Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “G2 Phase”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,063 records · Page 59Linked to original sources
PubMed1997

DNA gyrase involvement in chloroplast-nucleoid division in Cyanidioschyzon merolae.

Cyanidioschyzon merolae is a unicellular alga with one chloroplast and one mitochondrion per cell. Each of these organelles has a simple nucleoid located in the central area. Prior to mitosis and cytokinesis, both the division of organelle nucleoids and organellokinesis occur simultaneously. We investigated the effects of nalidixic acid, an inhibitor of DNA gyrase, on the replication and division of organelle nucleoids. Using microfluorometry, we monitored the synthesis of organelle DNA (organelle S phase) in synchronous cultures. In both the chloroplast and the mitochondrion, DNA synthesis occurred from the start of the light period and ceased within 3 h. The addition of nalidixic acid at the start of the light period inhibited DNA synthesis in both organelles. Next, we added nalidixic acid to the synchronous culture at the beginning (just before the organelle S phase) or at the end (organelle G2 phase) of the light period and monitored the division of organelle nucleoids in the dark period. Approximately 50 to 70% of chloroplast nucleoids divided unequally in both experiments. Despite the unequal division of the chloroplast nucleoids, electron microscopy revealed that the chloroplasts were divided into equal daughter chloroplasts by the chloroplast-divided ring. During the same treatments, all of the mitochondrial nucleoids underwent normal and equal division. Our results suggest the involvement of DNA gyrase in the replication of chloroplast and mitochondrial DNA and in the division of the chloroplast nucleoid, as well as a difference in mechanism between the division of chloroplast and mitochondrial nucleoids.

Anti-Infective Agents↗
PubMed1992

A new inhibitor of protein kinase C, RK-1409 (7-oxostaurosporine). I. Taxonomy and biological activity.

A novel inhibitor of protein kinase C was found in the fermentation of a soil actinomycete, strain RK-1409. According to the taxonomic studies, the producing strain was designated as Streptomyces platensis subsp. malvinus RK-1409. The protein kinase C inhibitor, RK-1409 (7-oxostaurosporine) inhibited the morphological change of a human chronic erythroleukemia cell, K-562, induced by phorbol 12,13-dibutyrate (PDBu) at the concentration of 10 ng/ml. The concentration of 3 ng/ml inhibited the activity of protein kinase C in vitro. RK-1409 inhibited the cell cycle progression at G2 phase of K-562 cells.

Alkaloids↗
PubMed1991

Radiation-induced chromatid aberrations in Cockayne syndrome and xeroderma pigmentosum group C fibroblasts in relation to cancer predisposition.

We showed previously that the persistence of chromatid breaks and gaps after G2 phase irradiation with X-rays or near-UV visible light characterizes skin fibroblasts from individuals with cancer-prone genetic diseases. This abnormal response appears to result from deficient DNA repair during G2 and to be associated with cancer proneness. We have, therefore, compared the responses of cells from two genetic disorders, Cockayne syndrome (CS) and xeroderma pigmentosum complementation group C(XP-C), both of which exhibit cellular hypersensitivity to sunlight, but only one of which, XP, manifests a high rate of sunlight-induced cancer. CS cells, in contrast to XP cells, showed a normal G2 response to irradiation with either X-rays or near-UV visible light. However, CS cells showed a deficiency in repair of DNA damage inflicted by light during S and G1 phases of the cell cycle. The present results support the concept that deficient DNA repair during G2 phase plays a role in carcinogenesis. This deficient repair in the presence of DNA damage and continuous cell cycling from activation of proto-oncogenes or loss of suppressor genes may be necessary and sufficient for cancer development.

Causality↗
PubMed1984

Spleen cell phosphorylation of salt soluble nuclear protein from isoproterenol treated and sorted nuclei.

The effect of isoproterenol (IPR) on phosphorylation of acidic nuclear proteins was investigated by two-dimensional gel autoradiography. Mouse spleen cells stimulated to divide by the mitogen concanavalin A (Con A) were separated according to cell cycle stage by flow microfluorometric technique. Exposure of cells for 48 h to 4 micrograms IPR/ml culture medium produced no significant change in the proportion of S and G2 phase cells, while a cumulative dose of 8 micrograms IPR/ml caused a significant repression in DNA synthesis and a reduction in the number of nuclei in G2 + M phase. Four micrograms IPR/ml stimulated the greatest amount of G0 + G1 phosphorylation of nuclear protein. Several proteins from G0 + G1 and S nuclei incorporated 32P after Con A + IPR administration, and one protein from S phase nuclei revealed intensified labeling at the 8 micrograms cumulative IPR dose but not at the 4 micrograms dose. The isolated proteins (W, X, Y, and Z) were reassociated with homologous DNA, centrifuged in a sucrose gradient and shown to co-sediment with DNA. S phase nuclear protein X-S, which was found to be a mixture of proteins (X0 and X1), was the only exception. One component of X-S, X0 bound to DNA, while component X1 failed to bind. Chymotryptic and V8 protease digests of all isolated proteins were made and analyzed by autoradiography. Proteins X0 and X1, recovered from the sucrose gradient, possessed dissimilar fragment patterns. It is concluded that protein X-S is composed of two proteins (X0 and X1), one of which (X1) appears during S phase during the 8 micrograms IPR induced nuclear repression.

Animals↗
PubMed2004

Macroscopic folding and replication of the homogeneously staining region in late S phase leads to the appearance of replication bands in mitotic chromosomes.

The chromosomal G/R bands are alternating domains differing in their nucleotide sequence biases. The bands are also related to the time of replication: pulse-labeling during S phase makes the replication sites as visible as replication bands that are close to the G/R bands in mitotic chromosomes. We previously showed that a plasmid bearing a mammalian replication origin efficiently generated a chromosomal homogeneously staining region (HSR). Here, we analyze the replication of this artificial HSR and show that it was replicated at the last stage of S phase. The HSR was composed of plasmid repeats only; nonetheless, we found that replication sites pulse-labeled during late S phase appeared as bands in the mitotic HSR and their number was dependent on the length of the HSR. Therefore, replication bands might not arise from sequence information per se. To understand the chronological order of appearance of replication sites, we performed a double pulse-chase experiment using IdU and CldU. Replication of the entire HSR required 100-120 minutes. During this period, the replicated sites appeared as bands at the first and last stages, but in between were apparently scattered along the entire HSR. An analysis of S-phase nuclei revealed that the replication started at the periphery of the globular HSR domain, followed by initiation in the internal domain. The replicated HSR appeared as a ring or a pair of extended spirals in late G2-phase nuclei. To account for these findings, we present a model in which the HSR is folded as a coiled-coil structure that is replicated from the outside to the inside in S phase nuclei.

Cell Line, Tumor↗
PubMed2003

p53-independent checkpoint controls in a plant cell model.

Allium cepa L. meristems were used as a plant model to study the p53-independent control of S and G2 phases by checkpoint pathways, in eukaryotic cells. Checkpoint blocks were induced at early and mid S by hydroxyurea. After their spontaneous override, cells became accumulated in G2-prophase, giving rise later on to a delayed mitotic wave. Cell growth was maintained during the checkpoint blocks, as the delayed mitoses were larger in size than the control ones. Under continuous hydroxyurea treatment, the delayed mitotic was formed by two subpopulations: normal mitoses corresponding to cells having properly recovered from the checkpoint block, and abnormal ones resulting from checkpoint adaptation. These latter cells displayed broken chromatids as they had unduly overriden the G2 checkpoint block, without completing DNA repair. The frequency of the checkpoint-adapted mitoses increased with the hydroxyurea concentration from 0.25 to 1.0 mM. However, from 1 mM hydroxyurea upwards, some of the cells lost their competence for checkpoint adaptation. Therefore, the dose of a genotoxic agent that still allows G2 checkpoint adaptation should always be applied in order to get rid of uncontrolled proliferating cells. This is specially suitable for cells lacking a functional p53 protein.

Cell Cycle↗
PubMed1993

Potentiating effects of organomercuries on clastogen-induced chromosome aberrations in cultured Chinese hamster cells.

Mercury compounds are among the most serious environmental pollutants. In this communication, the potentiating effects of organic and inorganic mercuries on clastogen-induced chromosome aberrations were studied in Chinese hamster CHO K1 cells. Post-treatment with monoalkylated mercuries--methyl mercuric chloride (MeHgCl) and ethyl mercuric chloride (EtHgCl)--increased the number of breakage- and exchange-type aberrations induced by 4-nitroquinoline 1-oxide (4NQO) and methyl methanesulfonate. With the DNA crosslinking agents mitomycin C (MMC) and cisplatin, MeHgCl enhanced both types of aberrations while EtHgCl enhanced breakage-type aberrations only. Since these monoalkylated mercuries did not show clastogenic effects by themselves under the present experimental conditions, the increases in chromosome aberrations were not additive. Dialkylated mercuries (dimethyl mercury and diethyl mercury) and inorganic mercuries (HgCl and HgCl2) did not show any potentiating effects. When MMC- or 4NQO-treated cells were post-treated with MeHgCl during the G1 phase, both breakage- and exchange-type aberrations were enhanced. Treatment with EtHgCl during the G1 phase also enhanced both types of aberrations induced by 4NQO. With MMC, however, G1 treatment with EtHgCl did not show any potentiating effect. MeHgCl and EtHgCl treatments during the G2 phase enhanced breakage-type aberrations only. Based on these results, the following possible mechanisms for potentiation of clastogenicity by monoalkylated mercuries were suggested; (1) they interfere with repair of base lesions induced by 4NQO and MMS during the pre-replicational stage, thereby increasing unrepaired DNA lesions which convert into DNA double-strand breaks in S phase, (2) MeHgCl (but not EtHgCl) also inhibits repair of crosslinking lesions during the pre-replicational stage, and (3) their G2 effects enhance breakage-type aberrations only.

4-Nitroquinoline-1-oxide↗
PubMed2004

Meiosis induced by inactivation of Pat1 kinase proceeds with aberrant nuclear positioning of centromeres in the fission yeast Schizosaccharomyces pombe.

Nuclear organization of chromosomes proceeds with significant changes during meiosis. In the fission yeast Schizosaccharomyces pombe, centromeres are clustered at the spindle-pole body (SPB) during the mitotic cell cycle; however, during meiotic prophase telomeres become clustered to the SPB and centromeres dissociate from the SPB. We followed the movement of telomeres, centromeres and sister chromatids in living S. pombe cells that were induced to meiosis by inactivation of Pat1 kinase (a key negative regulator of meiosis). Time-course observation in living cells determined the temporal order of DNA synthesis, telomere clustering, centromere separation and meiotic chromosome segregation. When meiosis was induced by Pat1 inactivation at the G1 phase of mitosis, telomeres clustered to the SPB as per normal meiosis, but in most cells the centromeres remained partially associated with the SPB. When meiosis was initiated at the G2 phase by Pat1 inactivation, both telomeres and centromeres retained their mitotic nuclear positions in the majority of cells. These results indicate that the progression of meiosis induced by Pat1 inactivation is aberrant from normal meiosis in some events. As Pat1 inactivation is often useful to induce S. pombe cells synchronously into meiosis, the temporal order of chromosomal events determined here will provide landmarks for the progression of meiosis downstream the Pat1 inactivation.

Cell Nucleus↗
PubMed1994

A topoisomerase II-dependent G2 cycle checkpoint in mammalian cells/.

The enzyme DNA topoisomerase II, which removes the catenations formed between the DNA molecules of sister chromatids during replication and is a structural component of chromosome cores, is needed for chromosome condensation in yeast and in Xenopus extracts. Inhibitors of topoisomerase II arrest mammalian cells before mitosis in the G2 phase of the cell cycle, but also produce DNA damage, which causes arrest through established checkpoint controls. It is open to question whether cells need topoisomerase II to leave G2, or control late-cycle progression in response to its activity. Bisdioxopiperazines are topoisomerase II inhibitors that act without producing direct DNA damage; the most potent, ICRF-193, blocks mammalian entry into but not exit from mitosis. Here we show that checkpoint-evading agents such as caffeine override this block to produce abortively condensed chromosomes, indicating that topoisomerase II is needed for complete condensation. We find that exit from G2 is regulated by a catenation-sensitive checkpoint mechanism which is distinct from the G2-damage checkpoint.

Animals↗
PubMed1996

UV-B-induced cell cycle perturbations, micronucleus induction, and modulation by caffeine in human keratinocytes.

UV-B-induced perturbations of cell cycle progression in asynchronous human keratinocytes were analysed during two cell cycles with respect to their cell cycle stage at the time of irradiation using BrdUrd/Hoechst flow cytometry. Exponentially growing SCL-2-keratinocytes exposed to UV-B radiation showed a short delay in G1-phase exit and were blocked in the S and G2/M phases of the first cell cycle. UV-A wavelengths did not show any detectable effect on cell cycle progression. In contrast, 137Cs-irradiation of these cells induced a temporary G2 block only. Micronucleus frequency increased in gamma-irradiated cells as soon as the cells started to divide and reached a plateau when most of the cells had divided. Continuous treatment with caffeine starting immediately after 137Cs gamma-irradiation prevented accumulation of cells in G2 phase, but did not influence the frequency of micronuclei. In UV-B-irradiated keratinocytes, however, the damage-induced cell cycle perturbations were merely reduced by caffeine, but not eliminated. Compared with gamma-irradiation a moderate induction of micronuclei was observed in UV-B-irradiated cells. Caffeine, however, potentiated the induction of micronuclei by UV-B. These different effects on cell cycle kinetics and micronucleus induction indicate different mechanisms of DNA damage caused by UV-B- and gamma-irradiation that may be repaired through different pathways.

Caffeine↗
PubMed1982

Effects of potassium dichromate on the cell cycle of an established human cell line (NHIK 3025).

The effect of K2Cr2O7 exposure on the cell cycle phases of the human cell line NHIK 3025 has been studied. Inhibition of cell proliferation was found to depend on the concentration and length of exposure. An effect on cell proliferation was observed 6-9 h after addition of K2Cr2O7, whereas cell death was not observed until 3-4 days later. The cells were most sensitive in G2 phase. After exposure to 8 mumol/l K2Cr2O7 the greatest prolongation of the cell cycle was in G2 + M phase, but an increase of S phase was also observed. No prolongation of G1 phase could be measured. Our results thus indicate that K2Cr2O7 delays progression through the cell cycle.

Cell Cycle↗
PubMed2002

Involvement of cell cycle control in bleomycin-induced mutagen sensitivity.

Bleomycin-induced chromosomal instability, generally referred to as mutagen sensitivity, is associated with an increased risk for the development of environmentally related cancer including head and neck squamous cell carcinoma and lung cancer. On average, the cultured lymphocytes of patients with these types of cancer show an increased number of chromatid breaks per cell after bleomycin exposure in the late S or G2 phase of the cell cycle as compared to lymphocytes from control persons. The aim of the present study was to investigate whether cell cycle regulation is involved in mutagen sensitivity. We determined cell cycle arrest after bleomycin-induced DNA damage in 21 lymphoblastoid cell lines that varied in mutagen sensitivity score. An ataxia telangiectasia (AT) cell line was included for comparison. Using a cut-off point of 0.70 breaks per cell, eight cell lines were classified as insensitive and 13 cell lines showed the hypersensitive phenotype. Compared to insensitive cell lines, bleomycin-treated hypersensitive cells remained at a relatively high level of DNA synthesis, as measured by thymidine incorporation, and showed a decreased accumulation of cells in G2 and M phase, as measured by flow cytometry. AT cells showed an extremely high mutagen sensitivity score, a high level of DNA synthesis, and a strong G2 block. In conclusion, mutagen sensitivity is associated with "damage-resistant growth," which is indicative of impaired cell cycle arrest. By which specific pathway(s) this checkpoint defect is explained has yet to be elucidated; however, it is probably distinct from the checkpoint defect in AT cells. Environ. Mol. Mutagen. 40:79-84, 2002.

Bleomycin↗
PubMed1997

Myt1: a Wee1-type kinase that phosphorylates Cdc2 on residue Thr14.

Most somatic cell division cycles contain a gap period (G2 phase) between the completion of DNA synthesis and the initiation of mitosis. This delay of mitotic entry is controlled, at least in part, by the repression of Cdc2 kinase activity by the phosphorylation of two conserved residues (Thr14 and Tyr15) within the ATP-binding pocket of the Cdc2 catalytic subunit. The kinases responsible for these two phosphorylation events include the Myt1 and Wee1 kinases, which phosphorylate Cdc2 on Thr14 and Tyr15, respectively. In this discussion, we summarise our current knowledge of the Myt1 kinase and its regulation of Cdc2 kinase activity during the G2-to -M phase transition.

Amino Acid Sequence↗
PubMed2000

Bromodeoxyuridine specifically labels the regenerative stem cells of planarians.

The singular regenerative abilities of planarians require a population of stem cells known as neoblasts. In response to wounding, or during the course of cell turnover, neoblasts are signaled to divide and/or differentiate, thereby replacing lost cell types. The study of these pluripotent stem cells and their role in planarian regeneration has been severely hampered by the reported inability of planarians to incorporate exogenous DNA precursors; thus, very little is known about the mechanisms that control proliferation and differentiation of this stem cell population within the planarian. Here we show that planarians are, in fact, capable of incorporating the thymidine analogue bromodeoxyuridine (BrdU), allowing neoblasts to be labeled specifically during the S phase of the cell cycle. We have used BrdU labeling to study the distribution of neoblasts in the intact animal, as well as to directly demonstrate the migration and differentiation of neoblasts. We have examined the proposal that a subset of neoblasts is arrested in the G2 phase of the cell cycle by double-labeling with BrdU and a mitosis-specific marker; we find that the median length of G2 (approximately 6 h) is sufficient to account for the initial mitotic burst observed after feeding or amputation. Continuous BrdU-labeling experiments also suggest that there is not a large, slow-cycling population of neoblasts in the intact animal. The ability to label specifically the regenerative stem cells, combined with the recently described use of double-stranded RNA to inhibit gene expression in the planarian, should serve to reignite interest in the flatworm as an experimental model for studying the problems of metazoan regeneration and the control of stem cell proliferation.

Animals↗
PubMed1997

Monoclonal antibody raised against human mitotic cyclin B1, identifies cyclin B-like mitotic proteins in synchronized onion (Allium cepa L.) root meristem.

Cyclin B-like mitotic proteins have been detected in synchronized Allium cepa L. root tip cells by using mouse monoclonal anti-cyclin B1 antibody raised against human cyclin B1. Immunoblot shows two closely placed isoforms of cyclin B-like proteins having an apparent molecular weight around 54 kDa. In vivo [35S]-methionine labelling followed by immunoprecipitation and autoradiography indicates that cyclin B-like proteins are mainly synthesized in the G2 phase of the cell cycle and destroyed in late mitosis. Immunoblotting data depict that the level of cyclin B-like proteins reaches the maximum at the late G2 to early M phase; and it becomes degraded in the late hours of mitosis. Moreover, the cyclin B isoforms are stabilized in colchicine-arrested metaphase cells as already reported in animal cells.

Allium↗
PubMed1996

Regulation of the activity of M-phase promoting factor through protein kinase A-mediated pathway in LP1-1 cells.

Treatment of cAMP analogs, dibutyryl cAMP and 8-bromo-cAMP, was found to inhibit the proliferation of mouse fibroblast LP1-1 cells and the p34cdc2 kinase activity of M-phase promoting factor (MPF). However, it showed relatively little effect on expression of the cyclin B1 and cdc2 genes. On the other hand, when the nuclear extracts obtained from the cells at early G2 phase were treated with cAMP analogs, the kinase activity was significantly decreased as compared to the untreated control. Furthermore, the inhibitory effect of cAMP analogs could be reversed upon treating with okadaic acid even in the presence of the cAMP analogs, implying that cdc25 remains in an active form. In addition, the treatment of okadaic acid stimulated the cell progression. These results suggest that down-regulation of MPF activity through protein kinase A-mediated pathway is under post-translational control and cdc25 activation pathway involving okadaic acid-sensitive phosphatase play a role in the regulation of MPF activity.

Animals↗
PubMed1978

An immunological approach to the isolation of factors with mitotic activity from the plasmodial stage of the myxomycete Physarum polycephalum.

Nuclear divisions in plasmodia of Physarum polycephalum were advanced by applying immunologically purified plasmodial extracts of late G2 phase on the surface of plasmodia which were 1.5 h before the third mitosis. The purification of G2 extracts was achieved by interaction of antibodies prepared against the antigens of early S phase plasmodia with the antigens of late G2 plasmodia. There was no advancement of mitosis by extracts prepared from early S phase plasmodia. Untreated G2 extracts did not accelerate mitosis with the same effectiveness as did antibody purified G2 extracts.

Immunologic Techniques↗
PubMed1989

Cell cycle traverse in NHIK-3025 carcinoma of the uterine cervix after low-dose-rate irradiation.

Theoretically, fractionation schemes could be tailored to the individual pattern of radiation-induced synchronization of cells in the radiosensitive G2 phase, leading to more effective radiotherapy. Using a human cervical carcinoma xenografted to nude mice, the effects of low-dose-rate irradiation on the cell cycle distribution were studied. Flow cytometric analysis demonstrated that cells accumulated in G2 + M phase 35 h after a total dose of 10 Gy of 137Cs irradiation. This accumulation time corresponded closely to the cell cycle time (Tc) (31 h) of this tumor, as determined by autoradiography. Further experiments are planned to determine the potential of fractionation schemes adjusted to the Tc-related accumulation in G2 in improving the effectiveness of radiotherapy.

Animals↗