全文获取类型
收费全文 | 7522篇 |
免费 | 657篇 |
国内免费 | 4篇 |
专业分类
8183篇 |
出版年
2023年 | 58篇 |
2022年 | 142篇 |
2021年 | 273篇 |
2020年 | 148篇 |
2019年 | 176篇 |
2018年 | 199篇 |
2017年 | 160篇 |
2016年 | 335篇 |
2015年 | 509篇 |
2014年 | 521篇 |
2013年 | 532篇 |
2012年 | 754篇 |
2011年 | 645篇 |
2010年 | 375篇 |
2009年 | 363篇 |
2008年 | 489篇 |
2007年 | 417篇 |
2006年 | 375篇 |
2005年 | 287篇 |
2004年 | 289篇 |
2003年 | 262篇 |
2002年 | 238篇 |
2001年 | 53篇 |
2000年 | 30篇 |
1999年 | 38篇 |
1998年 | 60篇 |
1997年 | 29篇 |
1996年 | 22篇 |
1995年 | 27篇 |
1994年 | 23篇 |
1993年 | 27篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 22篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 24篇 |
1982年 | 8篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1978年 | 15篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1974年 | 8篇 |
排序方式: 共有8183条查询结果,搜索用时 15 毫秒
71.
72.
73.
Rapha?l Laurenceau Gérard Péhau-Arnaudet Sonia Baconnais Joseph Gault Christian Malosse Annick Dujeancourt Nathalie Campo Julia Chamot-Rooke Eric Le Cam Jean-Pierre Claverys Rémi Fronzes 《PLoS pathogens》2013,9(6)
Natural genetic transformation is widely distributed in bacteria and generally occurs during a genetically programmed differentiated state called competence. This process promotes genome plasticity and adaptability in Gram-negative and Gram-positive bacteria. Transformation requires the binding and internalization of exogenous DNA, the mechanisms of which are unclear. Here, we report the discovery of a transformation pilus at the surface of competent Streptococcus pneumoniae cells. This Type IV-like pilus, which is primarily composed of the ComGC pilin, is required for transformation. We provide evidence that it directly binds DNA and propose that the transformation pilus is the primary DNA receptor on the bacterial cell during transformation in S. pneumoniae. Being a central component of the transformation apparatus, the transformation pilus enables S. pneumoniae, a major Gram-positive human pathogen, to acquire resistance to antibiotics and to escape vaccines through the binding and incorporation of new genetic material. 相似文献
74.
Telomeres are chromosome end structures and are essential for maintenance of genome stability. Highly repetitive telomere sequences appear to be susceptible to oxidative stress-induced damage. Oxidation may therefore have a severe impact on telomere integrity and function. A wide spectrum of oxidative pyrimidine-derivatives has been reported, including thymine glycol (Tg), that are primarily removed by a DNA glycosylase, Endonuclease III-like protein 1 (Nth1). Here, we investigate the effect of Nth1 deficiency on telomere integrity in mice. Nth1 null (Nth1−/−) mouse tissues and primary MEFs harbor higher levels of Endonuclease III-sensitive DNA lesions at telomeric repeats, in comparison to a non-telomeric locus. Furthermore, oxidative DNA damage induced by acute exposure to an oxidant is repaired slowly at telomeres in Nth1−/− MEFs. Although telomere length is not affected in the hematopoietic tissues of Nth1−/− adult mice, telomeres suffer from attrition and increased recombination and DNA damage foci formation in Nth1−/− bone marrow cells that are stimulated ex vivo in the presence of 20% oxygen. Nth1 deficiency also enhances telomere fragility in mice. Lastly, in a telomerase null background, Nth1−/− bone marrow cells undergo severe telomere loss at some chromosome ends and cell apoptosis upon replicative stress. These results suggest that Nth1 plays an important role in telomere maintenance and base repair against oxidative stress-induced base modifications. The fact that telomerase deficiency can exacerbate telomere shortening in Nth1 deficient mouse cells supports that base excision repair cooperates with telomerase to maintain telomere integrity. 相似文献
75.
76.
Alexander Benedikt Leichtle Uta Ceglarek Peter Weinert Christos T. Nakas Jean-Marc Nuoffer Julia Kase Tim Conrad Helmut Witzigmann Joachim Thiery Georg Martin Fiedler 《Metabolomics : Official journal of the Metabolomic Society》2013,9(3):677-687
Metabolomics as one of the most rapidly growing technologies in the “-omics” field denotes the comprehensive analysis of low molecular-weight compounds and their pathways. Cancer-specific alterations of the metabolome can be detected by high-throughput mass-spectrometric metabolite profiling and serve as a considerable source of new markers for the early differentiation of malignant diseases as well as their distinction from benign states. However, a comprehensive framework for the statistical evaluation of marker panels in a multi-class setting has not yet been established. We collected serum samples of 40 pancreatic carcinoma patients, 40 controls, and 23 pancreatitis patients according to standard protocols and generated amino acid profiles by routine mass-spectrometry. In an intrinsic three-class bioinformatic approach we compared these profiles, evaluated their selectivity and computed multi-marker panels combined with the conventional tumor marker CA 19-9. Additionally, we tested for non-inferiority and superiority to determine the diagnostic surplus value of our multi-metabolite marker panels. Compared to CA 19-9 alone, the combined amino acid-based metabolite panel had a superior selectivity for the discrimination of healthy controls, pancreatitis, and pancreatic carcinoma patients $ [ {\text{volume under ROC surface}}\;\left( {\text{VUS}} \right) = 0. 8 9 1 { }\left( { 9 5\,\% {\text{ CI }}0. 7 9 4- 0. 9 6 8} \right)]. $ We combined highly standardized samples, a three-class study design, a high-throughput mass-spectrometric technique, and a comprehensive bioinformatic framework to identify metabolite panels selective for all three groups in a single approach. Our results suggest that metabolomic profiling necessitates appropriate evaluation strategies and—despite all its current limitations—can deliver marker panels with high selectivity even in multi-class settings. 相似文献
77.
78.
Rapid species diversifications provide fascinating insight into the development of biodiversity in time and space. Most biological radiations studied to date, for example that of cichlid fishes or Andean lupines, are confined to isolated geographical areas like lakes, islands or island-like regions. Using DNA sequence data of the ribosomal internal transcribed spacer (ITS) for many species of the Poa alliance, a group comprising about 775 C3 grass species, revealed rapid and parallel diversifications in various parts of the world. Some of these radiations are restricted to isolated areas like the Andes, whereas others are typical of the lowlands of mainly the northern hemisphere. These radiations thus are not restricted to island-like areas and are seemingly actively ongoing. The ages of the diversifying clades are estimated to be 2.5–0.23 million years (Myr). Conservative diversification rates in the Poa alliance amount to 0.89–3.14 species per Myr, thus are in the order of, or even exceeding, other instances of well-known radiations. The grass radiations of the mainly cold-adapted Poa alliance coincide with the Late Tertiary global cooling, which resulted in the retreat of forests and the subsequent formation of cold-adapted grasslands especially in the northern, but also in parts of the southern hemisphere. The cold tolerance, suggested to be one of the ecological key innovations, may have been acquired during the early diversification of the subfamily Pooideae, but became significant millions of years later during the Pliocene/Pleistocene radiation of the Poa alliance. 相似文献
79.
Majolo Bonaventura deBortoli Vizioli Aurora Martínez-Íñigo Laura Lehmann Julia 《International journal of primatology》2020,41(2):325-341
International Journal of Primatology - Intergroup encounters are common in nonhuman primates and can vary from affiliative to aggressive. We extracted data from the literature to test five... 相似文献
80.
Hernández-Ledesma Ana Laura Rodríguez-Méndez Adriana Jheny Gallardo-Vidal Lilia Susana García-Gasca Teresa Alatorre-Cruz Julia María García-Solís Pablo López Reyes Julián Solís-Saínz Juan Carlos 《Molecular biology reports》2020,47(12):9667-9676
Molecular Biology Reports - Although cognitive impairment (CI) is classically associated with aging, it has been proposed that neurological pathologies may increase the risk to suffer CI. Despite... 相似文献