全文获取类型
收费全文 | 38311篇 |
免费 | 3538篇 |
国内免费 | 5235篇 |
专业分类
47084篇 |
出版年
2024年 | 122篇 |
2023年 | 509篇 |
2022年 | 1202篇 |
2021年 | 1918篇 |
2020年 | 1401篇 |
2019年 | 1795篇 |
2018年 | 1642篇 |
2017年 | 1302篇 |
2016年 | 1745篇 |
2015年 | 2538篇 |
2014年 | 3035篇 |
2013年 | 3173篇 |
2012年 | 3792篇 |
2011年 | 3479篇 |
2010年 | 2239篇 |
2009年 | 1988篇 |
2008年 | 2245篇 |
2007年 | 2039篇 |
2006年 | 1799篇 |
2005年 | 1492篇 |
2004年 | 1239篇 |
2003年 | 1128篇 |
2002年 | 980篇 |
2001年 | 614篇 |
2000年 | 548篇 |
1999年 | 495篇 |
1998年 | 317篇 |
1997年 | 279篇 |
1996年 | 248篇 |
1995年 | 205篇 |
1994年 | 193篇 |
1993年 | 135篇 |
1992年 | 179篇 |
1991年 | 137篇 |
1990年 | 106篇 |
1989年 | 110篇 |
1988年 | 86篇 |
1987年 | 59篇 |
1986年 | 69篇 |
1985年 | 86篇 |
1984年 | 39篇 |
1983年 | 47篇 |
1982年 | 36篇 |
1981年 | 32篇 |
1980年 | 25篇 |
1979年 | 29篇 |
1978年 | 23篇 |
1974年 | 20篇 |
1973年 | 29篇 |
1972年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
精制白喉毒素加0.02Mβ—丙氨酸,再加甲醛溶液经适当的时间解毒,即可转化为完全类毒化且无毒性逆转的精制白喉类毒素。此精白类的脱毒试验、毒性逆转试验、安全试验及效力试验均符合《中国生物制品规程》要求。在脱毒过程中絮状单位的损失明显低于单纯甲醛脱毒者,纯度亦相应得到了提高。 相似文献
92.
利用人粒细胞集落刺激因子(hG-CSF)cDNA3′端非翻译区(3′-UTR)中存在的DraⅠ酶切位点,通过部分酶切与完全酶切,删除3′-UTR不同长度,构建了四种hG-CSFcDNA瞬时重组表达质粒。转染COS-7细胞后,生物活性测定结果提示,hG-CSFcDNA3′-UTR对其表达起负调控作用,其关键性序列位于紧接终止密码子TGA下游的65bp范围内,3′-UTR对hG-CSFcDNA表达的影响与转录水平的差别有一定关系。 相似文献
93.
We describe a new immunoassay which combines an immunosorbent assay, Immunogold silver staining and epipolarization microscopy. Our new assay procedure features multiple samples on a single microscope slide, and high sensitivity of epipolarization microscope for detection of silver-enhanced colloidal gold as a final immunoassay product. We call the new immunoassay “on slide immunogold assay” (OSIGA). This new method uses biotinylated antibody and streptavidin-gold reaction with silver enhancement technique. With OSIGA it is possible to investigate 30 samples on a single microscopic slide. Our preliminary studies used 10-20 μ1 samples and detected nanogram quantities of a standardized protein solution. Unlike enzyme linked immunosorbent assay (ELISA), which has a limited time for reading the final color products, the OSIGA specimens can be dried or resin mounted for longer storage and future reference. 相似文献
94.
95.
Branch-point attack in group II introns is a highly reversible transesterification, providing a potential proofreading mechanism for 5'-splice site selection. 总被引:3,自引:0,他引:3 下载免费PDF全文
By examining the first step of group II intron splicing in the absence of the second step, we have found that there is an interplay of three distinct reactions at the 5'-splice site: branching, reverse branching, and hydrolytic cleavage. This approach has yielded the first kinetic parameters describing eukaryotic branching and establishes that group II intron catalysis can proceed on a rapid timescale. The efficient reversibility of the first step is due to increased conformational organization in the branched intermediate and it has several important mechanistic implications. Reversibility in the first step requires that the second step of splicing serve as a kinetic trap, thus driving splicing to completion and coordinating the first and second step of splicing. Facile reverse branching also provides the intron with a proofreading mechanism to control the fidelity of 5'-splice site selection and it provides a kinetic basis for the apparent mobility of group II introns. 相似文献
96.
E. R. Chin H. J. Green F. Grange J. D. Mercer P. J. O'Brien 《Molecular and cellular biochemistry》1994,139(1):41-52
A multiple measurement system for assessing sarcoplasmic reticulum (SR) Ca++-ATPase activity and Ca++-uptake was used to examine the effects of SR fractionation and quick freezing on rat white (WG) and red (RG) gastrocnemius muscle.In vitro measurements were performed on whole muscle homogenates (HOM) and crude microsomal fractions (CM) enriched in SR vesicles before and after quick freezing in liquid nitrogen. Isolation of the CM fraction resulted in protein yields of 0.96±0.1 and 0.99±0.1 mg/g in WG and RG, respectively. The percent Ca++-ATPase recovery for CM compared to HOM was 14.5% (WG) and 10.1% (RG). SR Ca++-activated Ca++-ATPase activity was not affected by quick freezing of HOM or CM, but basal ATPase was reduced (P<0.05) in frozen HOM (5.12±0.18–3.98±0.20 mole/g tissue/min in WG and from 5.39±0.20–4.48±0.24 mole/g tissue/min in RG). Ca++-uptake was measured at a range of physiological free [Ca++] using the Ca++ fluorescent dye Indo-1. Maximum Ca++-uptake rates when corrected for initial [Ca++]f were not altered in HOM or CM by quick freezing but uptake between 300 and 400nM free Ca++ was reduced (P<0.05) in quick frozen HOM (1.30±0.1–0.66±0.1 mole/g tissue/min in WG and 1.04±0.2–0.60±0.1 mole/g tissue/min in RG). Linear correlations between Ca++-uptake and Ca++-ATPase activity measured in the presence of the Ca++ ionophore A23187 were r=+0.25, (P<0.05) and r=+0.74 (P<0.05) in HOM and CM preparations, respectively, and were not altered by freezing. The linear relationships between HOM and CM maximum Ca++-uptake (r=+0.44, P<0.05) and between HOM and CM Ca++-ATPase activity (r=+0.34, P<0.05) were also not altered by tissue freezing. These data suggest that alterations in maximal SR Ca++-uptake function and maximal Ca++-ATPase activity may be measured in both HOM and CM fractions following freezing and short term storage. (Mol Cell Biochem139, 41–52, 1994) 相似文献
97.
98.
On protein solubility in organic solvent 总被引:1,自引:0,他引:1
Solubility of a model protein, hen egg-white lysozyme, was investigated in a wide range of neat nonaqueous solvents and binary mixtures thereof. All solvents that are protic, very hydrophilic, and polar readily dissolve more than 10 mg/mL of lysozyme (lyophilized from aqueous solution of pH 6.0). Only a marginal correlation was found between the lysozyme solubility in a non-aqueous solvent and the letter's dielectric constant or Hildebrand solubility parameter, and no correlation was observed with the dipole moment. Lysozyme dissolved in dimethyl sulfoxide (DMSO) could be precipitated by adding protein nondissolving co-solvents, although the enzyme had a tendency to form supersaturated solutions in such mixtures. The solubility of lysozyme, both in an individual solvent (1,5-pentanediol) and in binary solvent mixtures (DMSO/acetonitrile), markedly increased when the pH of the enzyme aqueous solution prior to lyophilization was moved away from the proteins's isoelectric point. (c) 1994 John Wiley & Sons, Inc. 相似文献
99.
The objective of this study was to evaluate the growth and nutrient-uptake characteristics of Fe-deficiency resistant and susceptible subclover (Trifolium subterraneum L., T. yanninicum Katzn. and Morley, T. brachcalycinum Katzn. and Morley) cultivars on a calcareous soil. Ten subclover cultivars showing varying susceptibilities to Fe-deficiency chlorosis (Karridale, Nangeela, Geraldton, Mt. Barker, Woogenellup, Larisa, Trikkala, Rosedale, Koala and Clare) were grown on a low-Fe, calcareous soil (Petrocalcic Paleustoll) under moist (18% water content, 85% of water holding capacity) and water-saturated conditions using a Cone-tainer® culture system. Chlorosis and its correlation with growth traits and mineral nutrition of the 10 cultivars were examined. The Fe-deficiency susceptibilities of the 10 cultivars decreased in the above order under the moist condition, but in slightly different order under the saturated condition. Shoot and root dry weights, total dry weight, and root-to-shoot ratio were each negatively correlated with chlorosis under both soil-moisture conditions, as was total shoot content of P, Ca, Fe, Mn and Zn. Shoot P and Fe concentrations were each positively correlated with chlorosis under the moist soil condition. Iron and Cu utilization efficiencies (biomass per unit weight of nutrient) in the shoot were each negatively correlated with chlorosis under the moist soil condition. These results suggest that there may be several characteristics of Fe-deficiency chlorosis resistance in subclovers, such as a more effective soil-Fe mobilizing mechanism(s), more balanced nutrition, lower required Fe concentration in the shoot, higher shoot-Fe utilization efficiency, and higher root/shoot ratio under Fe-deficiency stress conditions. 相似文献
100.