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   5300篇
  免费   363篇
  国内免费   4篇
  5667篇
  2021年   47篇
  2020年   26篇
  2019年   33篇
  2018年   48篇
  2017年   45篇
  2016年   58篇
  2015年   124篇
  2014年   135篇
  2013年   302篇
  2012年   212篇
  2011年   229篇
  2010年   148篇
  2009年   131篇
  2008年   237篇
  2007年   279篇
  2006年   211篇
  2005年   220篇
  2004年   227篇
  2003年   212篇
  2002年   203篇
  2001年   213篇
  2000年   212篇
  1999年   169篇
  1998年   65篇
  1997年   74篇
  1996年   64篇
  1995年   47篇
  1994年   63篇
  1993年   42篇
  1992年   123篇
  1991年   118篇
  1990年   130篇
  1989年   130篇
  1988年   111篇
  1987年   76篇
  1986年   82篇
  1985年   99篇
  1984年   83篇
  1983年   59篇
  1982年   45篇
  1981年   53篇
  1980年   39篇
  1979年   54篇
  1978年   37篇
  1977年   33篇
  1976年   33篇
  1975年   36篇
  1974年   34篇
  1973年   40篇
  1971年   26篇
排序方式: 共有5667条查询结果,搜索用时 15 毫秒
151.
The genome sequence of Leifsonia xyli subsp. xyli, which causes ratoon stunting disease and affects sugarcane worldwide, was determined. The single circular chromosome of Leifsonia xyli subsp. xyli CTCB07 was 2.6 Mb in length with a GC content of 68% and 2,044 predicted open reading frames. The analysis also revealed 307 predicted pseudogenes, which is more than any bacterial plant pathogen sequenced to date. Many of these pseudogenes, if functional, would likely be involved in the degradation of plant heteropolysaccharides, uptake of free sugars, and synthesis of amino acids. Although L. xyli subsp. xyli has only been identified colonizing the xylem vessels of sugarcane, the numbers of predicted regulatory genes and sugar transporters are similar to those in free-living organisms. Some of the predicted pathogenicity genes appear to have been acquired by lateral transfer and include genes for cellulase, pectinase, wilt-inducing protein, lysozyme, and desaturase. The presence of the latter may contribute to stunting, since it is likely involved in the synthesis of abscisic acid, a hormone that arrests growth. Our findings are consistent with the nutritionally fastidious behavior exhibited by L. xyli subsp. xyli and suggest an ongoing adaptation to the restricted ecological niche it inhabits.  相似文献   
152.
The genetic resources of a particular species of flowering cherry, Cerasus jamasakura, have high conservation priority because of its cultural, ecological and economic value in Japan. Therefore, the genetic structures of 12 natural populations of C. jamasakura were assessed using ten nuclear SSR loci. The population differentiation was relatively low (F ST, 0.043), reflecting long-distance dispersal of seeds by animals and historical human activities. However, a neighbor-joining tree derived from the acquired data, spatial analysis of molecular variance and STRUCTURE analysis revealed that the populations could be divided into two groups: one located on Kyusyu Island and one on Honshu Island. Genetic diversity parameters such as allelic richness and gene diversity were significantly lower in the Kyushu group than the Honshu group. Furthermore, STRUCTURE analysis revealed that the two lineages were admixed in the western part of Honshu Island. Thus, although the phylogeographical structure of the species and hybridization dynamics among related species need to be evaluated in detail using several marker systems, the Kyusyu Island and Honshu Island populations should be considered as different conservation units, and the islands should be regarded as distinct seed transfer zones for C. jamasakura, especially when rapid assessments are required.  相似文献   
153.
A sensitive thymocyte co-stimulator assay of IL-1 using a beta-D-galactoside specific lectin (allo A) obtained from the beetle (Allomyrina dichotoma) is reported here. Allo A stimulated [3H]thymidine uptake of mouse thymocytes in the presence of IL-1. The allo A assay was more sensitive than the PHA or PNA- thymocyte assay, especially at low doses of IL-1. Optimal conditions for the allo A assay were as follows: allo A, 2.5-5.0 micrograms/ml; whole thymocytes, 0.5-1.0 x 10(6) cells/well; incubation time, 72-96 hr. The assay is sensitive and convenient and can easily be performed in any laboratory.  相似文献   
154.
155.
We describe here the discovery and biological profile of a series of isoindolinone derivatives as developed mGluR1 antagonists. Our combined strategy of rapid parallel synthesis and conventional medicinal optimization successfully led to N-cyclopropyl 22 and N-isopropyl isoindolinone analogs 21 and 23 with improved in vivo DMPK profiles. Moreover the most advanced analog 23 showed an oral antipsychotic-like effect at a dose of 1 mg/kg in an animal model.  相似文献   
156.
Dicer1, an essential component of RNA interference and the microRNA pathway, has many important roles in the morphogenesis of developing tissues. Dicer1 null mice have been reported to die at E7.5; therefore it is impossible to study its function in adult tissues. We previously reported that Dicer1-hypomorphic mice, whose Dicer1 expression was reduced to 20% in all tissues, were unexpectedly viable. Here we analyzed these mice to ascertain whether the down-regulation of Dicer1 expression has any influence on adult tissues. Interestingly, all tissues of adult (8–10 week old) Dicer1-hypomorphic mice were histologically normal except for the pancreas, whose development was normal at the fetal and neonatal stages; however, morphologic abnormalities in Dicer1-hypomorphic mice were detected after 4 weeks of age. This suggested that Dicer1 is important for maintaining the adult pancreas.  相似文献   
157.
Kano  Rui  Kimura  Utako  Kakurai  Maki  Hiruma  Junichiro  Kamata  Hiroshi  Suga  Yasushi  Harada  Kazutoshi 《Mycopathologia》2020,185(6):947-958
Mycopathologia - In this report, we describe the first isolation of two highly terbinafine (TRF)-resistant Trichophyton interdigitale-like strains from a Nepali patient and an Indian patient with...  相似文献   
158.
We have isolated four strains of Rhodococcus which specifically degrade estrogens by using enrichment culture of activated sludge from wastewater treatment plants. Strain Y 50158, identified as Rhodococcus zopfii, completely and rapidly degraded 100 mg of 17beta-estradiol, estrone, estriol, and ethinyl estradiol/liter, as demonstrated by thin-layer chromatography and gas chromatography-mass spectrometry analyses. Strains Y 50155, Y 50156, and Y 50157, identified as Rhodococcus equi, showed degradation activities comparable with that of Y 50158. Using the random amplified polymorphism DNA fingerprinting test, these three strains were confirmed to have been derived from different sources. R. zopfii Y 50158, which showed the highest activity among these four strains, revealed that the strain selectively degraded 17beta-estradiol during jar fermentation, even when glucose was used as a readily utilizable carbon source in the culture medium. Measurement of estrogenic activities with human breast cancer-derived MVLN cells showed that these four strains each degraded 100 mg of 17beta-estradiol/liter to 1/100 of the specific activity level after 24 h. It is thus suggested that these strains degrade 17beta-estradiol into substances without estrogenic activity.  相似文献   
159.
Endothelin (ET)-1 is a 21-amino acid peptide that induces a variety of biological activities, including vasoconstriction and cell proliferation, and its likely involvement in cardiovascular and other diseases has recently led to broad clinical trials of ET receptor antagonists. ET-1 is widely distributed in the central nervous system (CNS), where it is thought to regulate hormone and neurotransmitter release. Here we show that CNS responses to emotional and physical stressors are differentially affected in heterozygous ET-1-knockout mice, which exhibited diminished aggressive and autonomic responses toward intruders (emotional stressors) but responded to restraint-induced (physical) stress more intensely than wild-type mice. This suggests differing roles of ET-1 in the central pathways mediating responses to different types of stress. Hypothalamic levels of ET-1 and the catecholamine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) were both increased in wild-type mice subjected to intruder stress, whereas MHPG levels were not significantly affected in ET-1-knockout mice. Furthermore, immunohistochemical analysis showed that ET-1 and tyrosine hydroxylase, an enzyme in the catecholamine synthesis pathway, were colocalized within certain neurons of the hypothalamus and amygdala. Our findings suggest that ET-1 modulates central coordination of stress responses in close association with catecholamine metabolism.  相似文献   
160.
Embryos of the silkworm, Bombyx mori L., were dispersed by trypsin and the dissociated cells were cultured for infection with nuclear polyhedrosis virus (NPV) of the silkworm. The monolayer and suspension cultures were infected with NPV. RNA and DNA syntheses in the normal and NPV-infected cells were measured by incorporation of 32P into RNA and DNA fractions. RNA and DNA syntheses in the cells after infection significantly increased over those in control cells (mock infection). The effects of actinomycin D, chloramphenicol and mitomycin C on RNA and DNA syntheses in infected cells were examined. The syntheses were inhibited by the antibiotics. It was suggested that the cellular DNA synthesis was inhibited by the viral infection, because the mitomycin C-resistant DNA synthesis was found in the normal cells but not in the infected cells treated with mitomycin C. The rate of DNA synthesis induced by NPV was immediately dropped to that of control cells by addition of chloramphenicol, while the RNA synthesis induced by NPV was not affected for 6 hr after the addition of chloramphenicol. If the antibiotic did not affect the size of precursor pools, this event suggested that the RNA polymerase concerned with viral RNA synthesis was more stable than the DNA polymerase participating in the viral DNA synthesis. The viral DNA as templates for RNA and DNA syntheses was decomposed by mitomycin C.  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