首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   903篇
  免费   97篇
  2023年   2篇
  2022年   6篇
  2021年   14篇
  2020年   8篇
  2019年   9篇
  2018年   12篇
  2017年   17篇
  2016年   24篇
  2015年   21篇
  2014年   42篇
  2013年   49篇
  2012年   66篇
  2011年   66篇
  2010年   47篇
  2009年   39篇
  2008年   45篇
  2007年   71篇
  2006年   61篇
  2005年   61篇
  2004年   57篇
  2003年   61篇
  2002年   45篇
  2001年   11篇
  2000年   18篇
  1999年   11篇
  1998年   15篇
  1997年   8篇
  1996年   9篇
  1995年   15篇
  1994年   7篇
  1993年   3篇
  1992年   9篇
  1991年   8篇
  1990年   6篇
  1989年   7篇
  1988年   4篇
  1987年   4篇
  1986年   2篇
  1985年   6篇
  1984年   2篇
  1983年   3篇
  1982年   5篇
  1980年   2篇
  1979年   3篇
  1978年   2篇
  1976年   2篇
  1975年   2篇
  1974年   2篇
  1957年   1篇
  1916年   1篇
排序方式: 共有1000条查询结果,搜索用时 15 毫秒
71.
Patterns of multilocus DNA sequence variation within and between closely related taxa can provide insights into the history of divergence. Here, we report on DNA polymorphism and divergence at six nuclear loci in globally distributed samples of the ascomycete Venturia inaequalis, responsible for scab on apple, loquat, and pyracantha. Isolates from different hosts were differentiated but did not form diagnosable distinct phylogenetic species. Parameters of an Isolation-with-Migration model estimated from the data suggested that the large amount of variation shared among groups more likely resulted from recent splitting than from extensive genetic exchanges. Inferred levels of gene flow among groups were low and more concentrated toward recent times, and we identified two potentially recent one-off shifters from apple and pyracantha to loquat. These findings support a scenario of recent divergence in allopatry followed by introgression through secondary contact, with groups from loquat and pyracantha being the most recently differentiated.  相似文献   
72.
73.
Social grooming is ubiquitous among the captive chimpanzees at Chester Zoo. Seven individuals were chosen here for a study of third-party social dynamics. The grooming decisions of five adult males were analysed, but only insofar as they directed attention to a mother–daughter pair. Uniquely, the daughter was an unpopular and physically disabled subadult whose congenital motor impairments prevented her from grooming others effectively. The impetus for this study was the observation that some males increased their grooming towards the disabled daughter during days when the mother had a tumescent anogenital swelling (sexually attractive to males) compared to days when the mother was not tumescent (less attractive). Apparently, males were grooming the daughter with no possibility of payback (because the daughter could never “return the favour”). A “grooming rate” (avg. grooming time/hour) was calculated that showed the grooming efforts of all five males towards both mother and daughter. These rates were compared on days when (1) the mother’s anogenital swelling was tumescent, and (2) days when the swelling was not tumescent. Each male showed a different pattern of behaviour. Two males groomed the daughter significantly more when the mother was tumescent. Results for all males were graphed against the quality of the social relationship between each male and the mother. Apparently, only males that had a weaker relationship to the mother groomed the daughter more when the mother was tumescent. This pattern did not exist for males with a stronger relationship to the mother. Possibly, the insecure males were using the disabled daughter as a way to curry favour with the attractive mother. If this is confirmed, then this type of triadic situation is a possible setting for indirect reciprocity to occur.  相似文献   
74.
Almost all cardiovascular events occur according to a circadian rhythm with a greater frequency in the morning on waking and when resuming activity, the mechanism and precise triggering events for myocardial infarction (MI) are not yet fully known. Multiple biologic functions show a diurnal and/or seasonal variation that may contribute to adverse cardiac outomes. Exogenous factors may also modulates these variations. The MI peak usually occurs between 07:00 and 12:00 h. This timing corresponds to the concurrent increase in platelet aggregability, blood concentration of cortisol, catecholamines, angiotensin II, myocardial oxygen demand and coagulation activity, while fibrinolytic activity is decreased. In this review paper we will point out the biological rhythms of a number of functions involved in acute myocardial infarction e.g. blood pressure, hormonal determinants, cholesterol, among others.  相似文献   
75.
76.
Natural transformation is a mechanism for genetic exchange in many bacterial genera. It proceeds through the uptake of exogenous DNA and subsequent homology-dependent integration into the genome. In Streptococcus pneumoniae, this integration requires the ubiquitous recombinase, RecA, and DprA, a protein of unknown function widely conserved in bacteria. To unravel the role of DprA, we have studied the properties of the purified S. pneumoniae protein and its Bacillus subtilis ortholog (Smf). We report that DprA and Smf bind cooperatively to single-stranded DNA (ssDNA) and that these proteins both self-interact and interact with RecA. We demonstrate that DprA-RecA-ssDNA filaments are produced and that these filaments catalyze the homology-dependent formation of joint molecules. Finally, we show that while the Escherichia coli ssDNA-binding protein SSB limits access of RecA to ssDNA, DprA lowers this barrier. We propose that DprA is a new member of the recombination-mediator protein family, dedicated to natural bacterial transformation.  相似文献   
77.
The effects of Cry toxins from Bacillus thuringiensis (Berliner) (Bt) on the anthocorid Orius albidipennis Reuter were studied under laboratory conditions. Tritrophic experiments were performed, in which Orius nymphs were fed Helicoverpa armigera (Hübner) larvae reared on a diet with Cry1Ac, Cry1Ab, or Cry2Ab toxins at different concentrations (0, 1, and 10 microg/ml), when supplemented with Ephestia kuehniella Zeller eggs. In complementary experiments, the Bt Cry1Ac toxin was directly fed to Orius nymphs at a very high concentration (1 mg/ml). No effects on prey consumption, developmental time, nymph survival, fecundity, and egg hatching of O. albidipennis were found in either experiment. It can be concluded that the toxins tested do not seem to pose a risk for the anthocorid O. albidipennis, especially when it is exposed through the prey.  相似文献   
78.
Protonation/deprotonation equilibria are frequently linked to binding processes involving proteins. The presence of these thermodynamically linked equilibria affects the observable thermodynamic parameters of the interaction (K obs, ΔH obs0). In order to try and elucidate the energetic factors that govern these binding processes, a complete thermodynamic characterisation of each intrinsic equilibrium linked to the complexation event is needed and should furthermore be correlated to structural information. We present here a detailed study, using NMR and ITC, of the interaction between α-chymotrypsin and one of its competitive inhibitors, proflavin. By performing proflavin titrations of the enzyme, at different pH values, we were able to highlight by NMR the effect of the complexation of the inhibitor on the ionisable residues of the catalytic triad of the enzyme. Using ITC we determined the intrinsic thermodynamic parameters of the different equilibria linked to the binding process. The possible driving forces of the interaction between α-chymotrypsin and proflavin are discussed in the light of the experimental data and on the basis of a model of the complex. This study emphasises the complementarities between ITC and NMR for the study of binding processes involving protonation/deprotonation equilibria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
79.
80.
The molecular mechanisms of endothelial differentiation into a functional vascular network are incompletely understood. To identify novel factors in endothelial development, we used a microarray screen with differentiating embryonic stem (ES) cells that identified the gene for ankyrin repeat and SOCS box protein 4 (ASB4) as the most highly differentially expressed gene in the vascular lineage during early differentiation. Like other SOCS box-containing proteins, ASB4 is the substrate recognition molecule of an elongin B/elongin C/cullin/Roc ubiquitin ligase complex that mediates the ubiquitination and degradation of substrate protein(s). High levels of ASB4 expression in the embryonic vasculature coincide with drastic increases in oxygen tension as placental blood flow is initiated. However, as vessels mature and oxygen levels stabilize, ASB4 expression is quickly downregulated, suggesting that ASB4 may function to modulate an endothelium-specific response to increasing oxygen tension. Consistent with the hypothesis that ASB4 function is regulated by oxygen concentration, ASB4 interacts with the factor inhibiting HIF1alpha (FIH) and is a substrate for FIH-mediated hydroxylation via an oxygen-dependent mechanism. Additionally, overexpression of ASB4 in ES cells promotes differentiation into the vascular lineage in an oxygen-dependent manner. We postulate that hydroxylation of ASB4 in normoxia promotes binding to and degradation of substrate protein(s) to modulate vascular differentiation.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号