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   1288篇
  免费   18篇
  国内免费   5篇
系统科学   23篇
丛书文集   2篇
理论与方法论   31篇
现状及发展   711篇
研究方法   65篇
综合类   437篇
自然研究   42篇
  2019年   10篇
  2018年   24篇
  2017年   25篇
  2016年   27篇
  2015年   20篇
  2014年   25篇
  2013年   26篇
  2012年   40篇
  2011年   75篇
  2010年   36篇
  2009年   13篇
  2008年   47篇
  2007年   31篇
  2006年   42篇
  2005年   52篇
  2004年   42篇
  2003年   45篇
  2002年   37篇
  2001年   35篇
  2000年   23篇
  1999年   18篇
  1998年   13篇
  1991年   14篇
  1990年   16篇
  1989年   11篇
  1988年   10篇
  1987年   9篇
  1985年   14篇
  1984年   18篇
  1983年   9篇
  1982年   11篇
  1981年   17篇
  1980年   15篇
  1979年   27篇
  1978年   22篇
  1977年   16篇
  1976年   23篇
  1975年   10篇
  1974年   18篇
  1973年   23篇
  1972年   24篇
  1971年   25篇
  1970年   22篇
  1969年   24篇
  1968年   26篇
  1967年   29篇
  1966年   22篇
  1965年   21篇
  1964年   15篇
  1954年   8篇
排序方式: 共有1311条查询结果,搜索用时 15 毫秒
61.
Translation initiation is a critical step in protein synthesis. Previously, two major mechanisms of initiation were considered as essential: prokaryotic, based on SD interaction; and eukaryotic, requiring cap structure and ribosomal scanning. Although discovered decades ago, cap-independent translation has recently been acknowledged as a widely spread mechanism in viruses, which may take place in some cellular mRNA translations. Moreover, it has become evident that translation can be initiated on the leaderless mRNA in all three domains of life. New findings demonstrate that other distinguishable types of initiation exist, including SD-independent in Bacteria and Archaea, and various modifications of 5′ end-dependent and internal initiation mechanisms in Eukarya. Since translation initiation has developed through the loss, acquisition, and modification of functional elements, all of which have been elevated by competition with viral translation in a large number of organisms of different complexity, more variation in initiation mechanisms can be anticipated.  相似文献   
62.
A series of pharmacological and physiological studies have demonstrated the functional cross-regulation between MOR and NMDAR. These receptors coexist at postsynaptic sites in midbrain periaqueductal grey (PAG) neurons, an area implicated in the analgesic effects of opioids like morphine. In this study, we found that the MOR-associated histidine triad nucleotide-binding protein 1 (HINT1) is essential for maintaining the connection between the NMDAR and MOR. Morphine-induced analgesic tolerance is prevented and even rescued by inhibiting PKC or by antagonizing NMDAR. However, in the absence of HINT1, the MOR becomes supersensitive to morphine before suffering a profound and lasting desensitization that is refractory to PKC inhibition or NMDAR antagonism. Thus, HINT1 emerges as a key protein that is critical for sustaining NMDAR-mediated regulation of MOR signaling strength. Thus, HINT1 deficiency may contribute to opioid-intractable pain syndromes by causing long-term MOR desensitization via mechanisms independent of NMDAR.  相似文献   
63.
Plasticity is a well-known property of macrophages that is controlled by different changes in environmental signals. Macrophage polarization is regarded as a spectrum of activation phenotypes adjusted from one activation extreme, the classic (M1), to the other, the alternative (M2) activation. Here we show, in vitro and in vivo, that both M1 and M2 macrophage phenotypes are tightly coupled to specific patterns of gene expression. Novel M2-associated markers were characterized and identified as genes controlling the extracellular metabolism of ATP to generate pyrophosphates (PPi). Stimulation of M1 macrophages with PPi dampens both NLR and TLR signaling and thus mediates cytokine production. In this context extracellular PPi enhanced the resolution phase of a murine peritonitis model via a decrease in pro-inflammatory cytokine production. Therefore, our study reveals an additional level of plasticity modulating the resolution of inflammation.  相似文献   
64.
65.
66.
67.
Asynchronous reproduction is a common phenomenon in high-elevation populations of lizards from Central México. Sperm storage in the reproductive tract of females is the mechanism for making oocyte fertilization possible. Our study addresses questions related to functional oviductal sperm storage of females mating on different dates throughout the reproductive season. A population of Sceloporus mucronatus with copulation in the summer and ovulation in the fall was chosen for this experiment. Eleven females that copulated in the field during June and 13 females that copulated in captivity during August were maintained in the laboratory until parturition. The number of pregnant females and the litter sizes produced in each experimental group were indicative of the viability of the stored sperm. Sperm stored in the reproductive tract of females were able to fertilize eggs after 4 months. No significant differences were found in the number of pregnant females between the 2 experimental groups nor in the litter sizes that they produced. We found that the amount of time sperm were held in the female reproductive tract (ca. 3 months) had no effect on the capacity of sperm to fertilize eggs. Histological examination of 8 oviducts collected before the mating season eliminated the possibility of sperm storage from one year to the next. In this system, sperm retention could have evolved as a response mechanism to deal with the asynchrony between sexes in the reproductive cycles. However, we cannot rule out alternative hypotheses.  相似文献   
68.
We examined attributes of pi?on pine ( Pinus edulis ) associated with the probability of infestation by pi?on ips ( Ips confuses ) in an outbreak in the Coconino National Forest, Arizona. We used data collected from 87 plots, 59 infested and 28 uninfested, and a logistic regression approach to estimate the probability of infestation based on plot and tree-level attributes. Pi?on pine stand density index was a good predictor of the likelihood of infestation by pi?on ips at the plot level, and a cross-validation analysis confirmed that the model correctly classified 82% of the cases. Diameter at root collar and pi?on dwarf mistletoe infestation level were good predictors of individual tree infestation, and a cross-validation analysis indicated that the model correctly classified 72% of the cases. Results suggest that the occurrence of pi?on ips infestations may be related to stress factors associated with increased stocking and pi?on dwarf mistletoe infestations.  相似文献   
69.
70.
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