全文获取类型
收费全文 | 4730篇 |
免费 | 367篇 |
专业分类
5097篇 |
出版年
2023年 | 26篇 |
2022年 | 43篇 |
2021年 | 80篇 |
2020年 | 53篇 |
2019年 | 63篇 |
2018年 | 73篇 |
2017年 | 80篇 |
2016年 | 118篇 |
2015年 | 221篇 |
2014年 | 246篇 |
2013年 | 255篇 |
2012年 | 360篇 |
2011年 | 340篇 |
2010年 | 252篇 |
2009年 | 196篇 |
2008年 | 278篇 |
2007年 | 258篇 |
2006年 | 253篇 |
2005年 | 240篇 |
2004年 | 241篇 |
2003年 | 222篇 |
2002年 | 234篇 |
2001年 | 50篇 |
2000年 | 41篇 |
1999年 | 58篇 |
1998年 | 64篇 |
1997年 | 45篇 |
1996年 | 47篇 |
1995年 | 35篇 |
1994年 | 36篇 |
1993年 | 44篇 |
1992年 | 36篇 |
1991年 | 37篇 |
1990年 | 22篇 |
1989年 | 45篇 |
1988年 | 33篇 |
1987年 | 20篇 |
1986年 | 25篇 |
1985年 | 16篇 |
1984年 | 25篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 16篇 |
1978年 | 20篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1974年 | 20篇 |
1973年 | 15篇 |
排序方式: 共有5097条查询结果,搜索用时 15 毫秒
101.
102.
103.
Binding of nogalamycin and adriamycin with Sarcoma-180 ascites tumor cell chromatin was studied by a spectrofluorometric method. There was significant reduction in the number of available drug binding sites per nucleotide when the chromatin was digested with DNase I for a period which releases only 7% of the chromosomal DNA. Results indicate preferential binding of these drugs with DNase I hypersensitive sites of chromatin. The DNase-I hypersensitive sites of chromatin were shown to correlate to the sequences required for gene expression. Further digestion with DNase I increases availability of drug binding sites, probably due to relaxation of the compact chromatin. 相似文献
104.
Crystal structure of a secreted insect ferritin reveals a symmetrical arrangement of heavy and light chains 总被引:2,自引:0,他引:2
Hamburger AE West AP Hamburger ZA Hamburger P Bjorkman PJ 《Journal of molecular biology》2005,349(3):558-569
Ferritins are iron storage proteins made of 24 subunits forming a hollow spherical shell. Vertebrate ferritins contain varying ratios of heavy (H) and light (L) chains; however, known ferritin structures include only one type of chain and have octahedral symmetry. Here, we report the 1.9A structure of a secreted insect ferritin from Trichoplusia ni, which reveals equal numbers of H and L chains arranged with tetrahedral symmetry. The H/L-chain interface includes complementary features responsible for ordered assembly of the subunits. The H chain contains a ferroxidase active site resembling that of vertebrate H chains with an endogenous, bound iron atom. The L chain lacks the residues that form a putative iron core nucleation site in vertebrate L chains. Instead, a possible nucleation site is observed at the L chain 3-fold pore. The structure also reveals inter- and intrasubunit disulfide bonds, mostly in the extended N-terminal regions unique to insect ferritins. The symmetrical arrangement of H and L chains and the disulfide crosslinks reflect adaptations of insect ferritin to its role as a secreted protein. 相似文献
105.
Leila Rubia Latha Rangan Rimjhim Roy Choudhury Miroslav Kamínek Petre Dobrev Jiri Malbeck Mark Fowler Adrian Slater Nigel Scott John Bennett Shaobing Peng Gurdev S. Khush Malcolm Elliott 《Journal of Plant Growth Regulation》2014,33(1):66-76
This paper reports the ways that the differences in leaf senescence are related to grain filling, grain yield, and the dynamics of cytokinins (CKs) in the top three leaves of four field-grown new plant type (NPT) rice, a tropical japonica developed at the International Rice Research Institute, Philippines, to increase the yield potential of rice. The chlorophyll content in leaves decreased from flowering to maturity in all the NPT lines, whereas the grain filling percentage was higher in the fast-senescing NPT line than in slow-senescing NPT line. Grain yield was positively correlated with senescence in the flag leaf. Rapid changes in the CK levels were recorded in the leaves of the fast-senescing line, whereas the CK levels were relatively stable in leaves of the slow-senescing line, suggesting that the dynamics of CKs in the fast-senescing line are vital for fast senescence. There were no significant changes in bioactive CKs, CK O-glucosides (storage CKs), and cis-zeatin derivatives in different leaves of the slow-senescing NPT line between 0 and 3 weeks after flowering, suggesting that the content of these CKs is relatively stable during grain filling. A progressive increase in levels of bioactive CKs was positively correlated with gradual accumulation of CK N-glucosides (inactive CKs) in the top three leaves of the slow-senescing NPT line, whereas the decrease of bioactive CKs in the flag leaf of the fast-senescing line was accompanied by accumulation of CK O-glucosides. These results suggest that there is a higher rate of biosynthesis and/or import of bioactive CKs as well as their turnover which may favor delay of leaf senescence in the slow-senescing NPT line. 相似文献
106.
The introduction of the predatory Nile perch, Lates niloticus, into the Lake Victoria basin coincided with a dramatic decline in fish species richness and diversity. This study focused on interactions between Nile perch and indigenous fishes in Lake Nabugabo, Uganda, a small satellite lake of Lake Victoria. We evaluated how the foraging impact of juvenile Nile perch on prey fishes varied with the size of the predator. We also evaluated the role of wetland ecotones in minimizing interaction between Nile perch and indigenous fishes. Wetland ecotones in Lake Nabugabo were characterized by complex structure (e.g., dense vegetation) and lower dissolved oxygen levels than non-wetland (exposed) areas. Nile perch (8.6–42.2cm, TL) were 3.7 times more abundant in offshore exposed areas than in inshore areas near wetland ecotones, and the proportion of Nile perch using wetland and exposed areas was independent of their body size. However, species richness was higher in waters at wetland ecotones than in exposed areas. Nile perch (5–35cm, TL) exhibited a shift in diet at approximately 30cm TL from feeding primarily on invertebrates to piscivory. Although the shift to piscivory occurred at approximately the same body size for Nile perch from both wetland and exposed habitats, the shift to piscivory was less abrupt in Nile perch captured near wetland ecotones. Nile perch from wetland areas consumed a greater diversity and a larger percentage of fish prey than those from exposed sites. However, the low abundance of Nile perch in wetland ecotones suggested that interaction between predator and prey in these areas is much reduced. 相似文献
107.
Lactic acid extraction was carried out with trioctyl amine (TOA) in three diluents. The effect of initial lactic acid concentrations on the extraction efficiency was investigated. It was observed that although the percentage extraction remained constant or decreased but the loading ratio was increased in all the cases. The overloading was observed in the case of TOA in methyl isobutyl ketone (MIBK). The extraction of lactic acid was favored at a lower aqueous pH?in all the diluents. The improvement of the extraction efficiency at a higher aqueous pH?(=?6) was achieved by using the modified TOA (treated with HCl) in MIBK. However, the recovery of lactic was very poor in the case of modified TOA in MIBK, although the complete recovery was obtained for untreated TOA. 相似文献
108.
Identification of suitable endogenous control genes for microRNA gene expression analysis in human breast cancer 总被引:2,自引:0,他引:2
Pamela A Davoren Roisin E McNeill Aoife J Lowery Michael J Kerin Nicola Miller 《BMC molecular biology》2008,9(1):76
The discovery of microRNAs (miRNAs) added an extra level of intricacy to the already complex system regulating gene expression.
These single-stranded RNA molecules, 18–25 nucleotides in length, negatively regulate gene expression through translational
inhibition or mRNA cleavage. The discovery that aberrant expression of specific miRNAs contributes to human disease has fueled
much interest in profiling the expression of these molecules. Real-time quantitative PCR (RQ-PCR) is a sensitive and reproducible
gene expression quantitation technique which is now being used to profile miRNA expression in cells and tissues. To correct
for systematic variables such as amount of starting template, RNA quality and enzymatic efficiencies, RQ-PCR data is commonly
normalised to an endogenous control (EC) gene, which ideally, is stably-expressed across the test sample set. A universal
endogenous control suitable for every tissue type, treatment and disease stage has not been identified and is unlikely to
exist, so, to avoid introducing further error in the quantification of expression data it is necessary that candidate ECs
be validated in the samples of interest. While ECs have been validated for quantification of mRNA expression in various experimental
settings, to date there is no report of the validation of miRNA ECs for expression profiling in breast tissue. In this study,
the expression of five miRNA genes (let-7a, miR-10b, miR-16, miR-21 and miR-26b) and three small nucleolar RNA genes (RNU19, RNU48 and Z30) was examined across malignant, benign and normal breast tissues to determine the most appropriate normalisation strategy.
This is the first study to identify reliable ECs for analysis of miRNA by RQ-PCR in human breast tissue. 相似文献
109.
Sprague ER Reinhard H Cheung EJ Farley AH Trujillo RD Hengel H Bjorkman PJ 《Journal of virology》2008,82(7):3490-3499
Recognition of immunoglobulin G (IgG) by surface receptors for the Fc domain of immunoglobulin G (Fcgamma), FcgammaRs, can trigger both humoral and cellular immune responses. Two human cytomegalovirus (HCMV)-encoded type I transmembrane receptors with Fcgamma-binding properties (vFcgammaRs), gp34 and gp68, have been identified on the surface of HCMV-infected cells and are assumed to confer protection against IgG-mediated immunity. Here we show that Fcgamma recognition by both vFcgammaRs occurs independently of N-linked glycosylation of Fcgamma, in contrast with the properties of host FcgammaRs. To gain further insight into the interaction with Fcgamma, truncation mutants of the vFcgammaR gp68 ectodomain were probed for Fcgamma binding, resulting in localization of the Fcgamma binding site on gp68 to residues 71 to 289, a region including an immunoglobulin-like domain. Gel filtration and biosensor binding experiments revealed that, unlike host FcgammaRs but similar to the herpes simplex virus type 1 (HSV-1) Fc receptor gE-gI, gp68 binds to the C(H)2-C(H)3 interdomain interface of the Fcgamma dimer with a nanomolar affinity and a 2:1 stoichiometry. Unlike gE-gI, which binds Fcgamma at the slightly basic pH of the extracellular milieu but not at the acidic pH of endosomes, the gp68/Fcgamma complex is stable at pH values from 5.6 to pH 8.1. These data indicate that the mechanistic details of Fc binding by HCMV gp68 differ from those of host FcgammaRs and from that of HSV-1 gE-gI, suggesting distinct functional and recognition properties. 相似文献
110.
DnaG is the primase that lays down RNA primers on single-stranded DNA during bacterial DNA replication. The solution structure of the DnaB-helicase-binding C-terminal domain of Escherichia coli DnaG was determined by NMR spectroscopy at near-neutral pH. The structure is a rare fold that, besides occurring in DnaG C-terminal domains, has been described only for the N-terminal domain of DnaB. The C-terminal helix hairpin present in the DnaG C-terminal domain, however, is either less stable or absent in DnaB, as evidenced by high mobility of the C-terminal 35 residues in a construct comprising residues 1-171. The present structure identifies the previous crystal structure of the E. coli DnaG C-terminal domain as a domain-swapped dimer. It is also significantly different from the NMR structure reported for the corresponding domain of DnaG from the thermophile Bacillus stearothermophilus. NMR experiments showed that the DnaG C-terminal domain does not bind to residues 1-171 of the E. coli DnaB helicase with significant affinity. 相似文献