全文获取类型
收费全文 | 27598篇 |
免费 | 2659篇 |
国内免费 | 22篇 |
出版年
2021年 | 457篇 |
2020年 | 323篇 |
2019年 | 403篇 |
2018年 | 588篇 |
2017年 | 493篇 |
2016年 | 809篇 |
2015年 | 1293篇 |
2014年 | 1441篇 |
2013年 | 1672篇 |
2012年 | 2104篇 |
2011年 | 1979篇 |
2010年 | 1312篇 |
2009年 | 1154篇 |
2008年 | 1581篇 |
2007年 | 1467篇 |
2006年 | 1325篇 |
2005年 | 1200篇 |
2004年 | 1232篇 |
2003年 | 1035篇 |
2002年 | 934篇 |
2001年 | 589篇 |
2000年 | 549篇 |
1999年 | 444篇 |
1998年 | 254篇 |
1997年 | 233篇 |
1996年 | 183篇 |
1995年 | 193篇 |
1994年 | 183篇 |
1993年 | 175篇 |
1992年 | 287篇 |
1991年 | 284篇 |
1990年 | 270篇 |
1989年 | 276篇 |
1988年 | 255篇 |
1987年 | 236篇 |
1986年 | 223篇 |
1985年 | 218篇 |
1984年 | 174篇 |
1983年 | 140篇 |
1982年 | 135篇 |
1981年 | 122篇 |
1980年 | 124篇 |
1979年 | 156篇 |
1978年 | 151篇 |
1977年 | 98篇 |
1976年 | 130篇 |
1975年 | 130篇 |
1974年 | 126篇 |
1973年 | 116篇 |
1972年 | 95篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
32.
The role of duplication in the expression of a variable surface glycoprotein gene of Trypanosoma brucei 总被引:1,自引:0,他引:1
The variable surface glycoprotein (VSG) genes of Trypanosoma brucei have been classified into two groups depending upon whether or not duplication of the genes is observed when they are expressed. We report here the observation of duplication apparently linked to expression of the ILTaT 1.3 gene in the ETaR 1 trypanosome stock. In the ILTaR 1 stock, expression of the ILTaT 1.3 VSG did not involve a new duplication, but instead activation of a preexisting gene copy that had been apparently generated earlier by a duplication event analogous to that directly observed in the ETaR 1 trypanosomes. The results suggest that the well-characterised gene duplications found with other VSG genes are common to all VSG genes but are not directly responsible for controlling expression. All currently available data can be accommodated by a model that assumes that gene duplication and replacement occurs independently of antigenic switching. 相似文献
33.
Andrew W. Saxe Ji-Won Yoon Phillip Gorden Murray F. Brennan 《In vitro cellular & developmental biology. Plant》1982,18(10):884-890
Summary Dispersed cells from both fresh and cryopreserved human insulinoma have been maintained in cell culture. Initial yield of
viable cells was 50% for fresh and 25% for cryopreserved tissue. Viability of cells in culture was documented by increasing
numbers of cells (doubling time approximately 5 d initially and 2 d at the sixth subculture for both fresh and cryopreserved
tissue) and continued release of insulin over time (approximately 100 ng/ml per 105 cells at 10 d and 175 ng/ml per 105 cells at 30 d of culture for both fresh and cryopreserved tissue). Evidence that cells growing in culture were beta cells
was provided by: (a) recovery of intracellular and extracellular immunoreactive insulin (IRI), (b) electron microscopic morphology,
and (c) immunohistochemical staining. Cells from fresh insulinoma incubated with increasing concentrations of extracellular
glucose released increasing amounts of IRI up to approximately 15 mM glucose, which paralleled changes in plasma insulin obtained during a preoperative glucose tolerance test.
Under an Intergovernmental Personnel Act Exchange from the Department of Surgery, University of California, Davis, Sacramento
Medical Center. 相似文献
34.
Virion conformational forms and the complex inactivation kinetics of echovirus by chlorine in water 总被引:1,自引:0,他引:1
Aberrant inactivation kinetics were observed when monodispersed echovirus type 1 (Farouk) was inactivated with chlorine. An initial 1- to 2-log10-unit decrease in titer was followed by lag period, during which the titer stayed the same or increased, and this was followed by a final decline in titer. First-order kinetics were obtained with poliovirus type 1 under the same conditions. Isoelectric focusing studies of echovirus before chlorine treatment showed that the virus distributed into two pH-dependent and interconvertible isoelectric forms. After chlorine treatment all remaining virus infectivity was associated with a third pH-independent isoelectric form. The complex inactivation kinetics appeared to be due to shifts between these conformational forms during inactivation in certain ionic environments. Under certain conditions the conformational shifts resulted in substantial resistance of monodispersed echovirus to chlorine. 相似文献
35.
Young J. Oh Jonathan W. Francis George J. Markelonis Tae H. Oh 《Journal of neurochemistry》1992,58(6):2131-2138
Peripheral-type benzodiazepine binding sites (PTBBS) are markedly increased in the injured CNS. Astrocytes appear to be the primary cell type which express increased PTBBS. Because certain cytokines within the injured CNS are potent mitogens for astrocytes, we examined the effects of two such cytokines, interleukin (IL)-1 beta and tumor necrosis factor (TNF), on PTBBS in cultured astrocytes using [3H]Ro 5-4864 as the specific ligand. Purified cultures of either polygonal or process-bearing astrocytes were prepared from neonatal rat cerebral hemispheres. At a concentration of 1.8 nM, specific binding of the radioactive ligand to polygonal astrocytes reached equilibrium within 60 min and was half-maximal by 5-10 min. By contrast, specific binding to process-bearing astrocytes barely exceeded background levels. IL-1 and TNF increased PTBBS within polygonal astrocytes in both dose- and time-dependent manners. At 10-50 ng/ml, IL-1 beta and TNF-alpha elevated [3H]Ro 5-4864 binding in polygonal astrocyte cultures 65 and 87%, respectively, above the level in control cultures. However, no changes in PTBBS were seen within polygonal astrocytes after IL-2 treatment. Scatchard analysis of saturation binding experiments suggested that the increase in PTBBS promoted by TNF was due to an increased number of binding sites present in polygonal astrocytes and not due to an increase in receptor affinity. Binding data suggested that PTBBS within cultures of process-bearing astrocytes were virtually absent irrespective of the treatment. These in vitro data suggest that certain cytokines found in the injured brain may be involved in up-regulating PTBBS within a particular subtype of astrocyte. 相似文献
36.
37.
Previous studies by us and others established that cell-cell adhesion is mediated by specific carbohydrate-to-carbohydrate
interaction (CCI). Those previous studies were based on various biochemical and biophysical approaches, including the use
of labeled glycosyl epitopes with fluorescent tag. However, these methods ideally require that the glycosyl epitope must be
fixed to a solid phase molecule, preferably with multivalency. The purpose of the present study is to establish a CCI process
using specific glycosyl residues conjugated to biotinylated diaminopyridine (BAP), and to observe: (i) clear occurrence of
homotypic CCI between “Os Fr.B” having 5–6 GlcNAc termini, vs. absence of such homotypic CCI between “Os Fr.1” having 2 GlcNAc termini; (ii) occurrence of heterotypic CCI between GM3
ganglioside and Os Fr.B, vs. absence of such heterotypic CCI between GM3 and Os Fr.1. Interaction between Os Fr.B-BAP conjugate and Os Fr.B-ceramide
mimetic (Os Fr.B-mCer) was demonstrated based on two experiments: (i) dose-dependent binding of Os Fr.B-BAP conjugate to polystyrene
plates coated with Os Fr.B-mCer was observed in the presence of bivalent cation, a prerequisite for all CCI processes, and
such binding was abolished by EDTA; (ii) binding between equal nanomolar Os Fr.B-BAP and Os Fr.B-mCer was inhibited by mM
concentration Os Fr.B without conjugate, in dose-dependent manner. Thus, cell adhesion processes based on homotypic CCI between
N-linked glycans having multiple GlcNAc termini, and heterotypic CCI between GM3 and such glycans, were clearly observed using
BAP conjugates of glycosyl epitopes. 相似文献
38.
39.
The amounts of cAMP and cGMP in the rat pinealocyte are regulated by norepinephrine acting through synergistic dual receptor mechanisms involving alpha 1- and beta-adrenoceptors (Vanecek, J., Sugden, D., Weller, J.L., and Klein, D.C. (1985) Endocrinology 116, 2167-2173; Sugden, L., Sugden, D., and Klein, D.C. (1986) J. Biol. Chem. 261, 11608-11612). Based on the available evidence, it appears that Ca2+-phospholipid-dependent protein kinase is involved in the alpha 1-adrenergic potentiation of beta-adrenergic stimulation of cAMP, but not in the stimulation of cGMP (Sugden, D., Vanecek, J., Klein, D.C., Thomas, T.P., and Anderson, W.B. (1985) Nature 314, 359-361). In the present study the role of protein kinase C in the adrenergic stimulation of cGMP was reinvestigated, with the purpose of determining whether protein kinase C activators would potentiate the effects of beta-adrenergic agonists on cGMP if cells were also treated with agents known to elevate intracellular free Ca2+. The protein kinase C activator 4 beta-phorbol 12-myristate 13-acetate (PMA) markedly elevated the cGMP content of beta-adrenergically stimulated pinealocytes which had also been treated with 1 microM A23187, 15 mM K+, or 1 microM ouabain. The effects of A23187 were blocked by EGTA and those of K+ were blocked by nifedipine, establishing the involvement of Ca2+. The stimulatory effects of PMA on cGMP accumulation were mimicked by other protein kinase C activators. PMA also stimulated cGMP accumulation in cells treated with cholera toxin (1 microgram/ml) and A23187 (1 microM), but not in cells treated only with cholera toxin. These results suggest that protein kinase C, which is activated in the pinealocyte by the alpha-adrenergic agonist phenylephrine, is probably involved in the adrenergic regulation of cGMP accumulation at a step distal to receptor activation. 相似文献
40.