全文获取类型
收费全文 | 1058766篇 |
免费 | 122002篇 |
国内免费 | 600篇 |
专业分类
1181368篇 |
出版年
2018年 | 8611篇 |
2016年 | 12359篇 |
2015年 | 18336篇 |
2014年 | 21156篇 |
2013年 | 29355篇 |
2012年 | 33728篇 |
2011年 | 33729篇 |
2010年 | 22789篇 |
2009年 | 21326篇 |
2008年 | 30321篇 |
2007年 | 31314篇 |
2006年 | 29422篇 |
2005年 | 28240篇 |
2004年 | 27791篇 |
2003年 | 26773篇 |
2002年 | 26301篇 |
2001年 | 46014篇 |
2000年 | 46336篇 |
1999年 | 37269篇 |
1998年 | 14170篇 |
1997年 | 14654篇 |
1996年 | 14013篇 |
1995年 | 13178篇 |
1994年 | 13073篇 |
1993年 | 12847篇 |
1992年 | 31585篇 |
1991年 | 30672篇 |
1990年 | 29956篇 |
1989年 | 29060篇 |
1988年 | 26847篇 |
1987年 | 26019篇 |
1986年 | 24014篇 |
1985年 | 24015篇 |
1984年 | 19993篇 |
1983年 | 17401篇 |
1982年 | 13748篇 |
1981年 | 12243篇 |
1980年 | 11621篇 |
1979年 | 19207篇 |
1978年 | 15266篇 |
1977年 | 13864篇 |
1976年 | 12914篇 |
1975年 | 14197篇 |
1974年 | 15185篇 |
1973年 | 14927篇 |
1972年 | 13482篇 |
1971年 | 12273篇 |
1970年 | 10720篇 |
1969年 | 10111篇 |
1968年 | 9029篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
882.
Myocardial infarction in diabetics is often accompanied by poor diabetic control. An assessment of a low-dose insulin infusion regimen in 26 diabetic patients after myocardial infarction found this system to be simple, effective, and safe. 相似文献
883.
J. C. O’Hanlon 《Journal of Ethology》2011,29(1):47-54
Tree trunks are spatially separated habitat patches for the tree-running mantid Ciulfina biseriata (Mantodea: Liturgusidae). This paper investigates how intraspecific interactions may affect spacing patterns in C. biseriata. Many mantid species inhabit tree trunk surfaces, but little is known about how they interact with this type of habitat.
Behavioural observations and choice tests investigated the effect of a conspecific on habitat utilisation. Field surveys were
used to investigate patterns of movement in the field. The decision to inhabit a tree trunk may be affected by the presence
of a conspecific. Whereas adult males seek out females when selecting habitat, females will actively avoid males. Habitat
choice decisions are primarily based on aspects of their mating system rather than on competition for space or food resources.
The behavioural interactions of C. biseriata are described, including novel observations of same-sex mating behaviour in adult males. 相似文献
884.
Six acapsular strains ofCryptococcus neoformans obtained by chemical mutagenesis failed to produce a capsulein vivo and were avirulent in mice following high dose intramuscular, intraperitoneal or intravenous inoculation. Peritoneal granulomas were observed in all animals inoculated with the acapsular mutants. These granulomas were characterized by a large central mass consisting of intact, degenerating and necrotic yeast cells. This was surrounded by concentric layers of a broad band of histiocytes, a narrow band of fibroblasts, and around the periphery, a mass of lymphocytes and plasma cells. These isolates did not revert to an encapsulated or virulent state after more than a year of subculturing or 18 passages through mice. 相似文献
885.
886.
In Escherichia coli, two enzymes catalyze the synthesis of methionine from homocysteine using methyltetrahydrofolate as the donor of the required methyl group: cobalamin-dependent and cobalamin-independent methionine synthases. Comparison of the mechanisms of these two enzymes offers the opportunity to examine two different solutions to the same chemical problem. We initiated the research described here to determine whether the two enzymes were evolutionarily related by comparing the deduced amino acid sequences of the two proteins. We have determined the nucleotide sequence for the metE gene, encoding the cobalamin-independent methionine synthase. Our results reveal an absence of similarity between the deduced amino acid sequences of the cobalamin-dependent and cobalamin-independent proteins and suggest that the two have arisen by convergent evolution. We have developed a rapid one-step purification of the recombinant cobalamin-independent methionine synthase (MetE) that yields homogeneous protein in high yield for mechanistic and structural studies. In the course of these studies, we identified a highly reactive thiol in MetE that is alkylated by chloromethyl ketones and by iodoacetamide. We demonstrated that alkylation of this residue, shown to be cysteine 726, results in complete loss of activity. While we are unable to deduce the role of cysteine 726 in catalysis at this time, the identification of this reactive residue suggests the possibility that this thiol functions as an intermediate methyl acceptor in catalysis, analogous to the role of cobalamin in the reaction catalyzed by the cobalamin-dependent enzyme. 相似文献
887.
Indomethacin inhibition of ovulation in the cow 总被引:1,自引:0,他引:1
Indomethacin or saline was administered via intramuscular, intrauterine or intraovarian routes to dairy cows, within 24 h after standing oestrus was first observed. The incidence of ovulation was determined at slaughter. All of the saline-treated cows (18/18) ovulated. Ovulation was not blocked after intramuscular injection (0/6) or intrauterine infusion (0/6) of indomethacin. In all cows, ovulation was blocked after intraovarian injection (6/6) of indomethacin. These findings add support to the hypothesis that prostaglandins play an essential role in ovulation in the cow as in many other mammalian species. 相似文献
888.
The physical and covalent binding of the carcinogen benzo(a)pyrene-7,8-diol-9,10-oxide (BaPDE) to poly(dG-dC).(dG-dC) and poly(dG-m5dC).(dG-m5dC) in the B and Z forms were studied utilizing absorbance, fluorescence and linear dichroism techniques. In the case of poly(dG-dC).(dG-dC) the decrease in the covalent binding of BaPDE with increasing NaCl concentration (0.1-4 M) as the B form is transformed to the Z form is attributed to the effects of high ionic strengths on the reactivity and physical binding of BaPDE to the polynucleotides; these effects tend to obscure differences in reactivities with the B and Z forms of the nucleic acids. In the case of poly(dG-m5dC).(dG-m5dC) the B-to-Z transition is induced at low ionic strength (2 mM NaCl + 10 microM Co(NH3)6Cl3) and the covalent binding is found to be 2-3-times lower to the Z form than to the B form. Physical binding of BaPDE by intercalation, which precedes the covalent binding reaction, is significantly lower in the Z form than in the B form, thus accounting, in part, for the lower covalent binding. The linear dichroism characteristics of BaPDE covalently bound to the Z and B forms of poly(dG-m5dC).(dG-m5dC) are consistent with nonintercalative, probably external conformations of the aromatic pyrenyl residues. 相似文献
889.
Changes in protein phosphorylation during the cell cycle of Chinese hamster ovary cells 总被引:10,自引:0,他引:10
The phosphorylation patterns of proteins were examined during the cell cycle of Chinese hamster ovary cells. This was accomplished by labeling synchronized cells at various times with [32P]orthophosphate and separating the proteins by both isoelectric focusing and nonequilibrium pH gradient two-dimensional gel electrophoresis. The most dramatic changes occurred during late G2/M when approximately eight proteins (including vimentin, lamin B, and histones 1 and 3) showed increased phosphorylation. Ten other proteins appeared to be uniquely phosphorylated during late G2/M. Of these 10 proteins, seven were no longer phosphorylated shortly after mitosis. There is also at least one protein which showed a relative decrease in phosphorylation during late G2/M. 相似文献
890.
Structural difference of the insulin receptors from circulating monocytes and erythrocytes 总被引:1,自引:0,他引:1
A McElduff R J Comi G Grunberger 《Biochemical and biophysical research communications》1985,133(3):1175-1180
We compared insulin receptors obtained from cells widely used in human studies, the circulating monocytes and erythrocytes. Biochemically, these receptors possess both binding (alpha-subunit) and tyrosine kinase (beta-subunit) activities similar to insulin receptors from other sources. Subtle differences in molecular weight, however, were detected between the alpha-subunits of these two cell types when analyzed by NaDodSO4-PAGE. Crosslinked [125I]insulin-labeled alpha-subunit of the monocyte insulin receptor was of higher apparent molecular weight than the alpha-subunit derived from red cells. Neuraminidase treatment of the alpha-subunits from each cell type indicated more sialic acid residues were present on the monocyte than the red cell alpha-subunit. The structural properties of the insulin receptors of human circulating cells are similar but not identical to insulin receptors of other characterized systems. 相似文献