全文获取类型
收费全文 | 131648篇 |
免费 | 4400篇 |
国内免费 | 821篇 |
出版年
2022年 | 349篇 |
2021年 | 671篇 |
2020年 | 486篇 |
2019年 | 568篇 |
2018年 | 12365篇 |
2017年 | 11164篇 |
2016年 | 8389篇 |
2015年 | 2304篇 |
2014年 | 2149篇 |
2013年 | 2607篇 |
2012年 | 7003篇 |
2011年 | 15435篇 |
2010年 | 13641篇 |
2009年 | 9722篇 |
2008年 | 11828篇 |
2007年 | 13387篇 |
2006年 | 2367篇 |
2005年 | 2386篇 |
2004年 | 2834篇 |
2003年 | 2678篇 |
2002年 | 2411篇 |
2001年 | 904篇 |
2000年 | 762篇 |
1999年 | 643篇 |
1998年 | 439篇 |
1997年 | 374篇 |
1996年 | 348篇 |
1995年 | 350篇 |
1994年 | 314篇 |
1993年 | 334篇 |
1992年 | 433篇 |
1991年 | 392篇 |
1990年 | 356篇 |
1989年 | 370篇 |
1988年 | 381篇 |
1987年 | 347篇 |
1986年 | 268篇 |
1985年 | 298篇 |
1984年 | 310篇 |
1983年 | 286篇 |
1982年 | 259篇 |
1981年 | 231篇 |
1980年 | 210篇 |
1979年 | 215篇 |
1978年 | 181篇 |
1977年 | 183篇 |
1976年 | 183篇 |
1975年 | 203篇 |
1972年 | 386篇 |
1971年 | 397篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Background
Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation. 相似文献12.
Victoria V. Hargreaves Scarlet S. Shell Dan J. Mazur Martin T. Hess Richard D. Kolodner 《The Journal of biological chemistry》2010,285(12):9301-9310
Indirect evidence has suggested that the Msh2-Msh6 mispair-binding complex undergoes conformational changes upon binding of ATP and mispairs, resulting in the formation of Msh2-Msh6 sliding clamps and licensing the formation of Msh2-Msh6-Mlh1-Pms1 ternary complexes. Here, we have studied eight mutant Msh2-Msh6 complexes with defective responses to nucleotide binding and/or mispair binding and used them to study the conformational changes required for sliding clamp formation and ternary complex assembly. ATP binding to the Msh6 nucleotide-binding site results in a conformational change that allows binding of ATP to the Msh2 nucleotide-binding site, although ATP binding to the two nucleotide-binding sites appears to be uncoupled in some mutant complexes. The formation of Msh2-Msh6-Mlh1-Pms1 ternary complexes requires ATP binding to only the Msh6 nucleotide-binding site, whereas the formation of Msh2-Msh6 sliding clamps requires ATP binding to both the Msh2 and Msh6 nucleotide-binding sites. In addition, the properties of the different mutant complexes suggest that distinct conformational states mediated by communication between the Msh2 and Msh6 nucleotide-binding sites are required for the formation of ternary complexes and sliding clamps. 相似文献
13.
14.
15.
16.
Background
Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes. 相似文献17.
Food availability is important to the dynamics of animal social organizations or populations. However, the role of winter
food availability in animal population dynamics is still controversial. We carried out an experimental study to test Lack’s
hypothesis that reduced food in winter limits survival and spring numbers of breeding individuals of social groups, using
the Mongolian gerbil (Meriones unguiculatus) as model species. We established 24 gerbil social groups in 24, 10 × 10 m, pens in September 2008. We provided wheat seeds
as supplemental food in 12 enclosures from September 2008 to March 2009; the other 12 enclosures, not provided with supplemental
food, served as controls. We live-trapped gerbils at a 2-week interval from September to April. Supplemental food during winter
increased biweekly survival by 10% relative to that in control groups. Only four control social groups survived to the end
of our study whereas all 12 food-supplemented social groups survived through our study period. Supplemental food also increased
cumulative numbers of recruits and group sizes of gerbils. We conclude that winter food availability limits winter survival
and spring social groups or population sizes of Mongolian gerbils. 相似文献
18.
19.
Carbon and nitrogen stable isotope analyses have improved our understanding of food webs and movement patterns of aquatic
organisms. These techniques have recently been applied to diet studies of elasmobranch fishes, but isotope turnover rates
and isotope diet–tissue discrimination are still poorly understood for this group. We performed a diet switch experiment on
captive sandbar sharks (Carcharhinus plumbeus) as a model shark species to determine tissue turnover rates for liver, whole blood, and white muscle. In a second experiment,
we subjected captive coastal skates (Leucoraja spp.) to serial salinity reductions to measure possible impacts of tissue urea content on nitrogen stable isotope values.
We extracted urea from spiny dogfish (Squalus acanthias) white muscle to test for effects on nitrogen stable isotopes. Isotope turnover was slow for shark tissues and similar to
previously published estimates for stingrays and teleost fishes with low growth rates. Muscle isotope data would likely fail
to capture seasonal migrations or diet switches in sharks, while liver and whole blood would more closely reflect shorter
term movement or shifts in diet. Nitrogen stable isotope values of skate blood and skate and dogfish white muscle were not
affected by tissue urea content, suggesting that available diet–tissue discrimination estimates for teleost fishes with similar
physiologies would provide accurate estimates for elasmobranchs. 相似文献
20.
A newborn child was noted to have an ulcerated lesion on the vertex of her scalp, which was diagnosed as aplasia cutis congenita. As this disorder is relatively rare and frequently misdiagnosed, this case is reported and the relevant literature reviewed. 相似文献