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   1588749篇
  免费   157353篇
  国内免费   1562篇
  2021年   18139篇
  2019年   16222篇
  2018年   18177篇
  2017年   16906篇
  2016年   28582篇
  2015年   43110篇
  2014年   51274篇
  2013年   77521篇
  2012年   45126篇
  2011年   34869篇
  2010年   45749篇
  2009年   46503篇
  2008年   33084篇
  2007年   31508篇
  2006年   35495篇
  2005年   35963篇
  2004年   35348篇
  2003年   32818篇
  2002年   30778篇
  2001年   48326篇
  2000年   46185篇
  1999年   42010篇
  1998年   27359篇
  1997年   27374篇
  1996年   26633篇
  1995年   24801篇
  1994年   24479篇
  1993年   23865篇
  1992年   36106篇
  1991年   34368篇
  1990年   33110篇
  1989年   33395篇
  1988年   30606篇
  1987年   29723篇
  1986年   27813篇
  1985年   29423篇
  1984年   27668篇
  1983年   24346篇
  1982年   23402篇
  1981年   22200篇
  1980年   20768篇
  1979年   24266篇
  1978年   21744篇
  1977年   20480篇
  1976年   19473篇
  1975年   19462篇
  1974年   20368篇
  1973年   20562篇
  1972年   17713篇
  1971年   16385篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Oligonucleotide analogs consisting exclusively of alpha-anomeric deoxynucleoside units bridged with phosphorothioate linkages have been synthesized and tested in vitro as antiviral agents against human immunodeficiency virus (HIV) in human T cells. Two 28-mers, an homopolymer alpha-S-dC28 and an oligomer alpha-S-anti-rev complementary to the initiation site of the regulatory viral gene rev exhibited antiviral activities comparable to those reported for the corresponding beta-anomeric phosphorothioate analogs. In contrast, a nuclease-resistant homopolymer, alpha-dC28 was inactive. Their preliminary results would indicate that the origin of oligonucleotide phosphorothioate anti-HIV activity is not exclusively correlated with their higher nuclease resistance.  相似文献   
892.
893.
The photoacoustic spectroscopic studies of purple pigmented leaves revealed the occurrence of anthocyanins and betalains in some local weed species growing on soils with low moisture levels. The pigmentation intensities were higher in C4 plants than in C3 plants. An inverse correlation was observed between pigmentation intensities and soil moisture levels. This work is a part of the UNEP Research Project granted to Prof. D. O. Hall, UNEP Coordinator, Department of Plant Sciences, King’s College; London.  相似文献   
894.
The genome of Mus domesticus has multiple genes of the alpha 1-acid glycoprotein (AGP). Two cDNA clones were identified corresponding to AGP-1 and AGP-2. Moreover, two alleles of AGP-1 exist in inbred mice. The genomic DNA of the AGP-2 gene has been cloned and studied. Here we report the genomic organization of three M. domesticus AGP genes, the sequence analysis of the AGP-3 genomic DNA, and the expression of the AGP-3 gene. The major structural differences between AGP-2 and AGP-3 genes are located in introns 1 and 5. The low level of AGP-3 mRNA can be detected by the polymerase chain reaction (PCR). The molecular basis of the low level expression of AGP-3 and the possible classification of AGP-3 as a pseudogene are discussed.  相似文献   
895.
The kinetics of L-phenylalanine absorption across rat small intestine in sham and 50% distal resected animals, in vivo, have been studied by perfusing jejunal loops and monitoring the disappearance of the substrate from the perfusate. After 5 months postresection the total phenylalanine absorption was increased. The relationship between total absorption of substrate and its concentration in the bulk phase shows a non-saturable component and a saturable one that can be inhibited by methionine, both in control and remnant jejunum. The slope of the line that represents the non-saturable component is greater in remnant jejunum, indicating that the apparent mass-transfer coefficient, K'D, was increased by distal resection. The kinetic analysis of the saturable component shows that Jmax was unaltered and the apparent semisaturation constant, K'M, was slightly decreased by distal small intestine resection. Correction of the kinetic constant for the unstirred water layer effects shows that the differences between 'real' KD values of the two experimental groups increase whereas 'real' KM values do not change significantly. This indicates that the observed increase in total intestinal absorption in resected animals appears to result from an increase in the intestinal passive permeability.  相似文献   
896.
Data are presented to show how the number and growth of juvenile salmon in streams in the Scottish Highlands are influenced by various physical (temperature, water chemistry, depth and velocity, type of substratum) and biotic (food resources, competition, recruitment) factors.  相似文献   
897.
Cultured astrocytes from a syncytium after maturation   总被引:2,自引:0,他引:2  
The formation of functional gap junctions between astrocytes was investigated during differentiation of these cells in culture. Precursor cells of GFA (glial fibrillary acidic) protein-positive astrocytes were cultured in a chemically defined medium as a homogeneous population. These cells were rarely coupled to one neighbour, as revealed by electrical and dye coupling and never formed a large syncytium, as investigated by injection and spread of Lucifer Yellow. Differentiation with respect to GFA protein accumulation can be induced in these cells by culturing in horse serum-containing medium. The formation of functional junctions developed within 2 weeks in about 20% of the cells. Coupled cells formed a large syncytium. When the astrocytes were co-cultured with primary cerebellar cells (consisting predominantly of small neurons) after the switch to serum-containing medium the percentage of coupled astrocytes increased to about 65%. Again the coupled cells formed a large syncytium. Since no physical contact was possible between the astrocyte cultures and the primary cerebellar cells the stimulation of coupling had to be signalized by soluble factor(s).  相似文献   
898.
899.
Saccharomyces cerevisiae CDC8 gene and its product.   总被引:14,自引:6,他引:8  
  相似文献   
900.
B. Boesiger 《Genetica》1982,59(2):99-111
The influence of inbreeding on several morphological characters as proboscis, wings, leg and palp segments was examined in strictly inbred lines of C. pipiens autogenicus, starting with females from a natural population. The results show the importance of ccological conditions, especially temperature, during larval development of the population. We report a progressive average size reduction from the second inbred generation. The length variability increases in practically all cases, depending on the degree of inbreeding, which is often more marked in certain organs. The symmetry regulation is strongly affected. We observe highly significant individual values in the highly inbred generations. The distribution values clearly mark a left/right disturbance. The progression of asymmetry as a function of the degree of inbreeding is more marked in certain organs in the two sexes. The results show the importance of the advantage of heterozygotes.  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