全文获取类型
收费全文 | 990248篇 |
免费 | 118000篇 |
国内免费 | 2288篇 |
出版年
2016年 | 11471篇 |
2015年 | 17521篇 |
2014年 | 20120篇 |
2013年 | 27656篇 |
2012年 | 31635篇 |
2011年 | 31197篇 |
2010年 | 21131篇 |
2009年 | 19803篇 |
2008年 | 27766篇 |
2007年 | 28474篇 |
2006年 | 26555篇 |
2005年 | 25628篇 |
2004年 | 25168篇 |
2003年 | 24467篇 |
2002年 | 23677篇 |
2001年 | 40167篇 |
2000年 | 40560篇 |
1999年 | 32907篇 |
1998年 | 12773篇 |
1997年 | 13357篇 |
1996年 | 12866篇 |
1995年 | 12581篇 |
1994年 | 12444篇 |
1993年 | 12197篇 |
1992年 | 28213篇 |
1991年 | 27413篇 |
1990年 | 26884篇 |
1989年 | 26200篇 |
1988年 | 24303篇 |
1987年 | 23942篇 |
1986年 | 21946篇 |
1985年 | 22245篇 |
1984年 | 18645篇 |
1983年 | 16324篇 |
1982年 | 13147篇 |
1981年 | 11966篇 |
1980年 | 11257篇 |
1979年 | 18266篇 |
1978年 | 14652篇 |
1977年 | 13343篇 |
1976年 | 12721篇 |
1975年 | 13814篇 |
1974年 | 14865篇 |
1973年 | 14703篇 |
1972年 | 13265篇 |
1971年 | 12270篇 |
1970年 | 10548篇 |
1969年 | 10153篇 |
1968年 | 9126篇 |
1967年 | 8168篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
Cross-links in tendon collagen are essential for the biomechanical strength of healthy tissue. The nature and position of these cross-links has long been a subject for conjecture. We have approached this problem in a non-destructive manner, by studying neutron diffraction from collagen fibrils that have been specifically deuterated by reduction at keto-amine and Schiff base groups with sodium borodeuteride (NaB2H4). The intensities of the first 23 meridional reflections were recorded for both native and reduced tendons. These data were used to calculate the neutron-scattering density profile of the 67 nm (D) repeat of type I collagen fibrils in rat tail tendon. This approach not only succeeds in determining the location of the cross-linkage sites with respect to the fibril structure, as projected onto the fibre axis, but also presents a novel form of the isomorphous derivative solution to the phase problem. 相似文献
103.
Fusaric acid and other metabolite production in Fusarium oxysporum f. sp. vasinfectum 总被引:1,自引:0,他引:1
A thin layer chromatography system is described for the analysis of fusaric acid and other secondary metabolites, to assist in the identification of pathogenic races of Fusarium oxysporum f. sp. vasinfectum . There were differences in the metabolite profiles of the races grown on different media. 相似文献
104.
105.
K Hirayama K Fukuyama W L Epstein 《Comparative biochemistry and physiology. B, Comparative biochemistry》1990,96(3):553-557
1. Angiotensin I hydrolases, Mr 140,000 and Mr 70,000 were separated by gel filtration from Tris-HCl buffer extract of hepatic granulomas developed in mice with schistosomiasis. Two enzymes had different substrate specificity. 2. Mr 140,000 hydrolase activity was inhibited by captopril as reported for angiotensin converting enzyme (ACE), while that of Mr 70,000 hydrolase activity was inhibited by potato carboxypeptidase inhibitor. 3. An intermediary, des-Leu10-angiotensin I and then angiotensin II were formed from angiotensin I by Mr 70,000 hydrolase. 4. The findings suggest that Mr 70,000 enzyme is tissue carboxypeptidase A, and it generates angiotensin II in granulomatous inflammation as does ACE. 相似文献
106.
107.
108.
J J Legros V Geenen T Carvelli H Martens M Andre J L Corhay M Radermecker P F Zangerle G Sassolas C Gharib 《Hormone research》1990,34(3-4):151-155
Vasopressin-neurophysin (hNpI), oxytocin-neurophysin (hNpII) and blood osmolality were assayed before any treatment in basal conditions in 35 patients suffering from lung carcinoma (20 oat cell, 6 undifferentiated and 9 well-differentiated epidermoid cell carcinomas). Plasma vasopressin (antidiuretic hormone, ADH) was also assayed in 7 of the 20 patients suffering from oat cell carcinoma. We found a close correlation (r = 0.98) between plasma ADH and hNpI levels in the 7 patients. Further, hNpI was elevated in 13 out of the 20 oat cell carcinoma patients and in none of the epidermoid-cell carcinoma group; however, searching for an abnormality of ADH secretion as reflected by a detectable plasma hNpI level together with subnormal plasma osmolality revealed 2 additional positive results in the oat cell carcinoma group, and 2 out of the 6 in the undifferentiated-cell carcinoma group. hNpII was increased together with an increase in hNpI in 6 oat cell carcinoma patients; it was specifically increased without hNpI increment in 2 additional oat cell carcinoma patients and in 2 patients of the undifferentiated-cell carcinoma group (different from the 2 positive for the hNpI-osmolality ratio). hNpI and hNpII were normal in the majority of undifferentiated and all of the differentiated epidermoid-cell carcinoma group. Hence, our results show that simultaneous measurements of hNpI, hNpII, and blood osmolality could detect abnormalities in 17 out of 20 oat cell carcinoma patients, in 4 of the 9 undifferentiated-cell carcinoma patients, but in none of the differentiated epidermoid-cell carcinoma patients, suggesting that the neurophysin assay can be used for the early detection of oat cell- and possibly other neuroendocrine-derived carcinomas. 相似文献
109.
Cell ultrastructure was investigated during the dispersion phase of development in the annual fish Cynolebias. Three cellular populations encompass the yolk mass during dispersion, namely, 1) the yolk syncytial layer (YSL) or periblast, which lies directly over the surface of the yolk; 2) the deep blastomeres of the blastoderm, which engage in morphogenetic movements on the surface of the YSL and beneath the enveloping layer prior to forming the future embryo; and 3) the enveloping layer (EVL) of the blastoderm, which is a cohesive epithelium that forms the outermost cell layer of the blastoderm. Deep blastomeres contain numerous mitochondria and scattered glycogen rosettes that appear to function in the utilization of energy reserves. These cells also possess surface extensions such as filopodia and ruffles. Numerous microfilaments running parallel to the plasma membrane occur in cell extensions and in the cortical cytoplasm of neighboring blastomeres. In bleb-like extensions such as ruffles, microfilamentous stress fibers run parallel to the plane of the plasma membrane and prevent cellular organelles from entering the hyaline cap of the ruffle. Deep blastomeres also have basal projections that contain glycogen as well as pits in the basal membrane. Blastomeres move about using the YSL as a substrate. The YSL possesses specializations for nutrient uptake, storage, and transport such as numerous multivesicular bodies and large amounts of glycogen. Glycogen, in the rosette form, occurs in extraordinary amounts, virtually occluding the cytoplasm. Glycogen reserves are postulated to serve as an energy source during diapause. Glycogen is sometimes contained within villous projections that extend from the apical surface of the YSL. This configuration suggests the possibility of glycogen transport to the overlying deep blastomeres. Specializations of the EVL include apical tight junctions and basal lateral zonulae adherentes that interdigitate with those of adjacent EVL cells. The EVL serves as an impermeable membrane that protects the developing egg from the vicissitudes of its environment. 相似文献
110.
Phylogenetic analysis and secondary structure of the Bacillus subtilis bacteriophage RNA required for DNA packaging 总被引:4,自引:0,他引:4
S Bailey J Wichitwechkarn D Johnson B E Reilly D L Anderson J W Bodley 《The Journal of biological chemistry》1990,265(36):22365-22370
An unusual RNA molecule encoded by the Bacillus subtilis bacteriophage phi 29 is a structural component of the viral prohead and is required for the ATP-dependent packaging of DNA. Here we report a model of secondary structure for this prohead RNA developed from a phylogenetic analysis of the primary sequences of prohead RNAs of related phages. Twenty-nine phages related to phi 29 were found to produce prohead RNAs. These RNAs were analyzed by their ability to replace phi 29 RNA in in vitro phage assembly, by Northern blot hybridization with a probe complementary to phi 29 RNA, and by partial and complete sequence analyses. These analyses revealed four quite different sequences ranging in length from 161 to 174 residues. The secondary structure deduced from these sequences, in agreement with earlier observations, indicated that prohead RNA is organized into two domains. The larger 5'-domain (Domain I) is composed of 113-117 residues and contains four helices. Three of these helices appear to be organized into a central stem that is interrupted by two unpaired loops and the fourth helix and loop. The smaller 3'-domain (Domain II) is composed of 40-44 residues and consists of two helices. Domains I and II are separated by 8-13 unpaired residues. Nuclease cleavage occurs readily in this single-stranded joining region, and this cleavage allows the subsequent separation of the two RNA domains. The separated Domain I is fully active in DNA packaging in vitro. The functional significance and biological role of Domain II are unknown. The phylogenetic secondary structure model provides a basis for further analysis of the role of this RNA in bacteriophage morphogenesis. 相似文献