全文获取类型
收费全文 | 800篇 |
免费 | 99篇 |
专业分类
899篇 |
出版年
2021年 | 17篇 |
2020年 | 8篇 |
2019年 | 12篇 |
2018年 | 11篇 |
2017年 | 14篇 |
2016年 | 23篇 |
2015年 | 43篇 |
2014年 | 30篇 |
2013年 | 45篇 |
2012年 | 41篇 |
2011年 | 43篇 |
2010年 | 21篇 |
2009年 | 22篇 |
2008年 | 36篇 |
2007年 | 21篇 |
2006年 | 40篇 |
2005年 | 27篇 |
2004年 | 27篇 |
2003年 | 22篇 |
2002年 | 17篇 |
2001年 | 20篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1986年 | 17篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 11篇 |
1977年 | 14篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 14篇 |
1973年 | 10篇 |
1972年 | 10篇 |
1971年 | 9篇 |
1970年 | 6篇 |
1969年 | 9篇 |
1968年 | 8篇 |
1967年 | 10篇 |
1966年 | 6篇 |
排序方式: 共有899条查询结果,搜索用时 15 毫秒
1.
Activities of Escherichia coli DNA polymerase-I were examined in the presence of the anti-tumor drug cis-diaminedichloroplatinum(II) and its inactive geometric isomer trans-diaminedichloroplatinum(II). The trans-isomer did not inhibit the enzyme activity. The anti-tumor drug, on the other hand, retarded the enzyme in its ability to extend the primer strand of DNA. Two alternative mechanisms of inhibition, covalent binding of cis-diaminedichloroplatinum(II) to the polymerase and to the template DNA, were explored. Selective preincubations of the platinum drug with the polymerase and DNA reveal that the inhibition is primarily due to covalent binding to the enzyme. The rates of inhibition were found to be first order in enzyme and zeroth order in platinum in the concentration range 0.05-3.0 mM. A mechanism that deals with the formation of an initial platinum-polymerase-I complex with a binding constant > 10(5) M(-1) followed by a further reaction to form an inhibitory complex is consistent with the kinetic data. The rate limiting first order rate constant for the formation of the inhibitory complex is comparable to that observed for the thiol coordination of peptides containing cysteine residues. Analyses of known structures and functions of catalytic domains of various polymerases point to the direction that the inhibition is perhaps due to the distortion of the DNA binding domain of the enzyme due to platinum coordination. 相似文献
2.
3.
Reticuloendotheliosis virus strain T (REV-T)-transformed cells gave rise spontaneously to variants which secrete a factor that forms a distinct visible ring of precipitation (halo) surrounding colonies grown in soft agar. An Mr 15,000 protein was produced at higher levels by halo variants than by nonhalo-producing cells. An assay designed to detect the formation of precipitates enabled purification of an Mr 15,000 protein, p15, from serum-free medium conditioned by the growth of REV-T-transformed hematopoietic cells. Fractions enriched in p15 permitted the growth of REV-T-transformed cells under conditions where they normally failed to proliferate. 相似文献
4.
Adult male Wistar rats were exposed to methylisocyanate (MIC, 3.2 mg/l, single inhalation exposure for 8 min under static condition) or ethyl methanesulphonate (EMS, 150 mg/kg, single ip dose) for the assessment of germ cell mutagenicity and reproductive effects. Sequential matings of treated males with normal females on days 1-7, 8-14 and 15-21 post-exposure did not indicate any induction of dominant lethal mutation (increased frequency of preimplantation losses and early fetal deaths) by MIC but it was significantly induced by EMS as compared to respective controls. Males, necropsied after 21 days of exposure, showed no effect of MIC on epididymal sperm density and morphology. EMS also had no effect on sperm density but it significantly induced morphological abnormalities in sperm as compared to untreated controls. There was an acute and transitional reduction in reproductive performance (10-21%) of MIC-exposed males during days 1-14 post-exposure followed by recovery to the normal level during days 15-21 post-exposure. The progeny of MIC-exposed males was also normal in terms of litter size, litter weight, neonatal survival and body weight gain in litters up to 10 days post-partem. It is concluded with the evidence at hand that the observed failure of MIC to cause germ cell mutagenicity is related to its poor biodistribution to the target site(s) and a transient reduction in the reproductive performance of MIC-exposed males is a result of general stress and disconsolate copulation. 相似文献
5.
6.
Plasmid content of 5 hepatotoxin and 2 neurotoxin producing cyanobacterial strains were analyzed. Among the hepatotoxin-producing strains, Microcystis aeruginosa PCC7820, M. aeruginosa M228 and M. aeruginosa UV027 were found to carry plasmids, whereas other hepatotoxin and neurotoxin producing strains did not harbor any plasmids. Correlations were sought between toxicity and the presence of plasmids in toxic cyanobacteria as a function of age. Aged cultures of M. aeruginosa PCC7820 exhibited toxicity and harbored plasmids. In other cyanobacterial strains, plasmids were not detected. The data add to and support the current understanding that plasmids are probably not involved in toxin production in cyanobacteria.Author for Correspondence 相似文献
7.
Avian reticuloendotheliosis virus (REV-T) is the most virulent of all retroviruses, inducing an invariably fatal leukemia in chickens with a latent period of 7-10 days. Unlike avian cells transformed by other acutely transforming viruses, lymphoid cells transformed by REV-T are immortalized. Furthermore, in vitro derived, REV-T transformed cells which do not produce virus are tumorigenic and induce lethal reticuloendotheliosis when injected into histocompatible birds. Thus REV-T transforms its target cell both in vitro and in vivo. In addition this transformation is independent of any helper virus functions. Like other acute leukemia viruses, REV-T is replication-defective and must co-replicate with a reticuloendotheliosis associated virus (REV-A). During evolution, a substantial portion of its genome has been deleted and replaced with a host-derived genetic sequence, designated v-rel. Presumably, the v-rel oncogene was transduced from a normal turkey DNA locus, c-rel. There are 9 regions of homology between c-rel and v-rel, however, several differences exist between these genes, suggesting that transformation by REV-T results from the production of an altered v-rel protein. The v-rel sequence is distinct from other known oncogenes and encodes a 57-kDa phosphoprotein. In REV-T transformed cells, this pp57v-rel protein is localized in the cytoplasm. The product of the v-rel oncogene is present at a low level, representing only about 0.003% of total methionine-labelled protein. In addition, pp57v-rel is relatively stable, having an estimated half-life of 4-10 h. The v-rel protein when purified close to homogeneity is complexed with a 40-kDa cellular phosphoprotein in transformed lymphoid cells and possesses serine kinase activity. This review discusses the molecular aspects of transformation by REV-T in the context of other oncogene-encoded proteins. 相似文献
8.
Antone L. Brooks Scott W. Jordan Kallol K. Bose Jennifer Smith David C. Allison 《Cell biology and toxicology》1988,4(1):31-40
The cytogenetic and hepatotoxic effects of 2,3,7,8-tetrachlorodibenzo p-dioxin (TCDD) on mouse liver cells were investigated. Male C57BL/6J strain mice, which have TCDD receptors, were given single intraperitoneal injections of 25, 37.5, 75 and 150 g of TCDD/kg body weight or corn oil carrier alone. Two-thirds hepatectomies were carried out at 1 or 7 days after injection and chromosomal aberrations and mitotic indexes of the regenerating hepatocytes were scored 54 hr after hepatectomy. Liver sections from additional intact mice were studied for TCDD-hepatotoxicity at 1, 7 and 30 days after injection. The three high doses of TCDD caused hepatotoxicity with necrosis of liver cells and focal architectural collapse by 30 days after injection. No evidence was obtained of an increase in the frequency of chromosomal structural aberrations at doses that allowed sufficient mitotic activity for cytogenetic evaluation. We conclude that TCDD is not a clastogen for mouse hepatocytes, although high doses cause marked hepatocellular necrosis.Abbreviations CSD
chromosome deletion
- META
metacentric chromosome
- TCDD
2,3,7,8-tetrachlorobenzo-p-dioxin 相似文献
9.
Scattered light intensity fluctuation in the canine ventricular myocardium: correlation with inotropic drug effect 总被引:2,自引:0,他引:2
D Bose T Kobayashi R A Bouchard L V Hryshko 《Canadian journal of physiology and pharmacology》1988,66(9):1232-1238
Scattered light intensity fluctuation (SLIF) of coherent light by a strip of ventricular muscle during diastole is believed to be due to asynchronous cellular motion within the myocyte as a result of spontaneous release of Ca from the sacoplamic reticulum. Previous studies have shown a correlation between inotropic agents, such as ouabain and elevated extracellular Ca or decreased extracellular Na, and SLIF. The purpose of this study was to see if this correlation could be extended to other inotropic agents. The digitalis genin, ouabagenin, produces inotropy by increasing intracellular free Ca. In toxic concentrations the drug produces abnormal aftercontractions by spontaneous Ca release from the sarcoplasmic reticulum. On the other hand, the Ca channel agonist BAY k 8644 is also positively inotropic, but its effect is associated with a decrease in Ca release from the sarcoplasmic reticulum, manifested by conversion of "rest potentiation" to "rest depression." The effects of these inotropic agents on the power spectra of SLIF were dissimilar. Both frequency and amplitude of SLIF were increased after ouabagenin (1 microM), but these changes were most marked after the onset of toxicity, at which time contractility was decreased, rather than during the positive inotropic response. In contrast, BAY k 8644 (1 microM) decreased SLIF at all levels of inotropic response. The beta-adrenoceptor stimulant drug, dobutamine, and the adenylate cyclase activator, forskolin, produced minimal increase in SLIF at inotropic concentrations but caused a large increase in SLIF only after the onset of toxicity. These results suggest that SLIF is a better indicator of intracellular Ca overload and toxic oscillatory contractions in the presence of an inotrope and not of increased inotropy, per se. 相似文献
10.
We report on a study of the cytogenetic and hepatotoxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Mice of the C57B1/6J (with high-affinity TCDD receptor) or DBA/2J (with low-affinity TCDD receptor) strains were given single intraperitoneal injections of 50, 100 or 150 micrograms of TCDD/kg body weight. At various times (8-48 h) after injection, we examined bone marrow cells for cytogenetic effects by performing structural aberration, sister-chromatid exchange, and micronucleus tests. 1 month after exposure, liver sections were studied for hepatotoxic effects. We found no evidence of chromosome damage by TCDD given in doses that cause liver damage in both strains of mice. 相似文献