首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   14177篇
  免费   1177篇
  国内免费   758篇
  16112篇
  2024年   20篇
  2023年   232篇
  2022年   427篇
  2021年   710篇
  2020年   448篇
  2019年   567篇
  2018年   604篇
  2017年   396篇
  2016年   595篇
  2015年   868篇
  2014年   944篇
  2013年   1089篇
  2012年   1307篇
  2011年   1175篇
  2010年   697篇
  2009年   601篇
  2008年   705篇
  2007年   680篇
  2006年   601篇
  2005年   493篇
  2004年   434篇
  2003年   346篇
  2002年   289篇
  2001年   233篇
  2000年   182篇
  1999年   199篇
  1998年   127篇
  1997年   141篇
  1996年   124篇
  1995年   102篇
  1994年   100篇
  1993年   62篇
  1992年   90篇
  1991年   66篇
  1990年   62篇
  1989年   51篇
  1988年   40篇
  1987年   31篇
  1986年   24篇
  1985年   29篇
  1984年   21篇
  1983年   16篇
  1982年   12篇
  1981年   9篇
  1980年   7篇
  1978年   6篇
  1977年   6篇
  1974年   8篇
  1973年   9篇
  1964年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Comment on: Rokavec M, et al. Mol Cell 2012; 45:777-89.  相似文献   
3.
Due to the fact that the flood data series of small drainage basins is relatively short, available data are often not sufficient for flood risk analysis. This presents the problem of risk analysis using very small data samples. One method that can be applied is to regard the available small samples as fuzzy information and optimize them using information diffusion technology to yield analytical results with greater reliability. In this article a risk analysis method based on information diffusion theory is applied to create a new flood risk analysis model. Application of the model is illustrated taking the Jinhuajiang and Qujiang drainage basins as examples. This is a new attempt at applying information diffusion theory in flood risk analysis. Computations based on this analytical flood risk model can yield an estimated flood damage value that is relatively accurate. This study indicates that the aforementioned model exhibits fairly stable analytical results, even when using a small set of sample data. The results also indicate that information diffusion technology is highly capable of extracting useful information and therefore improves system recognition accuracy. This method can be easily applied and the analytical results produced are easy to understand. Results are accurate enough to act as a guide in disaster situations.  相似文献   
4.
Digital images of ex situ germplasm stored in the Sardinian Germplasm Bank (BG-SAR) were used for the application of image analysis techniques at the Stazione Sperimentale di Granicoltura per la Sicilia. The analysed accessions refer to 148 taxonomic units belonging to 102 genera and 47 families, typical of the Sardinian flora, and of the Mediterranean basin in general.The images of diaspores were acquired by a flatbed scanner and elaborated with a macro specially developed for the morphometric and colorimetric measurements. This method allowed carrying out a database for the characterization of autochthonous germplasm in entry to the bank and the realization of statistic classifiers for the discrimination of genera and species within the following families: Apiaceae, Boraginaceae, Caryophyllaceae, Cistaceae, Fabaceae and Scrophulariaceae. Such classifiers, based on the linear discriminant analysis (LDA) technique and checked by cross-validation, showed a performance included between 74.3% and 96.4%.In addition, for the genus Astragalus, it was possible to elaborate a classifier able to identify very similar taxa of a species complex, obtaining a performance between 83.7% and 100%. Such analysis proved the validity of the methodology also from the taxonomic point of view.Suggestions for subsequent methodological progress, which could offer applications in other research issues, such as ecological analysis, soil seed bank and archaeological botany are proposed.  相似文献   
5.
Interplant communication has been widely demonstrated in plants, especially in herbaceous plants. In this study, mechanical damage was shown to affect the levels of pyrochatechol, chlorogenic acid, gallic acid and p-hydroxyl benzoic acid in poplar (Populus simonii × P. pyramidalis ‘Opera 8277’) cuttings, indicating the activation of defense response. In neighboring intact cuttings, the levels of those phenolics also varied when compared to the control, suggesting the interplant communication between poplar cuttings. Three volatiles, methyl jasmonate, methyl salicylate and benzothiazole, were detected in volatiles emitted from mechanically damaged poplar cuttings. All of them can induce changes in the levels of four phenolics. Therefore, they could act as airborne signals between P. simonii × P. pyramidalis ‘Opera 8277’ cuttings. The different change patterns of phenolic contents induced by different volatiles imply that the defense response activated in neighboring plants may be regulated by multiple signaling pathways. The results also suggest that the entire defense response of plants involves a variety of airborne signals in wound-induced volatiles.  相似文献   
6.
Ageing is an unavoidable corollary to being alive; the most intuitive interpretation of ageing being that it is the consequence of progressive body degeneration. In agreement with this, current models propose that ageing occurs through a stepwise accumulation of DNA damage, which ultimately limits the regenerative capacity of tissues. On the other hand, there is increasing evidence that fetal distress can influence the development of disease in adult life, a phenomenon known as ‘intrauterine programming’. The extent to which an intrauterine exposure to DNA damage can compromise lifespan remains unclear. My group has recently generated a murine model of a human syndrome linked to defective DNA repair and observed that these animals age prematurely, but the accumulation of DNA damage is restricted mostly to the embryonic period. Here, I discuss the implications of this finding and propose that ageing can be influenced by fetal distress.  相似文献   
7.
The amino-terminal domain of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) contains a serine/threonine-specific protein kinase that has characteristics of a growth factor receptor (Chung, T. D., Wymer, J. P., Smith, C. C., Kulka, M., and Aurelian, L. (1989) J. Virol. 63, 3389-3398; Chung, T. D., Wymer, J. P., Kulka, M. Smith, C. C., and Aurelian, L. (1990) Virology 179, 168-178). To characterize this protein kinase (PK) domain further we constructed a bacterial expression vector (pJL11) containing DNA sequences encoding ICP10 amino acid residues 1-445. Bacteria containing pJL11 were induced to express a 29-kDa protein (designated pp29la1) that represents a truncated portion of the ICP10-PK domain (includes PK catalytic motifs I-V) as demonstrated by immunoprecipitation with antibodies that recognize different antigenic domains, competition studies with extracts of ICP10-positive eukaryotic cells, and peptide mapping.pp29la1 has autophosphorylating and transphosphorylating activity for calmodulin. The enzyme is activated by Mn2+ but not by Mg2+ ions, and autophosphorylation is inhibited by histone. It differs from the authentic ICP10-PK in that phosphorylation is specific only for threonine.  相似文献   
8.
An attempt has been made to build a model of human rhinovirus 2 (HRV2) based on the known human rhinovirus 14 (HRV14) structure. HRV2 was selected because its amino acid sequence is known and because it belongs to the minor rhinovirus receptor class as compared to HRV14, which belongs to the major class. Initial alignment of HRV2 with HRV14 based on the primary sequence and the knowledge of the three-dimensional structure of HRV14 showed that the most probable position of the majority of insertions and deletions occurred in the vicinity of the neutralizing immunogenic sites (NIm). Out of a total of 855 amino acids present in one copy of each of the capsid proteins VP1 through VP4 of HRV14, 411 are different between the two viruses. There are also 6 amino acid residues inserted and 14 residues deleted in HRV2 relative to HRV14. Examination of amino acid interactions showed several cases of conservation of function, e.g., salt bridges or the filling of restricted space. The largest variation amongst the residues lining the canyon, the putative receptor binding site, was in the carboxy-terminal residues of VP1.  相似文献   
9.
The isomeric composition of type-III heptacarboxylic porphyrinogens derived from decarbosylation of uroporphyrinogen III by erythrocyte uroporphyringogen decarboxylase was analysed by h.p.l.c. with electrochemical detection. All four possible isomers were identified, and there were little differences in the proportion of isomers formed by erythrocytes from normal subjects and from patients with sporadic porphyria cutanea tarda. The results provide conclusive evidence that the normal decarboxylation pathway is random in nature, and the fourth isomer only increases when enzyme abnormality is found.  相似文献   
10.
本文研究了中国广东汉族健康人群apoAI-CⅢ-AIV基因簇DNA限制性内切酶PstI、SstI和EcoRI片段长度多态性。其中等位基因P_1,P_2,S_1,S_2,R_1和R_2的频率分别为0.98,0.02,0.96,0.04,0.90和0.10。经卡方检验符合Hardy-Weinbery氏遗传平衡,与其他种族比较,本文结果显示中国广东汉族人P_2等位基因频率低于日本人、亚洲印第安人和高加索人,S_2等位基因频率低于日本人、菲律宾人、沙特阿拉伯人和亚洲印第安人,而与高加索人相近,R_2等位基因频率稍高于高加索人。不同种族间apoAI-CⅢ-AIV基因簇DNA多态频率无疑存在差异,这种差异可能是由于遗传漂变和自然选择单独或联合作用所致。对P_1、P_2,S_1、S_2和R_1、R_2构成的单倍型和连锁平衡程度进行了分析,结果显示这些单倍型处于连锁不平衡状态。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号