全文获取类型
收费全文 | 4047篇 |
免费 | 464篇 |
国内免费 | 5篇 |
出版年
2021年 | 48篇 |
2020年 | 31篇 |
2019年 | 45篇 |
2018年 | 49篇 |
2017年 | 60篇 |
2016年 | 61篇 |
2015年 | 133篇 |
2014年 | 161篇 |
2013年 | 162篇 |
2012年 | 205篇 |
2011年 | 220篇 |
2010年 | 129篇 |
2009年 | 113篇 |
2008年 | 176篇 |
2007年 | 162篇 |
2006年 | 178篇 |
2005年 | 143篇 |
2004年 | 157篇 |
2003年 | 137篇 |
2002年 | 147篇 |
2001年 | 112篇 |
2000年 | 128篇 |
1999年 | 98篇 |
1998年 | 64篇 |
1997年 | 42篇 |
1996年 | 45篇 |
1995年 | 44篇 |
1994年 | 42篇 |
1992年 | 83篇 |
1991年 | 78篇 |
1990年 | 75篇 |
1989年 | 54篇 |
1988年 | 64篇 |
1987年 | 63篇 |
1986年 | 53篇 |
1985年 | 68篇 |
1984年 | 61篇 |
1983年 | 48篇 |
1982年 | 40篇 |
1981年 | 33篇 |
1980年 | 43篇 |
1979年 | 62篇 |
1978年 | 30篇 |
1977年 | 43篇 |
1976年 | 31篇 |
1975年 | 37篇 |
1974年 | 47篇 |
1973年 | 44篇 |
1971年 | 29篇 |
1967年 | 34篇 |
排序方式: 共有4516条查询结果,搜索用时 31 毫秒
21.
Structural studies of the capsular polysaccharide from Haemophilus pleuropneumoniae serotype 2 总被引:3,自引:0,他引:3
The capsular polysaccharide of Haemophilus pleuropneumoniae serotype 2 (ATCC 27089) is composed of D-glucose (two parts), D-galactose (one part), glycerol (one part), and phosphate (one part). Hydrolysis, dephosphorylation, methylation, enzymic studies, and 1H and 13C nuclear magnetic resonance experiments showed that the polysaccharide is a high molecular weight polymer of a tetrasaccharide repeating units, linked by monophosphate diester and having the following structure: (Formula: see text). 相似文献
22.
23.
Structural elucidation of the Brucella melitensis M antigen by high-resolution NMR at 500 MHz 总被引:7,自引:0,他引:7
The Brucella M antigen from the species type strain Brucella melitensis 16M has been identified as a component of the cell wall lipopolysaccharide (LPS). O polysaccharide liberated from this LPS by mild acid hydrolysis exhibited M activity in serological tests and was shown to be a homopolymer of 4-formamido-4,6-dideoxy-alpha-D-mannopyranosyl residues arranged in an oligosaccharide repeating unit as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the native lipopolysaccharide. Structural analysis of the O polysaccharide by NMR methods was difficult due to apparent microheterogeneity of the repeating unit, which was in fact caused by the presence of rotational isomers of the N-formyl moiety. This problem was resolved by chemical modification of the polysaccharide to its amino and N-acetyl derivatives, the 500-MHz 1H and 125-MHz 13C NMR spectra of which could be analyzed in terms of a unique structure through application of pH-dependent beta-shifts and two-dimensional techniques that included COSY, relayed COSY, and NOESY experiments together with heteronuclear C/H shift correlation spectroscopy. On the basis of these experiments and supported by methylation and periodate oxidation data, the structure of the M polysaccharide was determined as a linear polymer of unbranched pentasaccharide repeating units consisting of four 1,2-linked and one 1,3-linked 4,6-dideoxy-4-formamido-alpha-D-mannopyranosyl residues. The marked structural similarity of the M antigen and the A antigen, which is known to be a 1,2-linked homopolysaccharide of 4,6-dideoxy-4-formamido-alpha-D-mannopyranosyl units, accounts for cross-serological reactions of the two and the long-standing confusion surrounding the nature of their antigenic determinants. Structural and serological considerations in conjuction with the sodium dodecyl sulfate banding pattern of Brucella A LPS suggest that its biosynthesis differs appreciably from that of the M antigen, which appears to be synthesized by regulated assembly of preformed oligosaccharide repeating units. Temperature, lysogenic phage may be responsible for such biosynthetic and structural variations. 相似文献
24.
25.
Actinomycetes were isolated from the upper 1 - 3 cm of the soil layer in a well-developed forest and in an adjacent clearcut
area where Douglas-fir [Pseudotsuga menziesii (MIRB.) Franco] regeneration had been impaired for two decades. The population density in the clearcut area was two times
as high as that in the forested area. The percentage of actinomycetes that inhibited seed germination of the test plants was
significantly higher in isolates obtained from the clearcut area than in those obtained from the forested area, and isolates
from the clearcut showed five times the phytotoxic effect of those from the forest. Some actinomycete isolates, 4 % from the
clearcut and 2.6 % from the forest, significantly reduced in vitro growth of two common ectomycorrhizal fungi of Douglas-fir,Laccaria laccata andHebeloma ovstuliniforme. Two actinomycete isolates from the clearcut reduced fungal growth by 40 % and 73 %. Reduction of the nutrient in the growth
medium did not affect the antifungal activity of the actinomycetes. The data support the idea that, along with other factors,
phytotoxic and antifungal actinomycetes may suppress natural regeneration or establishment of planted seedlings - either directly
or. indirectly - through inhibition of seed germination or of mycorrhizal fungi. 相似文献
26.
Ronald J. Hauptman Brian A. Perry David G. Capco 《Development, growth & differentiation》1989,31(2):157-164
Proteins of the detergent-resistant cytoskeleton fraction and the detergent-soluble fraction from Xenopus oocytes and embryos are examined using a procedure which allows rapid and uniform extraction of tissues and large, single cells. SDS-polyacrylamide gels reveal only a few prominent cytoskeletal proteins in the early embryo, however qualitatively different proteins begin to appear after gastrulation. Incorporation of [35 S]-methionine into newly synthesized proteins indicates that there is synthesis and assembly of proteins into the cytoskeleton, but the amount remains low until after gastrulation. The use of nucleic acid probes for alpha-tubulin and actin mRNA indicates that about 80% of these mRNAs in the oocyte and meiotically mature egg are bound to the detergent-resistant cytoskeleton. 相似文献
27.
Soluble derivatives of the beta amyloid protein precursor of Alzheimer's disease are labeled by antisera to the beta amyloid protein 总被引:6,自引:0,他引:6
M R Palmert S L Siedlak M B Podlisny B Greenberg E R Shelton H W Chan M Usiak D J Selkoe G Perry S G Younkin 《Biochemical and biophysical research communications》1989,165(1):182-188
The amyloid deposited in Alzheimer's disease (AD) is composed primarily of a 39-42 residue polypeptide (beta AP) that is derived from a larger beta amyloid protein precursor (beta APP). In previous studies, we and others identified full-length, membrane-associated forms of the beta APP and showed that these forms are processed into soluble derivatives that lack the carboxyl-terminus of the full-length forms. In this report, we demonstrate that the soluble approximately 125 and approximately 105 kDa forms of the beta APP found in human cerebrospinal fluid are specifically labeled by several different antisera to the beta AP. This finding indicates that both soluble derivatives contain all or part of the beta AP sequence, and it suggests that one or both of these forms may be the immediate precursor of the amyloid deposited in AD. 相似文献
28.
The combined effects of chlorine and ammonia on litter breakdown in outdoor experimental streams 总被引:2,自引:0,他引:2
The response of Potamogeton crispus L. breakdown to controlled doses of different levels of chlorine and chlorine + ammonia was investigated over two years in
outdoor experimental streams. In 1985, downstream riffles of 2 streams were dosed (observed in-stream concentrations) at ca.
10 μg/L Total Residual Chlorine (TRC), one stream at 64 μg/L TRC and one stream at 230 μg/L TRC. Two control streams were
not dosed and the upstream riffles of each stream served as within stream controls. In 1986, the downstream riffle of one
stream was dosed at 70 μg/L TRC and a second stream was dosed at 200 μg/L TRC. Four streams were also dosed with 2.5 mg/L
NH3-N: one stream with no chlorine, one stream with ca. 10 μg/L TRC, one with 56 μg/L TRC, and one with 150 μg/L TRC. A seventh
stream was dosed for 2 h at 2000 μg/L TRC and 2.5 mg/L ammonia and then allowed to recover (recovery stream). Each year, litter
decomposition (degree day k values) was measured during two 35 day trials (Jun–Jul and Aug–Sep). In 1985, when streams were dosed with chlorine alone,
decomposition was significantly reduced with the high (230 μg/L TRC) chlorine dose. Downstream decomposition was 27% (Jun–Jul)
and 59% (Aug–Sep) of the upstream (control) rate. No other chlorine effects were found during this period. In Jun–Jul 1986,
there was significantly lower decomposition in the downstream dosed sites of the 200 μg/L TRC alone stream, the 146 μg/L TRC
+ ammonia stream and the recovery stream; downstream decay rates were (respectively) 56%, 42% and 64% of the upstream control
sites. No other up-down pairs were different in July 1986. In Aug–Sep, all three streams with chlorine + ammonia (6, 56 and
146 μg/L TRC + 2,5 mg/L ammonia) and the 70 μg/L TRC alone stream had significantly lower decomposition rates in the downstream
dosed sites. For these streams, downstream decay rates ranged from 46% (high chlorine + ammonia) to 73% (low chlorine + ammonia)
of the upstream control rates. No other up-down pairs were different during this trial. Up and downstream sites of the stream
dosed with 2.5 mg/L ammonia alone were nearly identical for both trials (< 3% difference). These results indicate that TRC
at less than 250 μg/L can significantly reduce litter decomposition and strongly suggest that addition of ammonia to chlorinated
water can increase the toxic effect of chlorine.
currently at the Department of Fisheries and Wildlife
currently at the Department of Fisheries and Wildlife 相似文献
29.
We have cloned the mouse IL2 gene and sequenced 2800 bp of 5' flanking DNA. Comparison to the previously reported human sequence revealed extensive identity (approximately 86%) between the two genes from +1 to -580 with additional small islands of homology further upstream. Proximal sites which have been shown to be important in regulation of the human IL2 gene are well conserved in sequence and location. Transfection experiments using hybrid gene constructs containing varying lengths of the mouse 5' flanking DNA linked to a CAT reporter gene have demonstrated the presence of several novel positive and negative regulatory elements. One negative regulatory region lying between -750 and -1000 consists primarily of alternating purines and pyrimidines and is absent from the human gene. The conserved region from -321 and -578, an upstream segment from -1219 to -1332, and another region of approximately 450 bp from -1449 to -1890, which contained a well-conserved sequence of 60 bp, were each associated with enhanced levels of expression. We found no evidence for intragenic or downstream enhancer elements in this gene. All the elements identified affect only the magnitude of the inducible response, for no region when deleted had the effect of altering either the need for induction, the kinetics of stimulation, or the cell-type specificity of expression. Deletion studies suggest a strong requirement for NFAT binding even in the presence of extensive 5' flanking sequence. Therefore we conclude that IL2 gene expression is controlled primarily through a central TH1-specific signaling pathway, which acts through proximal elements, while distal cis-elements exert a secondary modulating effect. 相似文献
30.