全文获取类型
收费全文 | 20465篇 |
免费 | 1889篇 |
国内免费 | 19篇 |
出版年
2022年 | 126篇 |
2021年 | 270篇 |
2020年 | 134篇 |
2019年 | 213篇 |
2018年 | 273篇 |
2017年 | 211篇 |
2016年 | 389篇 |
2015年 | 689篇 |
2014年 | 764篇 |
2013年 | 943篇 |
2012年 | 1227篇 |
2011年 | 1298篇 |
2010年 | 784篇 |
2009年 | 682篇 |
2008年 | 987篇 |
2007年 | 1090篇 |
2006年 | 1015篇 |
2005年 | 998篇 |
2004年 | 989篇 |
2003年 | 963篇 |
2002年 | 940篇 |
2001年 | 221篇 |
2000年 | 162篇 |
1999年 | 220篇 |
1998年 | 270篇 |
1997年 | 196篇 |
1996年 | 175篇 |
1995年 | 213篇 |
1994年 | 190篇 |
1993年 | 175篇 |
1992年 | 180篇 |
1991年 | 156篇 |
1990年 | 147篇 |
1989年 | 145篇 |
1988年 | 150篇 |
1987年 | 146篇 |
1986年 | 144篇 |
1985年 | 168篇 |
1984年 | 195篇 |
1983年 | 159篇 |
1982年 | 224篇 |
1981年 | 231篇 |
1980年 | 227篇 |
1979年 | 151篇 |
1978年 | 156篇 |
1977年 | 141篇 |
1976年 | 171篇 |
1975年 | 135篇 |
1974年 | 141篇 |
1973年 | 159篇 |
排序方式: 共有10000条查询结果,搜索用时 240 毫秒
971.
Lasha Gogokhia Kate Buhrke Rickesha Bell Brenden Hoffman D. Garrett Brown Christin Hanke-Gogokhia Nadim J. Ajami Matthew C. Wong Arevik Ghazaryan John F. Valentine Nathan Porter Eric Martens Ryan O’Connell Vinita Jacob Ellen Scherl Carl Crawford W. Zac Stephens Sherwood R. Casjens June L. Round 《Cell host & microbe》2019,25(2):285-299.e8
972.
Polly Campbell Lena Arvalo Heather Martin Charles Chen Shuzhen Sun Ashlee H. Rowe Michael S. Webster Jeremy B. Searle Bret Pasch 《Ecology and evolution》2019,9(22):12886-12896
Behavioral barriers to gene flow often evolve faster than intrinsic incompatibilities and can eliminate the opportunity for hybridization between interfertile species. While acoustic signal divergence is a common driver of premating isolation in birds and insects, its contribution to speciation in mammals is less studied. Here we characterize the incidence of, and potential barriers to, hybridization among three closely related species of grasshopper mice (genus Onychomys). All three species use long‐distance acoustic signals to attract and localize mates; Onychomys arenicola and Onychomys torridus are acoustically similar and morphologically cryptic whereas Onychomys leucogaster is larger and acoustically distinct. We used genotyping‐by‐sequencing (GBS) to test for evidence of introgression in 227 mice from allopatric and sympatric localities in the western United States and northern Mexico. We conducted laboratory mating trials for all species pairs to assess reproductive compatibility, and recorded vocalizations from O. arenicola and O. torridus in sympatry and allopatry to test for evidence of acoustic character displacement. Hybridization was rare in nature and, contrary to prior evidence for O. torridus/O. arenicola hybrids, only involved O. leucogaster and O. arenicola. In contrast, laboratory crosses between O. torridus and O. arenicola produced litters whereas O. leucogaster and O. arenicola crosses did not. Call fundamental frequency in O. torridus and O. arenicola was indistinguishable in allopatry but significantly differentiated in sympatry, a pattern consistent with reproductive character displacement. These results suggest that assortative mating based on a long‐distance signal is an important isolating mechanism between O. torridus and O. arenicola and highlight the importance of behavioral barriers in determining the permeability of species boundaries. 相似文献
973.
974.
Refinement of the AMBER force field for nucleic acids: improving the description of alpha/gamma conformers
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Pérez A Marchán I Svozil D Sponer J Cheatham TE Laughton CA Orozco M 《Biophysical journal》2007,92(11):3817-3829
We present here the parmbsc0 force field, a refinement of the AMBER parm99 force field, where emphasis has been made on the correct representation of the alpha/gamma concerted rotation in nucleic acids (NAs). The modified force field corrects overpopulations of the alpha/gamma = (g+,t) backbone that were seen in long (more than 10 ns) simulations with previous AMBER parameter sets (parm94-99). The force field has been derived by fitting to high-level quantum mechanical data and verified by comparison with very high-level quantum mechanical calculations and by a very extensive comparison between simulations and experimental data. The set of validation simulations includes two of the longest trajectories published to date for the DNA duplex (200 ns each) and the largest variety of NA structures studied to date (15 different NA families and 97 individual structures). The total simulation time used to validate the force field includes near 1 mus of state-of-the-art molecular dynamics simulations in aqueous solution. 相似文献
975.
The transformation of D-xylose and L-arabinose, the two major components of wheat straw and bran, into a unique multifunctional, optically pure, five-carbon synthon has been achieved. The synthetic sequence requires three steps: suitable protection of the hydroxyl groups of the pentoses, introduction of an iodide at the C-5 position and zinc-mediated opening of the furanose ring leading to the formation of a common substituted pent-4-enal. 相似文献
976.
In addition to the well-known roles of indoleacetic acid and cytokinin in crown gall formation, the plant hormone ethylene also plays an important role in this process. Many plant growth-promoting bacteria (PGPB) encode the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which can degrade ACC, the immediate precursor of ethylene in plants, to alpha-ketobutyrate and ammonia and thereby lower plant ethylene levels. To study the effect of ACC deaminase on crown gall development, an ACC deaminase gene from the PGPB Pseudomonas putida UW4 was introduced into Agrobacterium tumefaciens C58, so that the effect of ACC deaminase activity on tumour formation in tomato and castor bean plants could be assessed. Plants were also coinoculated with A. tumefaciens C58 and P. putida UW4 or P. putida UW4-acdS- (an ACC deaminase minus mutant strain). In both types of experiments, it was observed that the presence of ACC deaminase generally inhibited tumour development on both tomato and castor bean plants. 相似文献
977.
978.
979.
Charles M. Peters Andrew Henderson U Myint Maung U Saw Lwin U Tin Maung Ohn U Kyaw Lwin U Tun Shaung 《Economic botany》2007,61(1):3-13
Although Myanmar exports millions of dollars of rattan cane each year, the last systematic treatment of rattans in this country
was done over 100 years ago, and virtually nothing has been written about the collection and trade of this important forest
resource. Here we report the results from a study of rattans in the Hukaung Valley Tiger Reserve in northern Myanmar. A total
of 15 species of rattan were encountered; seven species are new records for Myanmar and two species are new to science. Inventory
transects revealed that the density of commercial rattans in local forests averages 40.5 canes ≽4m long/hectare. Populations
of all species appear to be actively regenerating. The current pattern of rattan exploitation, however, is largely uncontrolled
and will eventually lead to resource depletion unless some form of management is implemented. 相似文献
980.