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   1456581篇
  免费   138919篇
  国内免费   1436篇
  2021年   18072篇
  2019年   16231篇
  2018年   17982篇
  2017年   16674篇
  2016年   28470篇
  2015年   42888篇
  2014年   50948篇
  2013年   77371篇
  2012年   40539篇
  2011年   29590篇
  2010年   44541篇
  2009年   45710篇
  2008年   27850篇
  2007年   26239篇
  2006年   30883篇
  2005年   31901篇
  2004年   31207篇
  2003年   28640篇
  2002年   26626篇
  2001年   38600篇
  2000年   36359篇
  1999年   35186篇
  1998年   26113篇
  1997年   26109篇
  1996年   25559篇
  1995年   23727篇
  1994年   23539篇
  1993年   22726篇
  1992年   30956篇
  1991年   29567篇
  1990年   28226篇
  1989年   28667篇
  1988年   26546篇
  1987年   25279篇
  1986年   24133篇
  1985年   25996篇
  1984年   25108篇
  1983年   22232篇
  1982年   22011篇
  1981年   21273篇
  1980年   19815篇
  1979年   21792篇
  1978年   19859篇
  1977年   18926篇
  1976年   18028篇
  1975年   17863篇
  1974年   18314篇
  1973年   18602篇
  1972年   16162篇
  1971年   14533篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
63.
64.
Studies of association between candidate genes and disease can be designed to use cases with disease, and in place of nonrelated controls, their parents. The advantage of this design is the elimination of spurious differences due to ethnic differences between cases and nonrelated controls. However, several statistical methods of analysis have been proposed in the literature, and the choice of analysis is not always clear. We review some of the statistical methods currently developed and present two new statistical methods aimed at specific genetic hypotheses of dominance and recessivity of the candidate gene. These new methods can be more powerful than other current methods, as demonstrated by simulations. The basis of these new statistical methods is a likelihood approach. The advantage of the likelihood framework is that regression models can be developed to assess genotype-environment interactions, as well as the relative contribution that alleles at the candidate-gene locus make to the relative risk (RR) of disease. This latter development allows testing of (1) whether interactions between alleles exist, on the scale of log RR, and (2) whether alleles originating from the mother or father of a case impart different risks, i.e., genomic imprinting.  相似文献   
65.
The Fis protein: it''s not just for DNA inversion anymore   总被引:36,自引:0,他引:36  
  相似文献   
66.
Five open reading frames designated nirB, nirD, nirE, nirC and cysG have been identified from the DNA sequence of the Escherichia coli nir operon. Complementation experiments established that the NirB, NirD and CysG polypeptides are essential and sufficient for NADH-dependent nitrite reductase activity (EC 1.6.6.4). A series of plasmids has been constructed in which each of the open reading frames has been fused in-phase with the beta-galactosidase gene, lacZ. Rates of beta-galactosidase synthesis during growth in different media revealed that nirB, -D, -E and -C are transcribed from the FNR-dependent promoter, p-nirB, located just upstream of the nirB gene: expression is co-ordinately repressed by oxygen and induced during anaerobic growth. Although the nirB, -D and -C open reading frames are translated into protein, no translation of nirE mRNA was detected. The cysG gene product is expressed from both p-nirB and a second, FNR-independent promoter, p-cysG, located within the nirC gene. No NADH-dependent nitrite reductase activity was detected in extracts from bacteria lacking either NirB or NirD, but a mixture of the two was as active as an extract from wild-type bacteria. Reconstitution of enzyme activity in vitro required stoichiometric quantities of NirB and NirD and was rapid and independent of the temperature during mixing. NirD remained associated with NirB during the initial stages of purification of the active enzyme, suggesting that NirD is a second structural subunit of the enzyme.  相似文献   
67.
Changes in the properties of adenylate cyclase from the lungs of tuberculotic guinea pigs were revealed. The number of beta-adrenergic receptors in the lungs was found to be reduced by 30% at the second and by 70% at the third stage of the disease. The degree and the value of Ka for adenylate cyclase activation by isoproterenol remained thereby unchanged. The basal activity of adenylate cyclase was increased by 20% against the control level at the second stage and decreased by 20% at the third stage of the disease. At these periods, the stimulating effects of guanylyl imidodiphosphate, NaF and forskolin on lung adenylate cyclase were diminished. The experimental results point to the significant role of the enzymes of cAMP metabolism and reflect the course of the tuberculosis process in experimental animals.  相似文献   
68.
The cytochrome o terminal oxidase from Escherichia coli was immunochemically purified and monospecific antiserum toward cytochrome o was obtained. This antiserum is able to precipitate 100% of the ubiquinol-1 oxidase activity in Triton X-100 extracts of membranes from an E. coli strain in which cytochrome o is the only terminal oxidase. Cytochrome o was analyzed and quantitated using crossed immunoelectrophoresis, rocket immunoelectrophoresis, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that cytochrome o is composed of four subunits of approximate equimolar stoichiometry with molecular weights of 51,000, 28,500, 18,000, and 12,700. The low temperature (77 K) reduced - oxidized spectrum of the immunoprecipitate shows two peaks at 555 and 562 nm, indicating b-type cytochromes. With the anti-cytochrome o and antiserum toward the cytochrome d terminal oxidase complex which was previously obtained, it is possible to immunochemically assay for all the cytochromes in the cytoplasmic membrane of aerobically grown E. coli. Preliminary results indicate that the biosynthesis of cytochrome o is repressed when cytochrome d is induced by lowering the dissolved oxygen concentration during cell growth.  相似文献   
69.
70.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982).  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