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1.
The kinetics of DNA replication were analyzed in the second S phase following UV irradiation of Chinese hamster ovary cells synchronized at the beginning of S phase. The cells were synchronized by treating cells selected in mitosis with hydroxyurea for 9 h. Following UV irradiation, the cells were allowed to progress until the next mitosis; at which time they were resynchronized at the beginning of the second S phase by the same procedure. The kinetics of DNA replication were determined by measuring the proportion of DNA which achieved hybrid buoyant density on CsCl density gradients as a function of the time of incubation in the presence of 5-bromodeoxyuridine.The results of these experiments showed that even though the rate of DNA replication is substantially depressed during the first S phase following UV irradiation with a fluence of 5 J/m2, the rate has recovered to the extent that it is indistinguishable from the unirradiated control by the time the cells have entered their second S phase. It was concluded from these observations that the lesions in DNA which caused the rate of DNA replication to be initially depressed during the first S phase have been either removed or modified such that they no longer are able to cause a reduction in the rate of DNA replication in the second S phase following UV irradiation.  相似文献   
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3.
The induction and rejoining of gamma-ray-induced DNA strand breaks were measured in a Chinese hamster ovary cell line, AA8, and in two radiosensitive clones (EM9 and NM2) derived from it. The kinetics of recovery from sublethal damage (SLD) and potentially lethal damage (PLD) has previously been characterized in each of these lines [vanAnkeren et al., Radiat. Res., 115, 223-237 (1988)]. No significant differences were observed among the cell lines in the yields of either DNA single-strand breaks (SSBs) or double-strand breaks (DSBs) as assayed by filter elution. Data for SSB rejoining in AA8 and NM2 cells irradiated with 7.5 Gy were fit by a biexponential process (t1/2 values of approximately 4 and 80 min). In comparison, SSB rejoining in EM9 cells was initially slower (t1/2 = 10 min) and a higher level of SSBs was unrejoined 6 h after irradiation. DSB rejoining in AA8 cells assayed at pH 9.6 was also biphasic (t1/2 values of 15 and 93 min), although when assayed at pH 7.0, most (approximately 80%) of the damage was rejoined at a constant rate (t1/2 = 45 min) during the first 2 h. EM9 cells exhibited a slower initial rate of DSB rejoining when assayed at pH 9.6 but showed no difference compared with AA8 cells in DSB rejoining when assayed at pH 7.0. These results indicate that radiosensitive EM9 cells, whose kinetics of recovery from SLD and PLD was the same as that of AA8 cells, have a defect in the fast phase of SSB rejoining but no measurable defect in DSB rejoining. Conversely, NM2 cells, which displayed a reduced shoulder width on their survival curve and decreased recovery from SLD, had no demonstrable defects in the rate or extent of rejoining of DSBs or SSBs. When compared with the SLD and PLD data reported previously, these results suggest that there is no direct correlation between either of these recovery processes and the rejoining of SSBs or DSBs as assayed here.  相似文献   
4.
MAJOR CLADES OF THE ANGIOSPERMS   总被引:2,自引:0,他引:2  
Abstract— Our knowledge of fundamental angiosperm interrelationships is still very incomplete. The absence of a narrowly circumscribed gymnosperm outgroup, ideally the sister group, makes character evaluation, necessary for a cladistic analysis, difficult. According to current views the superorder Magnoliiflorae with a number of other groups, for example the monocotyledons, may represent a complex of families near the base of the angiosperms. Interrelationships of groups within the monocotyledons are much better understood than those between groups within the dicotyledons. A cladogram of monocotyledon orders based on earlier work by R. Dahlgren, H. T. Clifford, and F. N. Rasmussen is presented. A data matrix for a sample of the angiosperms with 61 characters for 49 taxa, mostly magnoliifloran and related families, is presented. The characters are polarized mainly according to the current view that the primitive angiosperm morphotype is a woody dicotyledon with strobiloid flowers. As an alternative the matrix is adjusted following W. C. Burger's conjecture that the primitive angiosperm was a herbaceous monocotyledon with trimerous flowers. Both matrices were run in a computerized parsimony analysis, resulting in numerous equally parsimonious solutions. This result is illustrative of the great homoplasy in the available character information, and also of how little actually is known about fundamental angiosperm interrelationships or phylogeny.  相似文献   
5.
In order to study the relationships among mammalian alpha-globin genes, we have determined the sequence of the 3' flanking region of the human alpha 1 globin gene and have made pairwise comparisons between sequenced alpha-globin genes. The flanking regions were examined in detail because sequence matches in these regions could be interpreted with the least complication from the gene duplications and conversions that have occurred frequently in mammalian alpha-like globin gene clusters. We found good matches between the flanking regions of human alpha 1 and rabbit alpha 1, human psi alpha 1 and goat I alpha, human alpha 2 and goat II alpha, and horse alpha 1 and goat II alpha. These matches were used to align the alpha-globin genes in gene clusters from different mammals. This alignment shows that genes at equivalent positions in the gene clusters of different mammals can be functional or nonfunctional, depending on whether they corrected against a functional alpha-globin gene in recent evolutionary history. The number of alpha-globin genes (including pseudogenes) appears to differ among species, although highly divergent pseudogenes may not have been detected in all species examined. Although matching sequences could be found in interspecies comparisons of the flanking regions of alpha- globin genes, these matches are not as extensive as those found in the flanking regions of mammalian beta-like globin genes. This observation suggests that the noncoding sequences in the mammalian alpha-globin gene clusters are evolving at a faster rate than those in the beta-like globin gene clusters. The proposed faster rate of evolution fits with the poor conservation of the genetic linkage map around alpha-globin gene clusters when compared to that of the beta-like globin gene clusters. Analysis of the 3' flanking regions of alpha-globin genes has revealed a conserved sequence approximately 100-150 bp 3' to the polyadenylation site; this sequence may be involved in the expression or regulation of alpha-globin genes.   相似文献   
6.
Previous reports have suggested that the potentiation of cellular radiation sensitivity by hyperthermia may be due to its inhibition of the repair of single-strand breaks in DNA. Such inhibition could result in increased numbers of unrejoined breaks at long times following irradiation, lesions that are presumed to be lethal to the cell. As a test of this hypothesis, the amounts of residual strand-break damage in cells following combined hyperthermia and ionizing radiation were measured. The results show that hyperthermia does significantly enhance the relative number of unrejoined strand breaks as measured by the technique of alkaline elution and that the degree of enhancement is dependent on both the temperature and duration of the hyperthermia treatment. For example, compared to unheated cells, the proportion of unrejoined breaks measured 8 hr after irradiation was increased by a factor of 1.5 in cells that were treated for 30 min at 43 degrees C, by a factor of 6 for cells treated for 30 min at 45 degrees C, and by a factor of 4 for cells treated at 43 degrees C for 2 hr. In experiments in which the sequence of heat and irradiation were varied, a high degree of correlation was observed between the resulting level of cell killing and the relative numbers of unrejoined strand breaks. The greatest effects on both of these parameters were observed in those protocols in which the irradiation was delivered either during, just before, or just after the heat treatment.  相似文献   
7.
Summary Host-cell reactivation of gamma-irradiated phage T1 in strains of E. coli K-12 has been compared with HCR of UV-irradiated phage in these same strains and with the radiation sensitivities of these strains (Fig. 1–4). The pattern of the HCR of gammairradiated phage in these strains is like that of the HCR of UV-irradiated phage. HCR in strains whose genotype is uvr +rec- is like that of the wild type; whereas, HCR is minimal in strains which are uvr -. It is suggested that some type of gamma-ray-induced base damage in phage DNA is repaired in uvr + strains.This work was supported by the United States Atomic Energy Commission Contract No. AT(11-1)-1686. — This is report No. COO-1686-6.Supported in part by the United States Public Health Service Training Grant No. 5T1 RH-80-02(67).  相似文献   
8.
INTRODUCTI0NThedifferentiati0nofcelIsalongthemonocyte-macr0phagepathwayandthesig-nalsinvo1vedinthesecel1sacquiringtheabilitytokilltum0rcellsarenotfllllyundersto0d.Wehavebeenstudingamoleculewhichappearst0beanimportantmemberofthecytokinenetworkinvo1vedintheregulati0nmonocyteactivation.ThiscytokinetermedP48wasisolatedfr0mthehllmannullcellleukemiacell1ineReh.IthasbeenpurifiedtohomogeneityandfOundtobedistinctfrominterferongamma,col0nystimulatingfactors(CSFs)andTNFalphaalldbeta[1,2].Func-ti…  相似文献   
9.
Chinese hamster ovary (CHO) cells in vitro were treated with HgCl2 at various stages in the cell cycle and the effects of this chemical on cell survival, DNA replication, and cell division were observed. In terms of survival the early G1 cells were the most sensitive to treatment, followed by late G1 and early S, while mid S and late S-G2 treated cells were the least sensitive. Treatment with HgCl2 also resulted in reduced rates of DNA replication and delays in cell division. The early G1 treated cells showed substantially reduced rates of DNA replication followed by 4--5 h division delay. The early S and late S-G2 treated cells had some reduction in their rates of DNA replication followed by corresponding division delay of 2.5 h in the early S treated cells and 1 h in the late S-G2 treated cells.  相似文献   
10.
Growth, tolerance and zinc and cadmium hyperaccumulation of Thlaspi caerulescens populations from three metal contaminated soils and three normal soils were compared under controlled conditions. Individuals of six populations were cultivated on five soils with increasing concentrations of zinc (50–25000 μg g−1) and cadmium (1–170 μg g−1). There was no mortality of normal soil populations in the four metal-contaminated soils, but plant growth was reduced to half that of populations from metal-contaminated soils. However, in noncontaminated soil, the growth of individuals from normal soils was greater than that of individuals from metal-contaminated soils. Individuals from normal soils concentrated three times more zinc in the aboveground biomass than those from metal-contaminated soils, but the latter accumulated twice as much cadmium. We conclude that populations of T. caerulescens from both normal and metal-contaminated soils are interesting material for phytoextraction of zinc and cadmium, but to optimize the process of phytoextraction it is necessary to combine the extraction potentials of both type of populations.  相似文献   
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