全文获取类型
收费全文 | 20578篇 |
免费 | 1895篇 |
国内免费 | 22篇 |
专业分类
22495篇 |
出版年
2022年 | 137篇 |
2021年 | 276篇 |
2020年 | 144篇 |
2019年 | 217篇 |
2018年 | 271篇 |
2017年 | 220篇 |
2016年 | 398篇 |
2015年 | 688篇 |
2014年 | 755篇 |
2013年 | 950篇 |
2012年 | 1225篇 |
2011年 | 1296篇 |
2010年 | 785篇 |
2009年 | 687篇 |
2008年 | 991篇 |
2007年 | 1097篇 |
2006年 | 1011篇 |
2005年 | 1000篇 |
2004年 | 982篇 |
2003年 | 970篇 |
2002年 | 935篇 |
2001年 | 236篇 |
2000年 | 163篇 |
1999年 | 228篇 |
1998年 | 270篇 |
1997年 | 198篇 |
1996年 | 178篇 |
1995年 | 206篇 |
1994年 | 189篇 |
1993年 | 172篇 |
1992年 | 186篇 |
1991年 | 159篇 |
1990年 | 150篇 |
1989年 | 139篇 |
1988年 | 152篇 |
1987年 | 146篇 |
1986年 | 141篇 |
1985年 | 169篇 |
1984年 | 186篇 |
1983年 | 159篇 |
1982年 | 220篇 |
1981年 | 232篇 |
1980年 | 228篇 |
1979年 | 153篇 |
1978年 | 157篇 |
1977年 | 148篇 |
1976年 | 172篇 |
1975年 | 128篇 |
1974年 | 143篇 |
1973年 | 160篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Refinement of the AMBER force field for nucleic acids: improving the description of alpha/gamma conformers 下载免费PDF全文
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
992.
Frugivory by Small Vertebrates Within a Deforested, Dry Tropical Region of Central America 总被引:1,自引:0,他引:1
Small vertebrates were inventoried within three habitat types in a degraded dry forest region of Panama. Animals were classified as frugivorous if they were observed foraging on fruit or if fecal samples contained mostly or exclusively seeds. Overall, we found that eight bat species and 21 bird species consumed fruit. The greatest numbers of birds were observed within live fences and bird species richness was greatest within riparian forests. Bat assemblages were not significantly different between habitats. The implication is that ecosystem services such as seed dispersal may still be functional in this landscape. 相似文献
993.
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
994.
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
995.
996.
997.
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
998.
999.
Elena SF Wilke CO Ofria C Lenski RE 《Evolution; international journal of organic evolution》2007,61(3):666-674
It is often assumed that the efficiency of selection for mutational robustness would be proportional to mutation rate and population size, thus being inefficient in small populations. However, Krakauer and Plotkin (2002) hypothesized that selection in small populations would favor robustness mechanisms, such as redundancy, that mask the effect of deleterious mutations. In large populations, by contrast, selection is more effective at removing deleterious mutants and fitness would be improved by eliminating mechanisms that mask the effect of deleterious mutations and thus impede their removal. Here, we test whether these predictions are supported in experiments with evolving populations of digital organisms. Digital organisms are self-replicating programs that inhabit a virtual world inside a computer. Like their organic counterparts, digital organisms mutate, compete, evolve, and adapt by natural selection to their environment. In this study, 160 populations evolved at different combinations of mutation rate and population size. After 10(4) generations, we measured the mutational robustness of the most abundant genotype in each population. Mutational robustness tended to increase with mutation rate and to decline with population size, although the dependence with population size was in part mediated by a negative relationship between fitness and robustness. These results are independent of whether genomes were constrained to their original length or allowed to change in size. 相似文献
1000.
Patterns of sequence variability and divergence at the diminutive gene region of Drosophila melanogaster: complex patterns suggest an ancestral selective sweep 总被引:1,自引:0,他引:1 下载免费PDF全文
To identify putatively swept regions of the Drosophila melanogaster genome, we performed a microsatellite screen spanning a 260-kb region of the X chromosome in populations from Zimbabwe, Ecuador, the United States, and China. Among the regions identified by this screen as showing a complex pattern of reduced heterozygosity and a skewed frequency spectrum was the gene diminutive (dm). To investigate the microsatellite findings, nucleotide sequence polymorphism data were generated in populations from both China and Zimbabwe spanning a 25-kb region and encompassing dm. Analysis of the sequence data reveals strongly reduced nucleotide variation across the entire gene region in both the non-African and the African populations, an extended haplotype pattern, and structured linkage disequilibrium, as well as a rejection of neutrality in favor of selection using a composite likelihood-ratio test. Additionally, unusual patterns of synonymous site evolution were observed at the second exon of this locus. On the basis of simulation studies as well as recently proposed methods for distinguishing between selection and nonequilibrium demography, we find that this "footprint" is best explained by a selective sweep in the ancestral population, the signal of which has been somewhat blurred via founder effects in the non-African samples. 相似文献