全文获取类型
收费全文 | 100520篇 |
免费 | 5730篇 |
国内免费 | 25篇 |
专业分类
106275篇 |
出版年
2021年 | 895篇 |
2018年 | 1317篇 |
2017年 | 1295篇 |
2016年 | 1692篇 |
2015年 | 1766篇 |
2014年 | 2145篇 |
2013年 | 3015篇 |
2012年 | 3314篇 |
2011年 | 3402篇 |
2010年 | 2532篇 |
2009年 | 2148篇 |
2008年 | 3102篇 |
2007年 | 3054篇 |
2006年 | 2882篇 |
2005年 | 2702篇 |
2004年 | 2690篇 |
2003年 | 2597篇 |
2002年 | 2400篇 |
2001年 | 4416篇 |
2000年 | 4167篇 |
1999年 | 3241篇 |
1998年 | 1059篇 |
1997年 | 1041篇 |
1996年 | 928篇 |
1995年 | 848篇 |
1993年 | 835篇 |
1992年 | 2449篇 |
1991年 | 2464篇 |
1990年 | 2459篇 |
1989年 | 2218篇 |
1988年 | 2070篇 |
1987年 | 1982篇 |
1986年 | 1829篇 |
1985年 | 1826篇 |
1984年 | 1502篇 |
1983年 | 1305篇 |
1982年 | 937篇 |
1981年 | 880篇 |
1979年 | 1437篇 |
1978年 | 1135篇 |
1977年 | 1018篇 |
1976年 | 978篇 |
1975年 | 1162篇 |
1974年 | 1259篇 |
1973年 | 1338篇 |
1972年 | 1184篇 |
1971年 | 1045篇 |
1970年 | 926篇 |
1969年 | 955篇 |
1968年 | 840篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
581.
OBJECTIVE: To investigate the development of the intra-abdominal part of the umbilical vessels in human fetuses by light microscopy. MATERIALS AND METHODS: The location of the umbilicus and umbilical vessels in the abdominal cavity of 90 human fetuses of gestational ages 10-40 weeks was determined. The external vessel diameter, lumen diameter, wall thickness, tunica adventitia thickness, tunica media thickness and the number of vasa vasorum were recorded from cross-sectlons of the intra-abdominal part of the umbilical vessels. 1985). 1985). RESULTS: Umbilical artery agenesis was observed on the left side in two cases and on the right in one case. There was a positive correlation between gestational age and umbilical vessel measurements. There were differences between the vessel and lumen diameters, tunica media thicknesses of the vessels of the second and third trimesters, and the full-term period. There were also predictable differences between the vessel and lumen diameters, tunica media and tunica adventitia thicknesses of the umbilical vein and umbilical arteries. CONCLUSION: Detailed information on quantitative parameters of umbilical vessels at each gestational age may prove helpful in determining pathologies of umbilical vessels and illuminating certain syndromes. 相似文献
582.
Biophysics - Abstract—In this study we present a modified method for determination of the antioxidant capacity of biological fluids and tissues based on the use of a chemiluminescent model... 相似文献
583.
Gevorkian ES Demirkhanian LO Iavroian ZhV Artsruni IG Derziian NG 《Ukrainski? biokhimicheski? zhurnal》2001,73(4):29-32
It was shown that the total content of phospholipids of rat liver nuclear matrix increased after in vivo action of insulin (2 units per 100 g of animal weight). At the same time the sphyngomyelin content was increased and the content of phosphatydilcholine and phoshpatydilinositol was decreased significantly under the hormone action. The decreasing of monophosphoinositides amount was accompanied by the increasing of triphosphoinositides content which indicates the redistribution among the phosphoinositides fractions under the hormone action and may be the result of insulin initiation of phosphoinositide pathway in nuclear membrances. 相似文献
584.
P O Angrand N Daigle F van der Hoeven H R Schler A F Stewart 《Nucleic acids research》1999,27(17):e16
ET recombination is a way to engineer DNA in Escherichia coli using homologous recombination. Here we develop the potential of ET recombination in two ways relevant to complex engineering exercises such as building gene targeting constructs. First, a targeting construct was made in a single step. Second, ET recombination was used to place two unique restriction sites at precise positions in a large genomic clone. Subsequently a complex targeting construct was created by ligation with a multifunctional cassette. 相似文献
585.
586.
Nielsen JL Klausen C Nielsen PH Burford M Jørgensen NO 《FEMS microbiology ecology》2006,55(3):432-438
The abundance, identity and activity of uncultured Bacteria and Actinobacteria present in a drinking water reservoir (North Pine Dam, Brisbane, Australia) were determined using a combination of fluorescence in situ hybridization (FISH) alone or with catalysed reporter deposition (CARD-FISH) with microautoradiography. The CARD-FISH technique was modified relative to previous described procedures and performed directly on gelatine cover slips in order to allow simultaneous combination with microautoradiography. Almost twofold higher numbers of microorganisms could be identified as either Bacteria or Actinobacteria using the CARD-FISH technique as compared with the traditional FISH technique. A combination of FISH or CARD-FISH with microautoradiography showed generally higher activity among the Actinobacteria than among all Bacteria. Another important observation was that many cells within the FISH-negative populations of both Actinobacteria and Bacteria were actively assimilating thymidine. Thus, great care should be taken when extrapolating the active fraction of a prokaryotic community to be equivalent to the FISH-detectable population in such environments. Bacterial groups within Actinobacteria produce the odours geosmin and 2-methylisoborneol, which lower the quality of surface water when used for drinking. The results indicate that combined microautoradiography and CARD-FISH may serve as an effective tool when studying identity and activity of microorganisms within freshwater environments. 相似文献
587.
Thomas J Jurkiw Percy P Tumbale Matthew J Schellenberg Charlotte Cunningham-Rundles R Scott Williams Patrick J OBrien 《Nucleic acids research》2021,49(3):1619
Human DNA ligase I (LIG1) is the main replicative ligase and it also seals DNA breaks to complete DNA repair and recombination pathways. Immune compromised patients harbor hypomorphic LIG1 alleles encoding substitutions of conserved arginine residues, R771W and R641L, that compromise LIG1 activity through poorly defined mechanisms. To understand the molecular basis of LIG1 syndrome mutations, we determined high resolution X-ray structures and performed systematic biochemical characterization of LIG1 mutants using steady-state and pre-steady state kinetic approaches. Our results unveil a cooperative network of plastic DNA-LIG1 interactions that connect DNA substrate engagement with productive binding of Mg2+ cofactors for catalysis. LIG1 syndrome mutations destabilize this network, compromising Mg2+ binding affinity, decreasing ligation efficiency, and leading to elevated abortive ligation that may underlie the disease pathology. These findings provide novel insights into the fundamental mechanism by which DNA ligases engage with a nicked DNA substrate, and they suggest that disease pathology of LIG1 syndrome could be modulated by Mg2+ levels. 相似文献
588.
589.
D.R. Dufield O.V. Nemirovskiy M.D. Tortorella W.R. Mathews 《Analytical biochemistry》2010,406(2):113-123
The degradation of articular cartilage by aggrecanases (ADAMTS-4 and ADAMTS-5) plays a significant role in the pathology of osteoarthritis (OA). To monitor aggrecanase activity in OA, we have developed a sensitive, accurate, and versatile assay for detection of two specific cleavage sites on aggrecan. The assay uses an immunoaffinity-based liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to detect cleavage at the 374ARGS site and the 1820AGEG site. The dynamic range of the assay is more than three orders of magnitude, with interassay precision less than 15%. It has been successfully applied to various biological fluids and species, including rat, bovine, dog, and human. The assay has been analytically qualified for use in human urine and synovial fluid (SF). The limits of detection (LODs) for ARGS in urine and SF are 2.5 and 10 pg/ml, respectively, whereas the LOD for AGEG is 20 pg/ml in SF. Analysis of these biomarkers from OA subjects and normal healthy volunteers revealed a significant elevation of both markers in OA. Similarly, in a rat model of cartilage degradation, both ARGS and AGEG were elevated, demonstrating the utility of these biomarkers for translational research. These data suggest that the ARGS and AGEG biomarkers developed have potential as measures of aggrecanase activity in OA and may contribute to our understanding of OA pathology. 相似文献
590.