全文获取类型
收费全文 | 57931篇 |
免费 | 4618篇 |
国内免费 | 4462篇 |
专业分类
67011篇 |
出版年
2024年 | 141篇 |
2023年 | 791篇 |
2022年 | 1853篇 |
2021年 | 3049篇 |
2020年 | 2082篇 |
2019年 | 2506篇 |
2018年 | 2345篇 |
2017年 | 1807篇 |
2016年 | 2547篇 |
2015年 | 3633篇 |
2014年 | 4385篇 |
2013年 | 4438篇 |
2012年 | 5288篇 |
2011年 | 4767篇 |
2010年 | 2887篇 |
2009年 | 2599篇 |
2008年 | 2938篇 |
2007年 | 2636篇 |
2006年 | 2263篇 |
2005年 | 1890篇 |
2004年 | 1513篇 |
2003年 | 1421篇 |
2002年 | 1072篇 |
2001年 | 909篇 |
2000年 | 890篇 |
1999年 | 810篇 |
1998年 | 499篇 |
1997年 | 455篇 |
1996年 | 479篇 |
1995年 | 423篇 |
1994年 | 413篇 |
1993年 | 325篇 |
1992年 | 446篇 |
1991年 | 324篇 |
1990年 | 284篇 |
1989年 | 260篇 |
1988年 | 210篇 |
1987年 | 194篇 |
1986年 | 176篇 |
1985年 | 154篇 |
1984年 | 115篇 |
1983年 | 122篇 |
1982年 | 81篇 |
1981年 | 45篇 |
1980年 | 51篇 |
1979年 | 63篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1973年 | 45篇 |
1972年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Chen CT Peng HP Jian JW Tsai KC Chang JY Yang EW Chen JB Ho SY Hsu WL Yang AS 《PloS one》2012,7(6):e37706
Protein-protein interactions are key to many biological processes. Computational methodologies devised to predict protein-protein interaction (PPI) sites on protein surfaces are important tools in providing insights into the biological functions of proteins and in developing therapeutics targeting the protein-protein interaction sites. One of the general features of PPI sites is that the core regions from the two interacting protein surfaces are complementary to each other, similar to the interior of proteins in packing density and in the physicochemical nature of the amino acid composition. In this work, we simulated the physicochemical complementarities by constructing three-dimensional probability density maps of non-covalent interacting atoms on the protein surfaces. The interacting probabilities were derived from the interior of known structures. Machine learning algorithms were applied to learn the characteristic patterns of the probability density maps specific to the PPI sites. The trained predictors for PPI sites were cross-validated with the training cases (consisting of 432 proteins) and were tested on an independent dataset (consisting of 142 proteins). The residue-based Matthews correlation coefficient for the independent test set was 0.423; the accuracy, precision, sensitivity, specificity were 0.753, 0.519, 0.677, and 0.779 respectively. The benchmark results indicate that the optimized machine learning models are among the best predictors in identifying PPI sites on protein surfaces. In particular, the PPI site prediction accuracy increases with increasing size of the PPI site and with increasing hydrophobicity in amino acid composition of the PPI interface; the core interface regions are more likely to be recognized with high prediction confidence. The results indicate that the physicochemical complementarity patterns on protein surfaces are important determinants in PPIs, and a substantial portion of the PPI sites can be predicted correctly with the physicochemical complementarity features based on the non-covalent interaction data derived from protein interiors. 相似文献
992.
993.
Lin S Yu L Yang J Liu Z Karia B Bishop AJ Jackson J Lozano G Copland JA Mu X Sun B Sun LZ 《The Journal of biological chemistry》2011,286(51):44023-44034
Biomarkers are lacking for identifying the switch of transforming growth factor-β (TGF-β) from tumor-suppressing to tumor-promoting. Mutated p53 (mp53) has been suggested to switch TGF-β to a tumor promoter. However, we found that mp53 does not always promote the oncogenic role of TGF-β. Here, we show that endogenous mp53 knockdown enhanced cell migration and phosphorylation of ERK in DU145 prostate cancer cells. Furthermore, ectopic expression of mp53 in p53-null PC-3 prostate cancer cells enhanced Smad-dependent signaling but inhibited TGF-β-induced cell migration by down-regulating activated ERK. Reactivation of ERK by the expression of its activator, MEK-1, restored TGF-β-induced cell migration. Because TGF-β is known to activate the MAPK/ERK pathway through direct phosphorylation of the adaptor protein ShcA and MAPK/ERK signaling is pivotal to tumor progression, we investigated whether ShcA contributed to mp53-induced ERK inhibition and the conversion of the role of TGF-β during carcinogenesis. We found that mp53 expression led to a decrease of phosphorylated p52ShcA/ERK levels and an increase of phosphorylated Smad levels in a panel of mp53-expressing cancer cell lines and in mammary glands and tumors from mp53 knock-in mice. By manipulating ShcA levels to regulate ERK and Smad signaling in human untransformed and cancer cell lines, we showed that the role of TGF-β in regulating anchorage-dependent and -independent growth and migration can be shifted between growth suppression and migration promotion. Thus, our results for the first time suggest that mp53 disrupts the role of ShcA in balancing the Smad-dependent and -independent signaling activity of TGF-β and that ShcA/ERK signaling is a major pathway regulating the tumor-promoting activity of TGF-β. 相似文献
994.
基于生态足迹修正模型的江苏省海洋经济可持续发展分析 总被引:2,自引:0,他引:2
基于生态足迹的理论和方法,提出海洋生态足迹的概念,并根据江苏省海洋环境的特殊性,构建了渔业用海、交通运输用海、工程建筑用海、能源用海、滩涂5个海洋生态足迹子模型,采用改进熵值法确定5类用海的均衡因子,分析了2000-2008年江苏省海洋生态足迹和生态承载力.结果表明:2000-2008年间,江苏省人均海洋生态足迹增幅近7倍,从36.90 hm2上升到252.94 hm2;人均生态承载力从105.01 hm2增至185.49 hm2,呈平稳增长态势;2000年,江苏省海洋环境处于生态盈余状态,海洋经济呈弱可持续发展状态,2004年,海洋生态环境急剧恶化,生态赤字高达109660.5 hm2,海洋经济可持续能力下降.渔业用海的高生态足迹是造成江苏省海洋生态赤字的主要原因,滩涂是维持江苏省海洋经济可持续发展的重要后备资源. 相似文献
995.
气候变化背景下中国农业气候资源变化V.宁夏农业气候资源变化特征 总被引:1,自引:0,他引:1
基于1961-2009年宁夏21个气象站点的气象资料,分析了宁夏各区农业气候资源的时空变化趋势.结果表明:研究期间,宁夏各地气温逐渐升高,呈北高南低的空间分布特征,年均气温的气候倾向率为0.4℃·(10 a)-1;大部分地区年降水量呈逐渐减少趋势,年降水量的气候倾向率为4.26 mm·(10 a)-1;无霜期和作物生长季天数随着气候变暖逐渐延长;≥10℃积温在3200℃·d以上的区域向南扩展,宁夏适宜种植中晚熟水稻的区域有所扩大;2001-2009年,宁夏大部分地区适宜种植冬小麦,全区各地几乎都适宜种植春小麦;宁夏南部山区各地7月平均气温≤20℃的区域面积逐渐缩小,适宜种植马铃薯的地域也随之缩小. 相似文献
996.
997.
Longkun Yang Zhipeng Li Peijie Wang Lisheng Zhang Yan Fang 《Plasmonics (Norwell, Mass.)》2014,9(1):121-127
The optical properties of individual noncontinuous shells with different gold coverage are investigated by the single-particle dark field scattering measurements and single-particle surface-enhanced Raman scattering (SERS) measurements at different excitation wavelengths. By controlling the growth of gold seeds, multi-metallic nanogaps/crevices with different optical responses are assembled on silica mesospheres forming noncontinuous shells that can be confirmed through the transmission electron microscope images. We find the surface plasmon resonance of single shell red-shifts from 510 to 680 nm with the increase of gold coverage. At the excitation of 532 and 785 nm, the best enhancements about 2.0?×?105 and 1.1?×?107 are obtained on spheres with ~60 and 83 % gold coverage, respectively. The weak polarization-dependent SERS indicates that the enhancement is from the multi-gaps on single noncontinuous shell. This optical tunable and SERS active noncontinuous gold shell can be applied in biosensing, ultra trace detection, and molecule analysis needing multi-wavelengths excitation. 相似文献
998.
本文用一元线性模型理论分析了辽宁西部朝阳、建平、喀左和北票(市)4县17个乡范围内的农田防护林主要树种——杨树(Populus spp.)、白榆(Ulumus pumila)的优势高与地形地貌、土壤肥力等立地条件因子的关系,确定地貌类型、坡位和土层厚度是本区农田防护林立地条件的主导因子,然后以地貌类型为单元将本区农田防护林的立地条件划分为3个大类型。在此基础上,再用坡位和土层厚度分级与3个大类型组合细分为6个基本类型。最后确定各立地条件类型的适宜树种,并定量地评价主要树种对各立地条件类型的适宜性。 相似文献
999.
1000.
选用华南地区广泛应用的8个杂交稻不育系与10个恢复系,按NCII设计配制两套不完全双列杂交组合61个,通过在湛江的两年种植试验,结合62对与产量性状QTL连锁SSR标记,对18个亲本的产量配合力进行综合评价和标记位点鉴定。结果表明:供试亲本间一般配合力存在较大差异,其中不育系珍汕97A、万金A和恢复系直龙、广恢3550的一般配合力相对较高;两套组合共检测出36个与亲本产量配合力显著相关的标记位点,其中RM1、RM216、RM231和RM542为两套材料所共有;检测出的杂合标记位点中,23个为增效标记位点,13个为减效标记位点,其中杂合标记位点RM231、RM209可使F1的单株粒重增加13.50 %和16.43 %。该结果可为华南地区杂交稻育种的亲本改良及选配提供了理论指导。 相似文献