SEARCH · Search PubMed
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.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Sychronous divisions of Euglena gracilis strain Z can be obtained by various methods. When the cells are cultivated in a medium containing lactate as the sole carbon source, synchronous divisions are observed, independent of the conditions of illumination. Nevertheless, there exists a relationship between the phase of cell division and ther periods of light and darkness applied to the culture. During the cell cycle, the synthesis of macromolecules is discontinuous--this is true of nuclear and mitochondrial DNA, ribosomal and nonribosomal RNA, and certain proteins (cytochrome c 558). Cyclic variations in the structure of mitochondria and chloroplasts are observed. In the course of the cell cycle, sequential metabolic processes accompany structural modifications of the organelles. Also, at the beginning of the cycle, at the start of phase G1, the cytoplasmic ribosomes are synthesized, and then, in green euglenids, nonribosomal RNAs are formed. These syntheses of RNA precede enlargement of the chondriome and plastids. In mid-G1 phase, a new synthesis of RNA begins, which precedes synthesis of nuclear and mitochondrial DNA. At the end of G1 phase, division of organelles starts, beginning with the chondriome and plastids, arranged in a network.
Human embryonic lung fibroblasts (IMR-90 and WI-38) were arrested in the G1 phase of the cell cycle by serum deprivation and high population density. Within 1 hr after the addition of medium containing fresh serum, these cells showed an increase in rRNA synthesis. The inclusion of 100 micrograms per ml aminonucleoside of puromycin (AMS) in the fresh medium eliminated the serum stimulation of rRNA synthesis and prevented the cells from making the G1-resting phase to G1-prereplicative phase transition. AMS also prevented the synthesis of HnRNA normally found within 10 hr after serum stimulation. Serum-stimulated RNA synthesis in starved, SV-40 transformed fibroblasts (WI-38-VA-13 cells) was inhibited, but not completely prevented, by AMS indicating that transformed cells may produce specific RNA's that are not AMS-sensitive and that may be responsible for the failure of transformed cells to be arrested in G1.
The in vitro relationship between nutritional factors, proliferative status of tumor cells, and the cytotoxic action of cytosine arabinoside (ara-C) was investigated. The reduction in the concentration of only one essential amino acid, L-isoleucine, in the growth medium of A(T1)C1-3 hamster fibrosarcoma cells decreased DNA synthesis in this cell population and slowed the rate of progression of G1 phase cells into S phase of the cell cycle. The complete omission of isoleucine from the growth medium blocked the progression of G1 phase cells into S phase and prevented the cytotoxic action of ara-C. The addition of isoleucine to the isoleucine-deprived cells permitted these cells to enter the S phase and restored their sensitivity to the cytotoxic action of ara-C. When G1 phase cells were placed in a medium containing reduced levels of all the amino acids and vitamins there was a prolongation of the G1 phase. Since medium with low levels of amino acids produced a delay in the entry of G1 phase cells into the S phase, the time interval in which these cells were most sensitive to the cytotoxic action of ara-C was different for G1 phase cells placed in medium with adequate levels of all the amino acids. These in vitro data indicate that nutritional factors can markedly effect the proliferation of tumor cells and the cytotoxic action of ara-C.
Complete and robust human genome duplication requires loading minichromosome maintenance (MCM) helicase complexes at many DNA replication origins, an essential process termed origin licensing. Licensing is restricted to G1 phase of the cell cycle, but G1 length varies widely among cell types. Using quantitative single-cell analyses, we found that pluripotent stem cells with naturally short G1 phases load MCM much faster than their isogenic differentiated counterparts with long G1 phases. During the earliest stages of differentiation toward all lineages, MCM loading slows concurrently with G1 lengthening, revealing developmental control of MCM loading. In contrast, ectopic Cyclin E overproduction uncouples short G1 from fast MCM loading. Rapid licensing in stem cells is caused by accumulation of the MCM loading protein, Cdt1. Prematurely slowing MCM loading in pluripotent cells not only lengthens G1 but also accelerates differentiation. Thus, rapid origin licensing is an intrinsic characteristic of stem cells that contributes to pluripotency maintenance.
WI-38 and SV40WI-38 cells have been synchronized using centrifugal elutriation. This technique allows for the rapid harvesting of early G1 phase cells from exponentially growing populations of both the normal and transformed cell. Using these cells, as well as WI-38 cells synchronized by serum deprivation, we have examined the effects of extracellular Ca and Mg levels on the progression of cells through G1 phase. A differential sensitivity to both Ca and Mg deprivation is observed between normal and transformed cells. The WI-38 cell requires higher levels of both ions for traversal of G1 phase and for continued proliferation as compared to the transformed cell. The temporal nature of the Ca and Mg requirements for the WI-38 cell has been examined during G1 phase. Ca is strictly required during early and late G1 phase, but not necessarily throughout mid-G1. An early as well as a late G1 Ca requirement is also found in serum-stimulated WI-38 cells. In contrast, the Mg requirement of WI-38 cells does not appear to be temporally well-defined. Mg appears to be a permissive factor, required throughout G1 phase rather than at certain prescribed intervals. On the basis of these data, it seems unlikely that these two cations exert their effects on cell growth entirely through a common competitive mechanism. Ca would appear to be involved in early serum or growth factor-mediated G1 events and later pre-S-phase events, as suggested in previous studies on other cell lines.
Three lines of mouse mammary carcinoma growing in isogenic hosts and differing in degree of histological differentiation and rates of proliferation were used to study parenchymal cells with various types of nucleoli. The relative number of cells possessing trabeculate or ring-shaped nucleoli or nucleolar fragments was closely related to the growth rate and degree of differentiation of tumor lines tested. All three subpopulations increased with increasing age and with decelerated tumor growth. In some cells in late telophase, either trabeculate or ring-shaped nucleoli could be distinguished in mitotic poles. This demonstrated that cells with these nucleoli are detected at the beginning of G1 phase. Even low levels of DNA synthesis, which would indicate that some cells with trabeculate or ring-shaped nucleoli or possessing nucleolar fragments were in S phase, could not be demonstrated. Microfluorometric measurements have indicated that cells with trabeculate and ring-shaped nucleoli have a DNA content close to 2c, whereas cells with dense nucleoli have a DNA content corresponding to either 2c, 2 to 4c, 4c, or greater than 4c. On the basis of these observations, it is concluded that cells with trabeculate and ring-shaped nucleoli and cells with nucleolar fragments either proceed slowly through G1 or are arrested in this phase. Cells with trabeculate nucleoli were replaced steadily, having a transit time of no longer than 84 hr. These cells constituted a "fast" component of cell renewal of G1-confined cells. A "slow" component, cells bearing ring-shaped nucleoli or nucleolar fragments, were replaced after a lag of 24 to 48 hr. with residency time for some of these cells being in excess of 84 hr.
Chinese hamster ovary cells were synchronized by selective detachment of cells in mitosis. The adenosine 3':5'-cyclic monophosphate (cyclic AMP) intracellular concentrations and cyclic AMP-dependent protein kinase activities were measured as these cells traversed G1 phase and entered S phase. Protein kinase activity, assayed in the presence or absence of saturating exogenous cyclic AMP in the reaction mixture, was lowest in early G1 phase (2 h after mitosis), increased 2-fold (plus exogenous cyclic AMP in reaction mixture) or 3.5-fold (minus cyclic AMP in reaction mixture) to maximum values in mid to late G1 phase (4-5 h after mitosis), and then decreased as cells entered S phase. Intracellular cyclic AMP concentrations were minimal 1 h after mitosis, increased 5-fold to maximum levels at 4-6 after mitosis, and decreased as cells entered S phase. Similar to the fluctuations in intracellular cyclic AMP, the cyclic AMP-dependent protein kinase activity ratio increased more than 40% in late G1 or early S phase. Puromycin (either 10 mug/ml or 50 mug/ml) administered 1 h after mitosis inhibited cyclic AMP-dependent protein kinase activity up to 50% by 5 h after mitosis, while similar treatment (10 mug/ml) had no effect on the increase in cyclic AMP formation. These data demonstrate that: (1) total protein kinase activity changed during G1 phase and this increase was dependent on new protein synthesis; (2) the increased intracellular concentrations of cyclic AMP were not dependent on new protein synthesis; and (3) the activation of cyclic AMP-dependent protein kinase was temporally coordinated with increased intracellular concentration of cycli AMP as Chinese hamster ovary cells traversed G1 phase and entered S phase. These results suggest that cyclic AMP acts during G1 phase to regulate the activation of cyclic AMP-dependent protein kinase.
CHO cells were synchronized 2 G1 phase and treated with UV light or HN2. These treatments resulted in a dose-dependent reduction in the rate of DNA replication and cell survival. Holding UV-irradiated cells in G1 phase (in HU medium) for an additional 10 h prior to their release into S phase did not assist recovery as measured by either of these criteria. The survival of cells treated with HN2 was also not enhanced by this recovery period. However, following 2 X 10(-5) M HN2 the rate of DNA replication increased from 30% to 70% of the control level when the period in HU medium was extended to 14 h. The induction of cross-links following HN2 treatment of asynchronous cells was shown to be dose dependent. Subsequent incubation in fresh medium resulted in complete recovery within 20 h at concentrations of HN2 up to 10(-5) M, and at 2 X 10(-5) M HN2, 75% of the cross-links were removed at 14 h post treatment.
Histone-gene expression was studied during the cell cycle of continuously dividing HeLa S3 cells and after stimulation of confluent monolayers of WI-38 human diploid fibroblasts to proliferate. The presence of histone-mRNA sequences was assayed by hybridization to a 3H-labelled single-stranded DNA complementary to histone-mRNA molecules. In HeLa S3 cells histone mRNA sequences were found in the nucleus and associated with polyribosomes during S-phase, but not during G1-phase. Transcripts of S-phase chromatin contained histone-mRNA sequences, but those of G1-phase chromatin did not. Similarly, in WI-38 cells association of histone-mRNA sequences with polyribosomes and transcription of histone-mRNA sequences from chromatin parallel DNA replication. Taken together these results suggest that the regulation of histone-gene expression resides, at least in part, at the transcriptional level. Chromatin-reconstitution studies provide evidence that non-histone chromosomal proteins play a key role in activation of histone-gene transcription during the period of the cell cycle when DNA is replicated. Phosphate groups associated with the S-phase non-histone chromosomal proteins appear to be functionally involved in the control of histone-gene readout.
The inhibiting effect of tissue extract from fully differentiated intestinal mucosa of adult animals on proliferation kinetics of exponentially growing embryonic epithelial gut cell populations was studied in the newt Pleurodeles waltlii. Crude extract was fractionated by G-200 Sephadex chromatography and the effect of fractions on cell proliferation was studied using both mitotic index and 3-H-thymidine incorporation methods. The inhibitions we obtained were then displayed by means of cytophotometric study of age distribution of intestinal gut cells around the cell cycle, measuring the Feulgen-DNA content. The results revealed the presence of two chalone-like substances in the intestine of adults. One (factor 1) is characterized by a molecular weight of between 120,000 and 150,000 and inhibits the cell cycle at the end of the G1 phase, the other (factor 2) is characterized by a molecular weight lower than 2000 and inhibits the cell cycle in the course of the G2 phase. The cells delayed in the G2 phase escape from inhibition but the cells delayed in the G1 phase do not, although availability time of both factor 1 and factor 2 is about 12 hr. It is thus thought that cells prevented from dividing in G1 phase are indefinitely delayed in this phase and possibly differentiate.
A temperature sensitive growth mutant, 13B11 derived from CHO-K1 has been partially characterized. Upon shift to the nonpermissive temperature (39 degrees C) DNA synthesis slows down, but it is partly resumed after prolonged incubation at 39 degrees C. The radioautographic results suggest that reduction in the rate of DNA synthesis is mainly due to decrease in the proportion of DNA synthesizing cells and that increase in that rate observed afterwards is due to appearance of cells that enter S. Synchronized mutant cells incubated at 39 degrees C from the beginning of G1 enter S with the delay of 6 hours. By incubating the cells at 39 degrees C for a restricted period during the G1 phase, some processes in late G1 are found to be susceptible to the high temperature. Mutant cells shifted up in the middle of S performed cell division whereas cells up-shifted in G1 phase did not, although about a half of these cells divided when they were shifted down in the presence of hydroxyurea. The analysis of DNA content of the cells cultured at 39 degrees C for more than one generation time, revealed the accumulation of nuclei containing DNA in amount of almost 4C level. Accordingly in this mutant the process necessary for cell division is also temperature sensitive. Considering the frequency of the appearance of spontaneous revertants (2 X 10(16)), the apparent two lesions of this mutant might be ascribed to the single mutation.
Mouse fibroblasts, subline L-929 F were synchronized by mitotic detachment. The synchronized cell cultures were irradiated with 200 kVp X-rays at different time after mitosis, and age response functions and dose effect curves were determined using the colony test. The cell age in the mitotic cycle was obtained from a computer analysis of flow cytometric DNA histograms. Both intrinsic radiosensitivity 1/D0 and extrapolation number n were found to vary during the cell cycle. The D0 has a maximum value of 176 +/- 1 rad in the middle of G1 phase and minimum of 71 +/- 1 rad at the S/G2 transition, while the extrapolation number is rather constant from the beginning of G1 phase (1.9 +/- 0.1) to the middle of S phase (2.3 +/- 0.1) and reaches a steep maximum of 9.3 +/- 1.1 at S/G2 transition. The values of n in the various phases of cell cycle are compared with the respective values of the recovery factor gamma determined after fractionated irradiation. --Cell survival after a single dose of 616 rad has minima for irradiation at G1/S transition and in early G2 phase; the survival in early G2 being about 40 times smaller than in early G1 phase. Implications for a cell cycle specific therapy are discussed.
Poly(A) polymerase activity was studied in lysates of cultured murine erythroleukemic cells (Friend cells). Incorporation of ATP into acid-precipitable products is dependendent on the presence of Mn2+ or Mg2+ and of an RNA primer. The reaction is specific for ATP as the substrate (KM=290 290 micron, it is not inhibited by actinomycin D and only slightly interferred with by ethidium bromide. Cordycepin 5'-triphosphate and sodium pyrophosphate inhibit the enzyme activity. The chain length of the products of the reaction is dependent on the primer concentration and reaches up to 30 nucleotides. Poly(A) polymerase activity is low in resting (G1 phase) cells 75 nmol ATP incorporated/h per 10(6) cells) and increases to a level about twice as high in early S phase of the cell cycle. A possible model for regulation of enzyme activity is discussed. Polymerase activity in the early phase of erythropoietic differentiation of the cells induced by butyric acid does not show any difference in comparison to untreated controls. A decrease in enzyme activity to levels characteristic for cells in G1 phase accompanies shutdown of cell growth in the course of the ongoing differentiation. Analysis of the DNA content of the cells revealed that erythropoietic differentiation of Friend cells induced by butyric acid is characterized by arrest of the cells in G1 phase of the cell cycle. Poly(A) polymerase activity in erythroleukemic cells is thus controlled only by the phase of the cell cycle; it is not affected by changes in gene expression during erythroid differentiation.
Nuclei were isolated from synchronized HeLa S3 cells and transcribed utilizing their endogenous RNA polymerases. Our data suggest that S phase nuclei are capable of synthesizing histone mRNA sequences while nuclei from G1 phase cells are not. Transcription of histone mRNA sequences by S phase nuclei can be abolished completely by low levels of alpha-amanitin (1.0 microgram/ml, a concentration which completely inhibits RNA polymerase II). From these results it appears that transcription of the histone mRNA sequences occurs during the S phase but not during the G1 phase of the cell cycle and that RNA polymerase II is responsible for histone gene readout.
The conditional cell division cycle yeast mutants cdc have been used to demonstrate that intragenic recombination induced by ultraviolet or gamma rays occurs in diploids arrested in G1, a short time after irradiation and before the initiation of the S phase. This implies that pairing of homologous chromosomes does not require duplicated chromatids.