全文获取类型
收费全文 | 1217篇 |
免费 | 155篇 |
国内免费 | 4篇 |
出版年
2022年 | 16篇 |
2021年 | 20篇 |
2020年 | 8篇 |
2019年 | 13篇 |
2018年 | 21篇 |
2017年 | 14篇 |
2016年 | 30篇 |
2015年 | 62篇 |
2014年 | 52篇 |
2013年 | 66篇 |
2012年 | 76篇 |
2011年 | 78篇 |
2010年 | 50篇 |
2009年 | 33篇 |
2008年 | 46篇 |
2007年 | 48篇 |
2006年 | 45篇 |
2005年 | 53篇 |
2004年 | 41篇 |
2003年 | 49篇 |
2002年 | 49篇 |
2001年 | 37篇 |
2000年 | 17篇 |
1999年 | 23篇 |
1998年 | 19篇 |
1997年 | 14篇 |
1996年 | 9篇 |
1994年 | 12篇 |
1992年 | 11篇 |
1991年 | 22篇 |
1990年 | 26篇 |
1989年 | 25篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 14篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1970年 | 8篇 |
1968年 | 9篇 |
1967年 | 7篇 |
1966年 | 8篇 |
排序方式: 共有1376条查询结果,搜索用时 15 毫秒
1.
Elena C. Guzman Alfonso Jimenez-Sanchez Elisha Orr Robert H. Pritchard 《Molecular & general genetics : MGG》1988,212(2):203-206
Summary A temperature shift-up accompanied by a reduction in RNA polymerase activity in Escherichia coli causes an increased rate of initiation leading to a 1.7- to 2.2-fold increase in chromosome copy number. A temperature shift-up without a reduction in polymerase activity induces only a transient non-scheduled initiation of chromosome replication caused by heat shock with no detectable effect on chromosome copy number. 相似文献
2.
Butow Barbara; Wynne David; Sukenik Assaf; Hadas Ora; Tel-Or Elisha 《Journal of plankton research》1998,20(2):355-369
Lipid peroxidation in Peridinium samples taken from two differentdepths in Lake Kinneret fluctuated throughout the spring withan overall increasing trend. Samples from 0.5 and 5 m showeda similar peroxidation pattern, which was maximal after thefall off in algal biomass. The rapid decline in Peridinium biomasscoincided with ambient lake temperatures of 2123C. Fattyacid composition profiles were similar at both depths, althoughafter the peak of the bloom, a significant increase in polyunsaturatedfatty acids and oleic acid was only found at 0.5 m, togetherwith a decrease in the percentage of polyunsaturated fatty acids.These effects were related to ambient light stress rather thana result of lipid peroxidation. Lake samples taken at differentperiods of the bloom and incubated at various temperatures showeddifferential peroxidation. Higher temperatures caused increasedlipid peroxidation, but this appeared to be dependent on thesampling period. Samples withdrawn from the lake at the beginningof the bloom showed little peroxidation after a 5 day incubationat 14C, room temperature (25C) or ambient lake temperature(16C) compared to mid-bloom samples in which there was a significantincrease in peroxidation when they were incubated at room temperature(25C) or ambient lake temperature (22C). Incubation at 14Cinhibited peroxidation; however, samples from mid-bloom againshowed enhanced peroxidation compared with those from the beginningof the bloom. These in situ results suggested a relationshipbetween temperature, another environmental variable during thebloom and lipid peroxidation in Peridinium. As total dissolvedinorganic carbon (DIC) concentrations fall significantly duringthe progress of the bloom and represent an important sourceof environmental stress, laboratory experiments were establishedto investigate the synergistic effect of temperature and carbonnutrition on lipid peroxidation in Peridinium cultures. Increasedtemperature alone caused a slight increase in lipid peroxidation,but this was greatly augmented by carbon limitation. Althoughcarbon limitation induced increased catalase activity, at highertemperatures activity declined after 48 h, allowing for thesubstantial increase in lipid peroxidation. 相似文献
3.
4.
E. coli cells containing a temperature-sensitivednaE mutation, in the α-subunit of holoenzyme DNA polymerase III, do not survive at the restrictive temperature. Such cells may
survive in the presence of thepcbA1 mutation, an allele of thegyrB gene. Such survival is dependent on an active DNA polymerase I. Evidence indicates that DNA polymerase I interacts directly
in the replisome (REP·A). Despite normal survival for cells using thepcbA replication pathway after some type of DNA damage, we have noted a failure of damage-induced mutagenesis. Here we present
evidence supporting a model of replisome pausing in cells dependent upon thepcbA replication pathway. The model argues that the (REP·A) complex pauses longer at the site of the lesion, allowing excision
repair to occur completely. In the normal replication pathway (REP·E) bypass of the lesion occurs, fixing the mutation. 相似文献
5.
6.
We have evaluated the ability of immortalized human fibroblasts to recombine transfected plasmid DNA. A number of cell lines from normal individuals and from patients with DNA damage-processing defects were examined. Two plasmid recombination substrates were derived from pSV2neo and contained nonoverlapping deletions in the aminoglycoside phosphotransferase II gene. Intermolecular recombination was assessed by two methods after cotransfection. In a short-term, extrachromosomal recombination assay, low molecular weight DNA was extracted from the human cells 48 h after transfection, and recombinant plasmids were detected by transformation into appropriate indicator bacteria. In a long-term stable recombination assay the fibroblasts were cotransfected and G418-resistant colonies allowed to form. By the former assay all but two cultures were recombination-proficient, whereas all were recombination-proficient by the latter assay. The efficiency of transfection of human cells with plasmids appears to be a major variable affecting recombination. Recombination can be stimulated by uv irradiation of plasmid DNA prior to transfection. Cells from patients with Fanconi anemia, ataxia telangiectasia, and xeroderma pigmentosum complementation groups A, C, D, E, and G are not defective at intermolecular plasmid recombination. 相似文献
7.
Geraldine E. Oliva George W. Rutherford Moses Grossman Janet Shalwitz Abigail English Frances Taylor David Werdegar 《The Western journal of medicine》1988,148(5):586-589
Although adolescents account for only 0.4% of reported cases of the acquired immunodeficiency syndrome (AIDS) in the United States, they are sexually active and, therefore, at risk of acquiring human immunodeficiency virus (HIV) infection. To address issues of HIV control in adolescents, we developed guidelines that emphasize education and medical care and deemphasize antibody testing. For adolescents known to be infected with HIV, we recommend no restrictions on access to educational or treatment programs except when their health providers recommend such restrictions to protect them from exposure to opportunistic infections. For adolescents of unknown antibody status with a possible previous exposure to HIV, we recommend that as long as the incidence of HIV infection and clinical AIDS remains low, there should be no restrictions on residential placements and no routine antibody testing. 相似文献
8.
Oxidative stress responses were tested in the unicellular cyanobacterium Synechococcus PCC 7942 (R2). Cells were exposed to hydrogen peroxide, cumene hydroperoxide and high light intensities. Activities of ascorbate peroxidase and catalase were correlated with the extent and time-course of oxidative stresses. Ascorbate peroxidase was found to be the major enzyme involved in the removal of hydrogen peroxide under the tested oxidative stresses. Catalase activity was inhibited in cells treated with high H2O2 concentrations, and was not induced under photo-oxidative stress. Regeneration of ascorbate in peroxide-treated cells was found to involve mainly monodehydroascorbate reductase and to a lesser extent dehydroascorbate reductase. The induction of the antioxidative enzymes was dependent on light and was inhibited by chloramphenicol. Peroxide treatment was found to induce the synthesis of eight proteins, four of which were also induced by heat shock.Abbreviations ASC
ascorbate
- DHA
dehydroascorbate
- MDA
monodehydroascorbate
- GSH
reduced glutathione
- GSSG
oxidized glutathione
- ASC Per
ascorbate peroxidase
- DHA red.
dehydroascorbate reductase
- MDA red.
monodehydroascorbate reductase
- GSSG red.
glutathione reductase
- HSP
heat shock proteins
- PSP
peroxide shock proteins
- Cm
chloramphenicol 相似文献
9.
The mechanism of resistance to gentamicin and tobramycin in a clinical isolate ofAcinetobacter baumannii, in which aminoglycoside-modifying enzymes were not detected, was investigated. For increase of the resistance gene product, DNA prepared from theA. baumannii isolate was cloned into pUC18 and introduced intoEscherichia coli by transformation. Gentamicin-resistant transformants were screened for aminoglycoside-modifying enzymes. This approach identified two genes encoding AAC(3) and AAD(2) activity, respectively. To determine whether both genes are expressed in the hostAcinetobacter strain, we extracted total cellular RNA from this strain, and Northern blots were hybridized with the cloned AAC(3) and AAD(2) structural genes. mRNA transcribed from the AAC(3) gene alone was detected. This shows that cloning a functional resistance gene is not sufficient in itself to investigate mechanisms of resistance in bacterial strains without detectable aminoglycoside-modifying activity. Furthermore, this study suggests a potential limitation of antibiotic resistance gene probes for studying mechanisms of resistance. 相似文献
10.
Phosphorylation of insulin-like growth factor I receptor by insulin receptor tyrosine kinase in intact cultured skeletal muscle cells 总被引:2,自引:0,他引:2
The interaction between insulin and insulin-like growth factor I (IGF I) receptors was examined by determining the ability of each receptor type to phosphorylate tyrosine residues on the other receptor in intact L6 skeletal muscle cells. This was made possible through a sequential immunoprecipitation method with two different antibodies that effectively separated the phosphorylated insulin and IGF I receptors. After incubation of intact L6 cells with various concentrations of insulin or IGF I in the presence of [32P]orthophosphate, insulin receptors were precipitated with one of two human polyclonal anti-insulin receptor antibodies (B2 or B9). Phosphorylated IGF I receptors remained in solution and were subsequently precipitated by anti-phosphotyrosine antibodies. The identities of the insulin and IGF I receptor beta-subunits in the two immunoprecipitates were confirmed by binding affinity, by phosphopeptide mapping after trypsin digestion, and by the distinct patterns of expression of the two receptors during differentiation. Stimulated phosphorylation of the beta-subunit of the insulin receptor correlated with occupancy of the beta-subunit of the insulin receptor by either insulin or IGF I as determined by affinity cross-linking. Similarly, stimulation of phosphorylation of the beta-subunit of the IGF I receptor by IGF I correlated with IGF I receptor occupancy. In contrast, insulin stimulated phosphorylation of the beta-subunit of the IGF I receptor at hormone concentrations that were associated with significant occupancy of the insulin receptor but negligible IGF I receptor occupancy. These findings indicate that the IGF I receptor can be a substrate for the hormone-activated insulin receptor tyrosine kinase activity in intact L6 skeletal muscle cells. 相似文献