全文获取类型
收费全文 | 3449篇 |
免费 | 89篇 |
国内免费 | 17篇 |
专业分类
系统科学 | 122篇 |
丛书文集 | 5篇 |
教育与普及 | 5篇 |
理论与方法论 | 36篇 |
现状及发展 | 1062篇 |
研究方法 | 61篇 |
综合类 | 2252篇 |
自然研究 | 12篇 |
出版年
2024年 | 17篇 |
2023年 | 27篇 |
2020年 | 14篇 |
2017年 | 16篇 |
2016年 | 31篇 |
2015年 | 59篇 |
2014年 | 30篇 |
2013年 | 27篇 |
2012年 | 161篇 |
2011年 | 175篇 |
2010年 | 13篇 |
2008年 | 23篇 |
2007年 | 22篇 |
2006年 | 38篇 |
2005年 | 214篇 |
2004年 | 524篇 |
2003年 | 481篇 |
2002年 | 173篇 |
2001年 | 98篇 |
2000年 | 154篇 |
1999年 | 82篇 |
1992年 | 39篇 |
1991年 | 28篇 |
1990年 | 37篇 |
1989年 | 37篇 |
1988年 | 35篇 |
1987年 | 18篇 |
1986年 | 26篇 |
1985年 | 38篇 |
1984年 | 28篇 |
1983年 | 24篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 17篇 |
1979年 | 61篇 |
1978年 | 39篇 |
1977年 | 31篇 |
1976年 | 30篇 |
1975年 | 40篇 |
1974年 | 59篇 |
1973年 | 51篇 |
1972年 | 52篇 |
1971年 | 74篇 |
1970年 | 58篇 |
1969年 | 69篇 |
1968年 | 59篇 |
1967年 | 42篇 |
1966年 | 56篇 |
1965年 | 41篇 |
1964年 | 11篇 |
排序方式: 共有3555条查询结果,搜索用时 281 毫秒
991.
A high-resolution stalagmite O-C isotope record from Nanjing and its rapid response to climatic events 总被引:3,自引:0,他引:3
Here we discussed rapid response of the cave temperature and vegetation to the four Dansgaard-Oeschger cold and warm cycles
during 50-40 kaBP based on results of oxygen and carbon stable isotopic compositions from a stalagmite in Tangshan, Nanjing.
It is found that the amount of C3 vegetation relative to C4-type declines during the D-O warm events, indicating the decrease of the effective meteoric precipitation. Compared with
O-isotope records of the Greenland ice core, the stalagmite record displays a very similar pattern to Greenland ice core record
over the decade-century time scale, suggesting that the changes of the East Asian monsoon climate are in accordance with the
high-latitude polar climate in the short-term time scale. The age of the ice-rafted H5 event in the stalagmite record, however,
preceded that of Greenland ice cores by 2 ka. This out of phase between the remote areas cannot be yet proven because the
two time scales were determined from different dating methods. 相似文献
992.
Identification of a mitochondrial receptor complex required for recognition and membrane insertion of precursor proteins 总被引:36,自引:0,他引:36
M Kiebler R Pfaller T S?llner G Griffiths H Horstmann N Pfanner W Neupert 《Nature》1990,348(6302):610-616
The mitochondrial import receptors MOM19 and MOM72 form a complex with two other proteins of the mitochondrial outer membrane, MOM38 and MOM22. This receptor complex is involved in recognition, membrane insertion and translocation of precursor proteins with MOM38 constituting (at least part of) the general insertion site GIP. 相似文献
993.
T von Schantz G G?ransson G Andersson I Fr?berg M Grahn A Helgée H Wittzell 《Nature》1989,337(6203):166-169
Recent theory on sexual selection suggests that females in species without paternal care choose mates by their secondary sexual characters because these indicate genotypic quality which will be transmitted to the offspring. These ideas are not yet empirically supported as data quantifying the relationship between female mate choice and female reproductive success are lacking. Only in one case, in Colias butterflies, has it been demonstrated unequivocally that females choose 'good genotypes' as mates and there is only one study, on Drosophila, demonstrating that mate choice increases one component of offspring fitness. Spur length of male pheasants (Phasianus colchicus) correlates with various fitness-related properties. We here present the first experimental field data showing that female pheasants select mates on the basis of male spur length and that female mate choice correlates with female reproductive success. 相似文献
994.
The diversification of the repertoire of T-cell antigen receptor (TCR) specificities is influenced by at least two selection processes which occur in the thymus. One of these, termed 'negative selection', is required to install a state of tolerance to self-antigens in the T-cell repertoire and is often achieved by clonal deletion. The second type of selection operating in the thymus results in preferential differentiation of T cells that have restriction specificity for thymic major histocompatibility complex glycoproteins, but the mechanisms leading to this selective process are not yet clear. One model used to describe this 'positive selection' proposes that only those T cells with sufficient avidity for the MHC glycoproteins expressed in the thymus are allowed to acquire functional competence. Here we directly investigate the generation of TCR specificities by following the fate of developing V beta 17+ CD4-CD8+ T cells under conditions where one of the main class I-MHC molecules, either H-2K or H-2D, was specifically blocked by in vitro monoclonal antibody treatment. The results show that development of V beta 17+ CD4-CD8+ T cells in the SJL H-2s mouse strain is selectively abrogated by blocking class I-Ks molecules but is unaffected by blocking class I-Ds molecules. These data directly demonstrate that generation of CD4-CD8+ T cells expressing a particular TCR V beta segment can be correlated with the expression of a particular class I-MHC molecule, thereby providing evidence for positive selection. 相似文献
995.
996.
High abundance of viruses found in aquatic environments 总被引:108,自引:0,他引:108
The concentration of bacteriophages in natural unpolluted waters is in general believed to be low, and they have therefore been considered ecologically unimportant. Using a new method for quantitative enumeration, we have found up to 2.5 x 10(8) virus particles per millilitre in natural waters. These concentrations indicate that virus infection may be an important factor in the ecological control of planktonic micro-organisms, and that viruses might mediate genetic exchange among bacteria in natural aquatic environments. 相似文献
997.
Primary structure and functional expression from complementary DNA of the rod photoreceptor cyclic GMP-gated channel 总被引:35,自引:0,他引:35
U B Kaupp T Niidome T Tanabe S Terada W B?nigk W Stühmer N J Cook K Kangawa H Matsuo T Hirose 《Nature》1989,342(6251):762-766
The complete amino-acid sequence of the cyclic GMP-gated channel from bovine retinal rod photoreceptors, deduced by cloning and sequencing its complementary DNA, shows that the protein contains several putative transmembrane segments, followed by a region that is similar to the cyclic GMP-binding domains of cyclic GMP-dependent protein kinase. Expression of the complementary DNA produces cyclic GMP-gated channel activity in Xenopus oocytes. 相似文献
998.
999.
1000.
Oncogene jun encodes a sequence-specific trans-activator similar to AP-1 总被引:166,自引:0,他引:166