of the cell cycle indicate that a certain synchronism of the cell cycles in the corpus takes place. These results are compared with those recently obtained by other authors working with different plants.

Mitosis in mammalian cells during exposure to anesthetics.

The effects of methoxflurane, trichloroethylene, chloroform, halothane and diethyl ether on the division of Chinese hamster fibroblasts in spinner culture have been studied. All agents caused dose-dependent inhibition of cell multiplication. Halothane increased the cell cycle time roughly in accordance with its effect on multiplication rate. There was no evidence that mitosis was greatly prolonged, and only small numbers of "c-metaphases" were seen. However, exposure to halothane resulted in a marked and rapid reduction in the prophase count, suggesting prolongation of G2 (the post-synthetic phase). Cine-photo-micrography showed frequent delay in division of cytoplasm at mitosis, and many binucleate cells were seen.

Localization in the cell cycle of the antimitotic action of the pyrrolizidine alkaloid, lasiocarpine and of its metabolite, dehydroheliotridine

The antimitotic action of the pyrrolizidine alkaloid lasiocarpine on rat liver parenchyma was investigated using as the experimental model the wave of mitosis produced in liver by a single dose of thioacetamide. A single low dose of lasiocarpine administered two weeks before the thioacetamide, almost completely inhibited the mitotic wave without inhibiting to the same extent the preceding wave of DNA synthesis. By the use of selective inhibitors and radioisotope labelling, the location of the mitotic block was found to be either in the latter half of the DNA synthetic phase, S, or early in G 2, the post-synthetic phase. The mitotic wave was similarly inhibited by pretreatment of the rats with a single injection of dehydroheliotridine, a pyrrolic metabolite of heliotridine-based pyrrolizidine alkaloids.

DURATION OF MITOTIC CYCLE AND REGULATION OF DNA REPLICATION INNICOTIANA PLUMBAGINIFOLIAAND A HYBRID DERIVATIVE OFN.TABACUMSHOWING CHROMOSOME INSTABILITY

The duration of the mitotic cycle with its component phases in Nicotiana plumbaginifolia (pbg, 2n = 20) and a hybrid derivative of N. tabacum (tbc, 2n = 48 and N. plumbaginifolia was determined from the root tips. A 30 minute pulse label of H3-thymidine was employed for the autoradiographic detection of the labeled prophases.The time intervals, for the total generation time (T), postsynthetic phase (G2), prophase, synthetic phase (S), and the combined presynthetic phase, metaphase, anaphase, telophase (G1 + M + A + T) in N. plumbaginifolia were estimated as 11.0, 3.4, 1.3, 5.7 and 0 6 hours, respectively. In the hybrid derivative the corresponding estimates were 9.0, 1.4, 0.8, 3.8 and 3.0 hours.The G1, S and G2periods of N. plumbaginifolia were found significantly different from those of the hybrid derivative, but the duration of the mitotic cycle did not show a significant difference. On the basis of the distribution of heterochromatin and its late replication in N. plumbaginifolia chromosomes, possible causes of instability of alien pbg chromosomes in N. tabacum nuclei are discussed.

Effect of Cycloheximide and Chloramphenicol on 6-Methylcoumarin Induced Chromosome Breakage inAllium Cepa

SUMMARYThe influence of the protein synthesis inhibitors cycloheximide (CHI) and chloramphenicol (CHL) on the chromosome breaking action of 6-methylcoumarin (6-MC) was investigated on Allium cepa root meristems.Both inhibitors, when applied as a post-treatment, strikingly increased the frequency of subchromatid exchanges, chromatid breaks and achromatic lesions induced by 6-MC. The possible implications of such findings in relation to the mechanism of induction of chromosome aberrations in G2 (DNA post-synthetic phase) and of chiasma formation in meiosis are discussed.

The effect of testosterone on cell renewal and mitotic cycles in sex accessory glands of castrated mice.

ABSTRACT The effect of testosterone propionate on cell renewal rate and mitotic cycle of castrated mice was investigated. 0.25 mg of the hormone was given thrice at 24 h intervals and 50 μc of 3H-thymidine was injected simultaneously with the last hormone injection. After this the mice were killed at 1 to 2 h intervals. Ventral prostates and seminal vesicles were weighed and the thymidine incorporation into their epithelia determined in radioautograms. A piece of proximal small intestine was used as reference tissue. The labelling index in the castrated control animals was 1.3 for the seminal vesicle; 68 h after testosterone treatment it was 44. The figures for the ventral prostate were 1.9 and 37, and for the small intestine 44 and 57. The mitotic index in the prostate of the experimental animals and that in the intestine of all the animals was 0.5; in the seminal vesicle it was 0.4. The mitotic index in the accessory glands of the control mice was very low. The specific activity of DNA per nucleus was higher in the hormone treated than in the control animals. The cell cycle (TC) in the prostate was shortened from about 900 h to 20 h after testosterone. The synthetic phase (TS) was shortened from 16 to 7 h, presynthetic (TG1) phase from 900 to 10 h, whereas the postsynthetic phase (TG2) remained unchanged. The corresponding figures for the seminal vesicle were: TG2 from 4.5 to 0.5 h, TS from 12 to 7 h, TG1 from 900 to 10 h and TC from 900 to 18 h. The changes in the intestinal epithelium were less marked: TC from 22 to 16 h, TS from 8.5 to 8 h, TG1 from 12 to 6 h, while TG2 remained unchanged.

A quantitative study of cell proliferation in the subependymal layer of the adult rat brain

The characteristics of the generation cycle of the subependymal layer of the adult rat brain have been studied by the autoradiographic method of Quastler and Sherman. A mean generation time of 18 hr, with a DNA synthesis period of 8 I 2 hr, and a postsynthetic phase of 1 I 2 hr, was found for the cells of the layer.