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   115196篇
  免费   8980篇
  国内免费   7766篇
  131942篇
  2024年   252篇
  2023年   1585篇
  2022年   3465篇
  2021年   5788篇
  2020年   3831篇
  2019年   4674篇
  2018年   4607篇
  2017年   3410篇
  2016年   4807篇
  2015年   6998篇
  2014年   8203篇
  2013年   8675篇
  2012年   10381篇
  2011年   9253篇
  2010年   5698篇
  2009年   4987篇
  2008年   5846篇
  2007年   5136篇
  2006年   4570篇
  2005年   3484篇
  2004年   3094篇
  2003年   2664篇
  2002年   2340篇
  2001年   2095篇
  2000年   1931篇
  1999年   1918篇
  1998年   1051篇
  1997年   1172篇
  1996年   1042篇
  1995年   936篇
  1994年   956篇
  1993年   684篇
  1992年   1010篇
  1991年   856篇
  1990年   621篇
  1989年   572篇
  1988年   493篇
  1987年   421篇
  1986年   396篇
  1985年   395篇
  1984年   215篇
  1983年   202篇
  1982年   139篇
  1981年   116篇
  1980年   107篇
  1979年   118篇
  1978年   80篇
  1977年   61篇
  1974年   76篇
  1972年   62篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Growth and death in overagitated microcarrier cell cultures   总被引:1,自引:0,他引:1  
The effects of hydrodynamic forces on cell growth are investigated for animal cells growing on microcarriers. A reduction in net growth was observed with high levels of agitation. DNA measurements indicated that the reduction in net growth was due entirely to cell death, from hydrodynamic forces. No inhibition or enhancement of cell replication appeared to occur with high levels of agitation.  相似文献   
32.
33.
The cytogenetics of domestic geese   总被引:5,自引:0,他引:5  
Hybrids were produced between an African male and several Pilgrim female domestic geese. Partial karyotypes revealed a difference in the fourth largest pair of autosomal chromosomes. This chromosome pair was metacentric in the African, submetacentric in the Pilgrim, and heteromorphic in the hybrids. A similar difference between the putative wild ancestors of the African and Pilgrim breeds has been reported by others. These findings provide cytological evidence to support the traditional opinion that the African breed was derived from the Asiatic swan goose (Anser cygnoides) and the Pilgrim breed was derived from the European greylag goose (Anser anser).  相似文献   
34.
Double-stranded DNA sequencing with T7 polymerase   总被引:11,自引:0,他引:11  
Y Wang 《BioTechniques》1988,6(9):843-845
  相似文献   
35.
36.
A chronic animal model is described which permits for the first time the continuous measurement of uterine artery blood flow velocity in the pregnant guinea-pig by using a miniaturized Doppler flow probe. Preliminary validation revealed that alterations in actual blood flow are directly and proportionally related to the change in the Doppler shift (r = 0.984) from 0 to 100 ml/h. The velocity signal baseline was as stable as that of systemic blood pressure. Depending upon the individual animal's flow velocity, a deviation of 2-5% from baseline was statistically significant. With experience, greater than 90% of preparations were successful and a 30-day interval was often available for study. Uterine artery flow velocity increased steadily between 45 and 55 days of gestation. Instrumentation did not result in fetal growth retardation. A reduction in flow velocity occurred during general anaesthesia using ketamine and the antianxietal xylazine. In agreement with the reports of other investigators using a different model, both hydralazine and angiotensin II increased uterine blood velocity and adrenaline reduced it.  相似文献   
37.
Pteroylpolyglutamate hydrolase was solubilized with Triton X-100 from human jejunal mucosal brush borders and purified approximately 5,000-fold using organomercurial affinity chromatography, DEAE-cellulose chromatography, and gel filtration. The apparent molecular weight of the purified enzyme in the Triton micelle was estimated as 700,000 using Bio-Gel A-1.5m gel filtration. Sodium dodecyl sulfate/urea-polyacrylamide gel electrophoresis followed by Coomassie stain demonstrated two polypeptide bands at 145,000 and 115,000 daltons. The purified enzyme had an isoelectric point of 7.2, was maximally active at pH 5.5, and was stable above pH 6.5 and at temperatures up to 65 degrees C for at least 90 min. Human jejunal brush-border pteroylpolyglutamate hydrolase is an exopeptidase which liberated [14C]Glu as the sole labeled product of PteGlu2[14C]Glue (where PteGlun represents pteroylpolyglutamate), failed to liberate a radioactive product from PteGlu2[14C]GluLeu2, and released all possible labeled PteGlun products during incubation with Pte[14C]GluGlu6 with the accumulation of Pte[14C]Glu. PteGlu2, PteGlu3, and PteGlu7 were substrates, each with Km = 0.6 microM, whereas PteGlu was a weak inhibitor of the hydrolysis of PteGlu3 with Ki = 20 microM. Components of the pteroyl moiety, Glu, and short chain Glun in alpha or gamma linkages were not inhibitory. The enzyme was activated by Zn2+ or Co2+. The properties of brush-border pteroylpolyglutamate hydrolase are different from those described for the soluble intracellular pteroylpolyglutamate hydrolase in other species and in human mucosa, yet are consistent with previous data on the process of hydrolysis of PteGlun in the intact human intestine.  相似文献   
38.
39.
40.
We have used single strand specific nucleases to map DNA distortion in the adult chicken beta A-globin gene. We have detected two structures of that kind and have mapped nuclease-cutting sites at one base resolution. One prominent site is centered at -190 relative to the RNA capping site and is positioned at the center of a stretch of contiguous C residues. The second site is near the first intron/exon junction (+620) and appears as a series of discrete 1-base-long enzyme-cutting sites. Based upon the pattern of nuclease cutting and the kinetics of nuclease cutting we conclude that the "poly(C)" stretch may assume a looped geometry in supertwisted DNA molecules which is similar to that proposed by Felsenfeld (Nickol, J. M., and Felsenfeld, G. (1983) Cell 35, 467-477). We show that S1 nuclease cuts within the intron occur mainly at the end points of polypurine segments and suggest that such end points may assume a distorted transitional geometry. We find that Neurospora crassa endonuclease cuts both the promotor and intron sites in linear DNA molecules but that in linear DNA the cutting process is limited by a first order conformation change of the DNA substrate. Based upon those kinetics we propose that in unstressed DNA, each of the two sites can convert between a distorted and undistorted geometry. In the enzyme assay buffer at 37 degrees C, the time constant for the equilibrium is nearly 10 h for the promotor site and 7 h for the intron.  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