全文获取类型
收费全文 | 1277593篇 |
免费 | 114066篇 |
国内免费 | 1615篇 |
专业分类
1393274篇 |
出版年
2021年 | 17751篇 |
2020年 | 12601篇 |
2019年 | 16152篇 |
2018年 | 17039篇 |
2017年 | 15801篇 |
2016年 | 27593篇 |
2015年 | 42084篇 |
2014年 | 50143篇 |
2013年 | 76387篇 |
2012年 | 34029篇 |
2011年 | 21539篇 |
2010年 | 42300篇 |
2009年 | 44033篇 |
2008年 | 21076篇 |
2007年 | 18647篇 |
2006年 | 24743篇 |
2005年 | 25874篇 |
2004年 | 25181篇 |
2003年 | 23006篇 |
2002年 | 21023篇 |
2001年 | 28413篇 |
2000年 | 25587篇 |
1999年 | 27181篇 |
1998年 | 24845篇 |
1997年 | 24693篇 |
1996年 | 24447篇 |
1995年 | 22485篇 |
1994年 | 22368篇 |
1993年 | 21438篇 |
1992年 | 24354篇 |
1991年 | 22795篇 |
1990年 | 21404篇 |
1989年 | 22455篇 |
1988年 | 20652篇 |
1987年 | 19860篇 |
1986年 | 18850篇 |
1985年 | 20833篇 |
1984年 | 21154篇 |
1983年 | 18862篇 |
1982年 | 19920篇 |
1981年 | 19306篇 |
1980年 | 17977篇 |
1979年 | 17691篇 |
1978年 | 16917篇 |
1977年 | 16284篇 |
1976年 | 15670篇 |
1975年 | 14961篇 |
1974年 | 15504篇 |
1973年 | 15779篇 |
1972年 | 13433篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
Genetic analysis of sterile mutants in the dpy-5 unc-13 (I) genomic region of Caenorhabditis elegans
Essential genes were identified in the 1.5-map unit dpy-5 unc-13 region of chromosome I in the Caenorhabditis elegans genome by rescuing lethal mutations using the duplication sDp2. In this paper, we report the mapping and complementation testing of lethal mutations, 45 of which identify 18 new, essential
genes. This analysis brings the number of essential genes defined by the sDp2 rescue of lethal mutants to 97; 64 of these map between dpy-5 and unc-13. 61% of these essential genes are identified by more than one allele. Positioning of the mutations was done using the breakpoints
of six duplications. The mutant phenotypes of 14 loci essential for fertility were characterized by Nomarski microscopy and
DAPI staining. None of the mutants were rescued by wild-type male sperm. The cytological data showed that four genes produced
mutants with defects in gonadogenesis, let-395, let-603, let-605 and let-610. Mutations in seven genes, let-355, let-367, let-384, let-513, let-544, let-545 and let-606, affected germ cell proliferation or gametogenesis. Mutants for the remaining three genes, let-370, let-599 and let-604, produced eggs that failed to develop or hatch, thereby acting as maternal effect lethals. We observed a nonrandom distribution
of arrest phenotypes with regard to map position.
Received: 8 May 1996 / Accepted : 27 January 1997 相似文献
13.
14.
15.
E Estébanez-Perpi?á A Bayés J Vendrell M A Jongsma D P Bown J A Gatehouse R Huber W Bode F X Avilés D Reverter 《Journal of molecular biology》2001,313(3):629-638
The cotton bollworm Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) is one of the most serious insect pests in Australia, India and China. The larva causes substantial economical losses to legume, fibre, cereal oilseed and vegetable crops. This pest has proven to be difficult to control by conventional means, mainly due to the development of pesticide resistance. We present here the 2.5 A crystal structure from the novel procarboxypeptidase (PCPAHa) found in the gut extracts from H. armigera larvae, the first one reported for an insect. This metalloprotease is synthesized as a zymogen of 46.6 kDa which, upon in vitro activation with Lys-C endoproteinase, yields a pro-segment of 91 residues and an active carboxypeptidase moiety of 318 residues. Both regions show a three-dimensional structure quite similar to the corresponding structures in mammalian digestive carboxypeptidases, the most relevant structural differences being located in the loops between conserved secondary structure elements, including the primary activation site. This activation site contains the motif (Ala)(5)Lys at the C terminus of the helix connecting the pro- and the carboxypeptidase domains. A remarkable feature of PCPAHa is the occurrence of the same (Ala)(6)Lys near the C terminus of the active enzyme. The presence of Ser255 in PCPAHa instead of Ile and Asp found in the pancreatic A and B forms, respectively, enlarges the S1' specificity pocket and influences the substrate preferences of the enzyme. The C-terminal tail of the leech carboxypeptidase inhibitor has been modelled into the PCPAHa active site to explore the substrate preferences and the enzymatic mechanism of this enzyme. 相似文献
16.
C Mapp J Hartiala O L Frick R L Shields W M Gold 《Journal of applied physiology》1986,61(4):1467-1474
The relationship between airway responsiveness to inhaled antigen and histamine, immunologic release of lung histamine, immunologic responsiveness of skin, and specific immunoglobulin E (IgE) antibodies were examined in 11 inbred allergic dogs immunized with extracts of ragweed and grass and 5 nonimmunized control dogs from the same colony. Airway responsiveness to antigen and histamine was characterized by the doses that increased the airflow resistance of the total respiratory system to twice the control values (ED200). Highly significant correlations were found between airway responsiveness and cutaneous responsiveness to antigen and other immunologic characteristics (e.g., IgE and histamine released from lung by inhaled antigen) in all dogs. In ragweed-sensitized dogs, there was an inverse correlation between immunologic responsiveness (reflected by the cutaneous response to antigen and histamine released from lung by inhaled antigen) and nonimmunologic responsiveness of airways (histamine ED200: r = 0.73, P less than 0.05 and r = 0.75, P less than 0.01, respectively). Antigen ED200 was also correlated with histamine release from lung after antigen inhalation (r = 0.74; P less than 0.01). We conclude that airway reactions to inhaled antigen in allergic dogs are dependent not only on immunologic factors but also on the degree of nonimmunologic airway responsiveness to histamine and that these factors are correlated inversely. 相似文献
17.
18.
19.
In vivo EMG biofeedback was found to be an effective pedagogical tool for removing unwanted left-hand tension in nine violin and viola players. Improvement occurred rapidly and persisted throughout a 5-month follow-up period. Further studies will be necessary to assess the effect of biofeedback independent of placebo effects. The brevity of the method and the magnitude of improvement warrant further investigation. 相似文献
20.
Unlikelihood that minimal phylogenies for a realistic biological study can be constructed in reasonable computational time 总被引:1,自引:0,他引:1
R. L. Graham 《Mathematical biosciences》1982,60(2):133-142
The problem of determining a phylogeny of maximum parsimony from a given set of protein sequences is defined. It is shown that this problem is what is called, in computer science, NP-complete. The implication of this result is that it is equivalent in difficulty to a host of other problems in combinatorial optimization which are notorious for their intractability. This implies that it is more fruitful to attempt to develop heuristic techniques (which do not guarantee maximum parsimony but which do run in reasonable computer time) than to try to develop exact algorithms for phylogeny construction 相似文献