全文获取类型
收费全文 | 30017篇 |
免费 | 3840篇 |
国内免费 | 29篇 |
出版年
2016年 | 268篇 |
2015年 | 356篇 |
2014年 | 442篇 |
2013年 | 714篇 |
2012年 | 700篇 |
2011年 | 732篇 |
2010年 | 514篇 |
2009年 | 440篇 |
2008年 | 626篇 |
2007年 | 680篇 |
2006年 | 646篇 |
2005年 | 651篇 |
2004年 | 660篇 |
2003年 | 660篇 |
2002年 | 690篇 |
2001年 | 1879篇 |
2000年 | 1876篇 |
1999年 | 1468篇 |
1998年 | 495篇 |
1997年 | 484篇 |
1996年 | 502篇 |
1995年 | 434篇 |
1994年 | 446篇 |
1993年 | 425篇 |
1992年 | 1089篇 |
1991年 | 1104篇 |
1990年 | 1094篇 |
1989年 | 1049篇 |
1988年 | 978篇 |
1987年 | 928篇 |
1986年 | 791篇 |
1985年 | 760篇 |
1984年 | 601篇 |
1983年 | 521篇 |
1982年 | 375篇 |
1981年 | 351篇 |
1980年 | 336篇 |
1979年 | 570篇 |
1978年 | 460篇 |
1977年 | 418篇 |
1976年 | 347篇 |
1975年 | 461篇 |
1974年 | 473篇 |
1973年 | 420篇 |
1972年 | 402篇 |
1971年 | 316篇 |
1970年 | 258篇 |
1969年 | 242篇 |
1968年 | 227篇 |
1967年 | 207篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
62.
63.
S Scarfi M Giovine A Gasparini G Damonte E Millo M Pozzolini U Benatti 《FEBS letters》1999,451(3):264-268
Overexpression of inducible nitric oxide synthase causes the production of high levels of nitric oxide, which, under pathological conditions, leads to immunosuppression and tissue damage. The results recently obtained using peptide nucleic acids, rather than traditional oligonucleotides as antigen and antisense molecules, prompted us to test their efficacy in the regulation of nitric oxide production, thereby overcoming the obstacle of cellular internalization. The cellular permeability of four inducible nitric oxide synthase antisense peptide nucleic acids of different lengths was evaluated. These peptide nucleic acids were covalently linked to a hydrophobic peptide moiety to increase internalization and to a tyrosine to allow selective 125I radiolabelling. Internalization experiments showed a 3-25-fold increase in the membrane permeability of the modified peptide nucleic acids with respect to controls. Inducible nitric oxide synthase inhibition experiments on intact stimulated macrophages RAW 264.7 after passive permeation of the two antisense peptide nucleic acids 3 and 4 demonstrated a significant decrease (43-44%) in protein enzymatic activity with respect to the controls. These data offer a basis for developing a good alternative to conventional drugs directed against inducible nitric oxide synthase overexpression. 相似文献
64.
Jennifer Beckensteiner David M. Kaplan Warren M. Potts Carmen V. Santos Michael R. O’Farrell 《PloS one》2016,11(2)
Excessive truncation of a population’s size structure is often identified as an important deleterious effect of exploitation, yet the effect on population persistence of size-structure truncation caused by exploitation is often not quantified due to data limitations. In this study, we estimate changes in eggs per recruit (EPR) using annual length-frequency samples over a 9 year period to assess persistence of the two most important recreational fishes in southern Angola: west coast dusky kob (Argyrosomus coronus) and leerfish (Lichia amia). Using a length- and age-structured model, we improve on an existing method to fit this type of model to length-frequency data and estimate EPR. The objectives of the methodological changes are to add flexibility and robustness to the approach for assessing population status in data-limited situations. Results indicate that dusky kob presents very low levels of EPR (5%-10% of the per recruit reproductive capacity in the absence of fishing) in 2013, whereas large inter-annual variability in leerfish estimates suggest caution must be applied when drawing conclusions about its exploitation status. Using simulated length frequency data with known parameter values, we demonstrate that recruitment decline due to overexploitation leads to overestimation of EPR values. Considering the low levels of EPR estimated for the study species, recruitment limitation is not impossible and true EPR values may be even lower than our estimates. It is, therefore, likely that management action, such as the creation of Marine Protected Areas, is needed to reconstitute the west coast dusky kob population. 相似文献
65.
66.
U Pfeffer A Di Vinci E Geido G Vidali W Giaretti 《Journal of cellular physiology》1991,149(3):567-574
We have recently described a novel nuclear antigen, AF-2, which is related to cell cycle dependent alterations of chromatin structure. We show by two parameter flow cytometry on a cell by cell basis that the antigen is accessible to specific monoclonal antibodies only in mitotic and postmitotic early G1-phase cells. The evaluation of nuclease susceptibility and AF-2 antigen accessibility reveals different subcompartments of the G1-phase of the cell cycle with distinct chromatin conformations. Digestion with DNase I seems to alter the chromatin structure according to concentration and this is reflected by an increase of the antigen accessibility. Chromatin in the more condensed early G1-phase is specifically digested by lower concentrations of the enzyme than chromatin in later stages of interphase. Chromatin from cells in the late-G1, S-, and G2-phases shows a higher relative resistance to DNase I and a reduced accessibility of the AF-2 antigen to monoclonal antibodies. Nuclease S1 has a similar effect on chromatin topology, as revealed by the reaction with anti-AF-2 antibodies, without digestion of detectable amounts of DNA. The antigen becomes available to the antibodies in almost all cells by digestion with high concentrations of DNase I or Nuclease S1. 相似文献
67.
N Brinkmann M Malissard M Ramuz U R?mer T Schumacher E G Berger L Elling C Wandrey A Liese 《Bioorganic & medicinal chemistry letters》2001,11(18):2503-2506
The alpha-Gal trisaccharide Gal(alpha)(1-->3)Galbeta(1-->4)GlcNAc 11 was synthesized on a homogeneously soluble polymeric support (polyethylene glycol, PEG) by use of a multi-enzyme system consisting of beta-1,4-galactosyltransferase (EC 2.4.1.38), alpha-1,3-galactosyltransferase (EC 2.4.1.151), sucrose synthase (EC 2.4.1.13) and UDP-glucose-4-epimerase (EC 5.1.3.2). In addition workup was simplified by use of dia-ultrafiltration. Thus the advantages of classic chemistry/enzymology and solid-phase synthesis could be united in one. Subsequent hydrogenolytic cleavage afforded the free alpha-Gal trisaccharide. 相似文献
68.
69.
During their 120 to 165 million years of isolation, the flora and fauna of Madagascar evolved, to a large extent, independently of the African mainland.1 In contrast to other oceanic islands, Madagascar is large enough to house the major components of tropical ecosystems, allowing tests of evolutionary hypotheses on the level of complete communities. Taking lemurs, the primates of Madagascar, as an example, evolutionary hypotheses correctly predict the organization of their community structure with respect to ecological correlates. Lemur social systems and their morphological correlates, on the other hand, deviate in some respects from those of other primates. Apparently, lemur social systems are influenced by several selection pressures that are weak or rare in other primates. These include variable activity patterns and avoidance of infanticide. The interspecific variation in lemur social systems therefore offers a unique opportunity for a comprehensive study of the determinants of primate social systems. 相似文献
70.
D A Nickson M W McBride S Zeinali C S Hawes A Petropoulos U W Mueller R G Sutcliffe 《Journal of reproduction and fertility》1991,93(1):149-156
The monoclonal antibody FDO161G reacts with a 43-kDa protein found in human extravillous trophoblast, syncytiotrophoblast, adrenal cortex, interstitial cells of the testis and ovarian follicle cumulus cells. cDNAs for this protein have been isolated from the lambda gt11 library, sequenced, and expressed in COS-7 cells. The protein was identified as 3 beta-hydroxy-5-ene steroid dehydrogenase (HSD). The sequence of the HSD protein raises questions about its association with cell membrane systems. The lack of reactivity of FDO161G with other tissues suggests that HSD has a limited tissue distribution and that other enzymes may exist in peripheral tissues, which can convert delta 5 3-hydroxysteroids to delta 4 3-ketosteroids. 相似文献