全文获取类型
收费全文 | 64347篇 |
免费 | 6967篇 |
国内免费 | 5213篇 |
出版年
2023年 | 523篇 |
2022年 | 1149篇 |
2021年 | 2159篇 |
2020年 | 1559篇 |
2019年 | 1996篇 |
2018年 | 1881篇 |
2017年 | 1494篇 |
2016年 | 2139篇 |
2015年 | 3165篇 |
2014年 | 3749篇 |
2013年 | 4076篇 |
2012年 | 4925篇 |
2011年 | 4476篇 |
2010年 | 2886篇 |
2009年 | 2618篇 |
2008年 | 3199篇 |
2007年 | 2967篇 |
2006年 | 2650篇 |
2005年 | 2310篇 |
2004年 | 2049篇 |
2003年 | 1986篇 |
2002年 | 1784篇 |
2001年 | 1467篇 |
2000年 | 1385篇 |
1999年 | 1209篇 |
1998年 | 699篇 |
1997年 | 638篇 |
1996年 | 598篇 |
1995年 | 501篇 |
1994年 | 498篇 |
1993年 | 456篇 |
1992年 | 814篇 |
1991年 | 733篇 |
1990年 | 689篇 |
1989年 | 736篇 |
1988年 | 624篇 |
1987年 | 642篇 |
1986年 | 526篇 |
1985年 | 623篇 |
1984年 | 501篇 |
1983年 | 414篇 |
1982年 | 427篇 |
1981年 | 381篇 |
1980年 | 338篇 |
1979年 | 455篇 |
1978年 | 391篇 |
1977年 | 343篇 |
1975年 | 339篇 |
1974年 | 390篇 |
1973年 | 372篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Wei CHEN Liqing PENG Lichun JIANG David A.PIKE Christopher R.FRIESEN Gregory BROWN 《亚洲两栖爬行动物研究(英文版)》2018,(1)
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes. 相似文献
2.
T Smith 《BMJ (Clinical research ed.)》1984,289(6449):909-910
3.
4.
Andrew C Smith 《BMJ (Clinical research ed.)》1981,283(6306):1596-1597
5.
Escherichia coli RecBC pseudorevertants lacking chi recombinational hotspot activity 总被引:24,自引:7,他引:17
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Pseudorevertants of an Escherichia coli exonuclease V (RecBC enzyme)-negative mutant have been isolated after ethyl methane sulfonate mutagenesis of a recC73 (presumed missense) mutant. The remedial mutations in each of the four pseudorevertants studied in detail map and complement as recC mutations. By several criteria, such as recombination proficiency, support of phage growth, RecBC nuclease activity, and cell viability, the pseudorevertants appear to have regained partially or completely various aspects of RecBC activity. However, chi recombinational hotspots, which stimulate exclusively the RecBC pathway of recombination, have no detectable activity in lambda vegetative crosses in the pseudorevertants. The properties of these mutants, in which the RecBC pathway of recombination is active yet in which chi is not active, are consistent with the hypothesis that wild-type RecBC enzyme directly interacts with chi sites; alternatively, the mutants may block or bypass the productive interaction of another recombinational enzyme with chi. 相似文献
6.
T Smith 《BMJ (Clinical research ed.)》1980,281(6252):1410-1411
7.
Selective diapedesis of Th1 cells induced by endothelial cell RANTES. 总被引:16,自引:0,他引:16
T Kawai M Seki K Hiromatsu J W Eastcott G F Watts M Sugai D J Smith S A Porcelli M A Taubman 《Journal of immunology (Baltimore, Md. : 1950)》1999,163(6):3269-3278
Differentiated CD4 T cells can be divided into Th1 and Th2 types based on the cytokines they produce. Differential expression of chemokine receptors on either the Th1-type or the Th2-type cell suggests that Th1-type and Th2-type cells differ not only in cytokine production but also in their migratory capacity. Stimulation of endothelial cells with IFN-gamma selectively enhanced transmigration of Th1-type cells, but not Th2-type cells, in a transendothelial migration assay. Enhanced transmigration of Th1-type cells was dependent on the chemokine RANTES produced by endothelial cells, as indicated by the findings that Ab neutralizing RANTES, or Ab to its receptor CCR5, inhibited transmigration. Neutralizing Ab to chemokines macrophage-inflammatory protein-1alpha or monocyte chemotactic protein-1 did not inhibit Th1 selective migration. Whereas anti-CD18 and anti-CD54 blocked basal levels of Th1-type cell adherence to endothelial cells and also inhibited transmigration, anti-RANTES blocked only transmigration, indicating that RANTES appeared to induce transmigration of adherent T cells. RANTES seemed to promote diapedesis of adherent Th1-type cells by augmenting pseudopod formation in conjunction with actin rearrangement by a pathway that was sensitive to the phosphoinositol 3-kinase inhibitor wortmannin and to the Rho GTP-binding protein inhibitor, epidermal cell differentiation inhibitor. Thus, enhancement of Th1-type selective migration appeared to be responsible for the diapedesis induced by interaction between CCR5 on Th1-type cells and RANTES produced by endothelial cells. Further evidence that CCR5 and RANTES play a modulatory role in Th1-type selective migration derives from the abrogation of this migration by anti-RANTES and anti-CCR5 Abs. 相似文献
8.
Food availability is important to the dynamics of animal social organizations or populations. However, the role of winter
food availability in animal population dynamics is still controversial. We carried out an experimental study to test Lack’s
hypothesis that reduced food in winter limits survival and spring numbers of breeding individuals of social groups, using
the Mongolian gerbil (Meriones unguiculatus) as model species. We established 24 gerbil social groups in 24, 10 × 10 m, pens in September 2008. We provided wheat seeds
as supplemental food in 12 enclosures from September 2008 to March 2009; the other 12 enclosures, not provided with supplemental
food, served as controls. We live-trapped gerbils at a 2-week interval from September to April. Supplemental food during winter
increased biweekly survival by 10% relative to that in control groups. Only four control social groups survived to the end
of our study whereas all 12 food-supplemented social groups survived through our study period. Supplemental food also increased
cumulative numbers of recruits and group sizes of gerbils. We conclude that winter food availability limits winter survival
and spring social groups or population sizes of Mongolian gerbils. 相似文献
9.
10.
S. Wehmeier A. S. Varghese S. S. Gurcha B. Tissot M. Panico P. Hitchen H. R. Morris G. S. Besra A. Dell M. C. M. Smith 《Molecular microbiology》2009,71(2):421-433
Previously mutations in a putative protein O -mannosyltransferase (SCO3154, Pmt) and a polyprenol phosphate mannose synthase (SCO1423, Ppm1) were found to cause resistance to phage, φC31, in the antibiotic producing bacteria Streptomyces coelicolor A3(2). It was proposed that these two enzymes were part of a protein O-glycosylation pathway that was necessary for synthesis of the phage receptor. Here we provide the evidence that Pmt and Ppm1 are indeed both required for protein O-glycosylation. The phosphate binding protein PstS was found to be glycosylated with a trihexose in the S. coelicolor parent strain, J1929, but not in the pmt − derivative, DT1025. Ppm1 was necessary for the transfer of mannose to endogenous polyprenol phosphate in membrane preparations of S. coelicolor . A mutation in ppm1 that conferred an E218V substitution in Ppm1 abolished mannose transfer and glycosylation of PstS. Mass spectrometry analysis of extracted lipids showed the presence of a glycosylated polyprenol phosphate (PP) containing nine repeated isoprenyl units (C45 -PP). S. coelicolor membranes were also able to catalyse the transfer of mannose to peptides derived from PstS, indicating that these could be targets for Pmt in vivo . 相似文献