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   2005篇
  免费   138篇
  国内免费   7篇
  2024年   3篇
  2023年   8篇
  2022年   30篇
  2021年   69篇
  2020年   33篇
  2019年   49篇
  2018年   57篇
  2017年   55篇
  2016年   65篇
  2015年   98篇
  2014年   101篇
  2013年   108篇
  2012年   142篇
  2011年   161篇
  2010年   92篇
  2009年   84篇
  2008年   107篇
  2007年   122篇
  2006年   120篇
  2005年   110篇
  2004年   112篇
  2003年   102篇
  2002年   90篇
  2001年   23篇
  2000年   12篇
  1999年   17篇
  1998年   12篇
  1997年   11篇
  1996年   7篇
  1995年   7篇
  1994年   11篇
  1993年   6篇
  1992年   13篇
  1991年   3篇
  1990年   6篇
  1989年   6篇
  1988年   12篇
  1987年   4篇
  1985年   12篇
  1984年   12篇
  1983年   8篇
  1982年   7篇
  1981年   3篇
  1980年   5篇
  1979年   11篇
  1977年   6篇
  1975年   3篇
  1974年   4篇
  1972年   2篇
  1966年   2篇
排序方式: 共有2150条查询结果,搜索用时 15 毫秒
1.
2.
3.
Measurements of the coefficient of water molecules self-diffusion (D) and the time of spin-lattice relaxation (T 1) in prosenchyme (elongated) plant cells, whose length significantly exceeding their transverse size, show that the orientation of plant tissues in the H 0field significantly affects the measured parameters. We conclude that this effect should be taken into account in experiments on the measurement of self-diffusion coefficients and time of proton spin-lattice relaxation in plant tissues containing prosenchyme cells.  相似文献   
4.
5.
Summary Escherichia coli cells, carrying a rifampicin sensitive RNA polymerase -subunit gene in the chromosome and a rifampicin resistant -subunit gene placed under the control of a strong promoter in a multicopy plasmid, are unable to grow in the presence of rifampicin, despite the accumulation of large quantities of the resistant subunit. A major portion of the overproduced subunit is found in an insoluble form. Conditions known to induce the heat shock proteins (hsps), e.g. elevated temperature or the presence of ethanol in the growth medium, increase the amount of the plasmid-borne -subunit which apparently assembles into active RNA polymerase and makes the plasmid bearing cells rifampicin resistant. Alternatively, plasmid-borne subunits assemble into RNA polymerase with low efficiency in rpoH mutant cells known to have reduced level of hsps. We suggest that the plasmid-borne subunit is poorly assembled into RNA polymerase and that hsps promote the assembly by interfering with -subunit aggregation.  相似文献   
6.
Summary Methylation protection experiments with four promoters (P1 and P2 of the pBR322 plasmid, lacUV5 and lambda P0) have shown that the RNA polymerases from Escherichia coli and Pseudomonas putida, while differing in the primary structure of the subunits involved in DNA binding, display identical patterns of DNA contacts. Nor do these enzymes differ in covalent cross-linking patterns with a partially apurinized promoter. We conclude that the two RNA polymerases have very similar structures of DNA binding centers. The evolutionary conservation of this structure may account for the fact that diverse RNA polymerases often recognize and efficiently use promoters of distant bacterial species.  相似文献   
7.
Summary The unusual recombinant plasmid pRC19 carrying the N-terminal fragment of the Escherichia coli RNA polymerase rpoB gene was found to specify high level rifampicin resistance of E. coli cells. Sequence analysis of this plasmid revealed one substitution only: transversion GT, leading to amino acid substitution Val146Phe. This mutational change marks the second domain of the subunit involved in rifampicin binding.  相似文献   
8.
Broad host range vectors derived from an RSF1010::Tn1 plasmid   总被引:2,自引:0,他引:2  
  相似文献   
9.
Evolutionary relationships of eukaryotic kingdoms   总被引:5,自引:0,他引:5  
The evolutionary relationships of four eukaryotic kingdoms—Animalia, Plantae, Fungi, and Protista—remain unclear. In particular, statistical support for the closeness of animals to fungi rather than to plants is lacking, and a preferred branching order of these and other eukaryotic lineages is still controversial even though molecular sequences from diverse eukaryotic taxa have been analyzed. We report a statistical analysis of 214 sequences of nuclear small-subunit ribosomal RNA (srRNA) gene undertaken to clarify these evolutionary relationships. We have considered the variability of substitution rates and the nonindependence of nucleotide substitution across sites in the srRNA gene in testing alternative hypotheses regarding the branching patterns of eukaryote phylogeny. We find that the rates of evolution among sites in the srRNA sequences vary substantially and are approximately gamma distributed with size and shape parameter equal to 0.76. Our results suggest that (1) the animals and true fungi are indeed closer to each other than to any other crown group in the eukaryote tree, (2) red algae are the closest relatives of animals, true fungi, and green plants, and (3) the heterokonts and alveolates probably evolved prior to the divergence of red algae and animal-fungus-green-plant lineages. Furthermore, our analyses indicate that the branching order of the eukaryotic lineages that diverged prior to the evolution of alveolates may be generally difficult to resolve with the srRNA sequence data.  相似文献   
10.
Successful progression through the cell cycle requires the coupling of mitotic spindle formation to DNA replication. In this report we present evidence suggesting that, inSaccharomyces cerevisiae, theCDC40 gene product is required to regulate both DNA replication and mitotic spindle formation. The deduced amino acid sequence ofCDC40 (455 amino acids) contains four copies of a β-transducin-like repeat. Cdc40p is essential only at elevated temperatures, as a complete deletion or a truncated protein (deletion of the C-terminal 217 amino acids in thecdc40-1 allele) results in normal vegetative growth at 23°C, and cell cycle arrest at 36°C. In the mitotic cell cycle Cdc40p is apparently required for at least two steps: (1) for entry into S phase (neither DNA synthesis, nor mitotic spindle formation occurs at 36°C and (2) for completion of S-phase (cdc40::LEU2 cells cannot complete the cell cycle when returned to the permissive temperature in the presence of hydroxyurea). The role of Cdc40p as a regulatory protein linking DNA synthesis, spindle assembly/maintenance, and maturation promoting factor (MPF) activity is discussed.  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