全文获取类型
收费全文 | 8825篇 |
免费 | 667篇 |
国内免费 | 662篇 |
专业分类
10154篇 |
出版年
2024年 | 19篇 |
2023年 | 114篇 |
2022年 | 282篇 |
2021年 | 492篇 |
2020年 | 294篇 |
2019年 | 371篇 |
2018年 | 377篇 |
2017年 | 300篇 |
2016年 | 366篇 |
2015年 | 566篇 |
2014年 | 683篇 |
2013年 | 701篇 |
2012年 | 828篇 |
2011年 | 717篇 |
2010年 | 430篇 |
2009年 | 401篇 |
2008年 | 461篇 |
2007年 | 377篇 |
2006年 | 293篇 |
2005年 | 245篇 |
2004年 | 191篇 |
2003年 | 186篇 |
2002年 | 160篇 |
2001年 | 143篇 |
2000年 | 128篇 |
1999年 | 107篇 |
1998年 | 71篇 |
1997年 | 83篇 |
1996年 | 81篇 |
1995年 | 58篇 |
1994年 | 47篇 |
1993年 | 45篇 |
1992年 | 64篇 |
1991年 | 49篇 |
1990年 | 50篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 26篇 |
1986年 | 23篇 |
1985年 | 36篇 |
1984年 | 33篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1978年 | 11篇 |
1974年 | 11篇 |
1973年 | 7篇 |
1971年 | 9篇 |
1967年 | 7篇 |
1966年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Nguyen Van Du 《Kew Bulletin》2008,63(3):491-493
Summary
Typhonium stigmatilobatum V. D. Nguyen, a new species from Vietnam, is described and illustrated. 相似文献
992.
J. S. Zheng X. L. Liao J. G. Tong H. J. Du M. C. Milinkovitch D. Wang 《Conservation Genetics》2008,9(4):1007-1009
The finless porpoise (Neophocaena phocaenoides) is one of the smallest cetacean species widely distributed in the shallow coastal waters of the Indo-Pacific Oceans. The
population size of the Yangtze subspecies (N. p. asiaeorientalis) has sharply decreased in the last two decades and access to objective data on its population structure and genetic diversity
would be of great assistance for their proper management. Here we report on the isolation of nine polymorphic microsatellite
using the “Fast Isolation by AFLP of Sequences Containing repeats” (FIASCO) protocol. Polymorphism was assessed using 30 porpoise individuals randomly sampled in the Yangtze River. The number
of alleles per locus varies from 2 to 9, with an average value of 5.56, whereas the ranges of observed and expected heterozygosities
were 0.300–0.633 (mean 0.496) and 0.473–0.804 (mean 0.659), respectively. 相似文献
993.
Li Z Lai Z Ya K Fang D Ho YW Lei Y Ming QZ 《Journal of cellular and molecular medicine》2008,12(2):569-579
Transferrin and transferrin receptor are two key proteins of iron metabolism that have been identified to be hypoxia-inducible genes. Divalent metal transporter 1 (DMT1) is also a key transporter of iron under physiological conditions. In addition, in the 5' regulatory region of human DMT1 (between -412 and -570), there are two motifs (CCAAAGTGCTGGG) that are similar to hypoxia-inducible factor-1 (HIF-1) binding sites. It was therefore speculated that DMT1 might also be a hypoxia-inducible gene. We investigated the effects of hypoxia and hypoxia/re-oxygenation on the expression of DMT1 and the content of HIF-1alpha in HepG2 cells. As we expected, a very similar tendency in the responses of the expression of HIF-1alpha, DMT1+IRE (iron response element) and DMT1-IRE proteins to chemical (CoCl(2)) or physical hypoxia was observed. A highly significant correlation was found between the expression of DMT1 proteins and the contents of HIF-1 in hypoxic cells. After the cells were exposed to hypoxia and subsequent normoxia, no HIF-1alpha could be detected and a significant decrease in DMT1+IRE expression (P<0.05), but not in DMT1-IRE protein (versus the hypoxia group), was observed. The findings implied that the HIF-1 pathway might have a role in the regulation of DMT1+IRE expression during hypoxia. 相似文献
994.
The optimization of a Diels-Alder reaction to prepare a novel ketone bearing a 2,3-dimethylnorbornyl group is presented together with the structure elucidation of the isomers. Employing this new ketone as starting material, derivatives with new woody odor notes as well as attempts to obtain ambery-musky odorants are reported. 相似文献
995.
Three meta-dicopper complexes, 1-3, based on 5-substituted 1,3-xylylene spacer have been synthesized. These complexes are capable of inducing the transformation of supercoiled DNA (pUC19) to its nicked and linear DNA form in the presence of ascorbate, and their DNA nicking efficiency can be correlated to their DNA-binding ability. The cleavage mechanism is similar to that of the non-substituted meta-dicopper complex A. Amongst the three complexes, 5-(aminomethyl)-substituted complex 3 displayed a higher DNA-binding ability and nicking efficiency than unsubstituted complex A. The CD-spectroscopic study and structural analysis imply that the different CuCu distances and DNA binding modes induced by different 5-substituents on benzene-1,3-bis(methylene) spacer may be responsible for the different DNA cleaving behavior of meta-dicopper complexes. 相似文献
996.
Ligularia virgaurea is a perennial herb that is widely distributed in the alpine meadow on the eastern Qinghai-Tibet plateau.We investigated the patterns of growth and reproduction of L.virgaurea under two contrasting levels of light conditions for two continuous growing seasons.Our results showed that the light affects on the maximum relative growth rate,the shoot weight ratio and the root weight ratio differed between the two growing seasons.L.virgaurea reproduced initially through rhizome in the second growing season,rather than sexual reproduction.The proportion of genets with clonal reproduction decreased under shaded conditions.A minimum genet size should be attained for clonal reproduction to begin under the shaded conditions.There was a positive linear relationship between clonal reproduction and genet size.Light level affected the allocation of total biomass to clonal structures,with less allocation under the full natural irradiance than under the shaded conditions.There seemed to be a trade-off between vegetative growth and clonal reproduction under the full natural irradiance,in terms of smaller relative growth rates of genets with clonal reproduction than those without clonal reproduction.L.virgaurea emphasized clonal reproduction under the full natural irradiance,while the plant emphasized vegetative growth under the shaded conditions. 相似文献
997.
B24 and B25 phenylalanines (Phe) play important roles in insulin structure and function. Insulin analogs with B24 Phe or B25 Phe replaced by biphenylalanine (Bip) were prepared by enzymatic semisynthesis. The biological activities were determined by receptor binding assay and in vivo mouse convulsion assay. The results showed that B25 Bip insulin has 139% receptor binding activity and 50% in vivo biological activity, whereas B24 Bip insulin is inactive, when compared with native insulin, suggesting that B24 Phe is crucial for insulin activity. The structures in solution were studied by circular dichroism and fluoremetry, and our results suggested that the insulin analogs with low activities tend to be more tightly packed. The association properties were studied by size exclusion chromatography. The Bip.amide replacement of B24 Phe in deshexapeptide insulin or B25 Phe in despentapeptide insulin will cause the monomeric B24 Phe-amide deshexapeptide insulin or B25 Phe-amide despentapeptide insulin to associate and form dimers, whereas the mutations of B24 Phe in insulin will make insulin dimers dissociate into insulin monomers. 相似文献
998.
Haiyan Bu Qingji Ren Xiuli Xu Kun Liu Peng Jia Shujun Wen Dashuai Sun Guozhen Du 《Frontiers of Biology in China》2008,3(2):187-193
The germination experiment was started on March 3, 2004, and seeds were collected from July to October in 2003. We analyzed
the percentage of germination, days to first germination, germination period and days to 50% germination. Among the 54 examined
species, 26 species exceeded 80% germination, 11 species exceeded 60%–80% germination, 8 exceeded 40%–0%, 5 exceeded 20%–40%,
and 4 showed less than 20%. A principal-component analysis revealed that the species were distributed along two statistically
independent axes, the first primarily represented the germination rate and the second represented the time of germination
process. Based on scores of germination characteristics, cluster analysis of the 54 gramineous species could be divided into
4 distinct groups: rapid, slow, intermediate germinating (germination percentage > 50%), and low germinating (germination
percentage < 50%). The meaning of different groups to the vegetation regeneration was discussed.
__________
Translated from Journal of Plant Ecology, 2006, 30(4): 624–632 [译自: 植物生态学报] 相似文献
999.
Qiuliang Yan Yinghan Zhang Hongbin Li Caihong Wei Lili Niu Shan Guan Shangang Li Lixin Du 《遗传学报》2008,35(5):261-266
To identify EST-SSR molecular markers, 41,986 cattle UniGene sequences from NCBI were mined for analyzing SSRs. A total of 1,831 SSRs were identified from 1,666 ESTs, which represented an average density of 19.88 kb per SSR. The frequency of EST-SSRs was 4.0%. The dinucleotide repeat motif was the most abundant SSR, accounting for 54%, followed by 22%, 13%, 7% and 4%, respec-tively, for tri-, hexa-, penta- and tetra-nucleotide repeats. Depending upon the length of the repeat unit, the length of microsatellites varied from 14 to 86 bp. Among the di- and tri-nucleotide repeats, AC/TG (57%) and AGC (12%) were the most abundant type. Annotation of EST-SSRs was also carried out. Three hundred primer pairs were randomly designed using Prime Premier 5.0 program and Oligo 5.0 for further experimental validation. 相似文献
1000.
Zhenming Du Yangzhong Liu Yuchuan Zheng Scott McCallum John Dansereau Victoria Derbyshire Marlene Belfort Georges Belfort Patrick Van Roey Chunyu Wang 《Biomolecular NMR assignments》2008,2(2):111-113
The backbone and side chain resonance assignments of an engineered intein based on Mycobacterium tuberculosis RecA have been determined based on triple-resonance experiments with the uniformly [13C,15N]-labeled protein. 相似文献